obj_spec.md 132 KB
Wavefront OBJ File Format Specification
Despite how widely supported .obj is, this spec is ridiculously hard to come by. Most search results lead to ads or paywalls, and .edu links keep disappearing...
It's an extremely simple format, most people have figured out what v, vn, vt
and f lines are for on their own, yet it's great to have this spec because this
format has a lot of secret features. I've also appended the material library
file format (.mtl) specification at the end, so this is the fullest spec there
can be.
My one and only addition to the format is the annotation labels:
a (vertex index, same as in face) (an utf-8 string without control characters)
B1. Object Files (.obj)
Object files define the geometry and other properties for objects in Wavefront's Advanced Visualizer. Object files can also be used to transfer geometric data back and forth between the Advanced Visualizer and other applications.
Object files can be in ASCII format (.obj) or binary format (.mod). This appendix describes the ASCII format for object files. These files must have the extension .obj.
In this release, the .obj file format supports both polygonal objects and free-form objects. Polygonal geometry uses points, lines, and faces to define objects while free-form geometry uses curves and surfaces.
About this section
The .obj appendix is for those who want to use the .obj format to translate geometric data from other software applications to Wavefront products. It also provides information for Advanced Visualizer users who want detailed information on the Wavefront .obj file format.
If you are a 2.11 user and want to understand the significance of the 3.0 release and how it affects your existing files, you may be especially interested in the section called "Superseded statements" at the end of the appendix. The section, "Patches and free-form surfaces," gives examples of how 2.11 patches look in 3.0.
How this section is organized
Most of this appendix describes the different parts of an .obj file and how those parts are arranged in the file. The three sections at the end of the appendix provide background information on the 3.0 release of the .obj format.
The .obj appendix includes the following sections:
o File structure
o General statement
o Vertex data
o Specifying free-form curves/surfaces
o Free-form curve/surface attributes
o Elements
o Free-form curve/surface body statements
o Connectivity between free-form surfaces
o Grouping
o Display/render attributes
o Comments
o Mathematics for free-form curves/surfaces
o Superseded statements
o Patches and free-form surfaces
The curve and surface extensions to the .obj file format were
developed in conjunction with mental images GmbH&Co.KG, Berlin,
Germany, as part of a joint development project to incorporate
free-form surfaces into Wavefront's Advanced Visualizer.
File structure
The following types of data may be included in an .obj file. In this list, the keyword (in parentheses) follows the data type.
Vertex data
o geometric vertices (v)
o texture vertices (vt)
o vertex normals (vn)
o parameter space vertices (vp)
Free-form curve/surface attributes
o rational or non-rational forms of curve or surface type:
basis matrix, Bezier, B-spline, Cardinal, Taylor (cstype)
o degree (deg)
o basis matrix (bmat)
o step size (step)
Elements
o point (p)
o line (l)
o face (f)
o curve (curv)
o 2D curve (curv2)
o surface (surf)
Free-form curve/surface body statements
o parameter values (parm)
o outer trimming loop (trim)
o inner trimming loop (hole)
o special curve (scrv)
o special point (sp)
o end statement (end)
Connectivity between free-form surfaces
o connect (con)
Grouping
o group name (g)
o smoothing group (s)
o merging group (mg)
o object name (o)
Display/render attributes
o bevel interpolation (bevel)
o color interpolation (c_interp)
o dissolve interpolation (d_interp)
o level of detail (lod)
o material name (usemtl)
o material library (mtllib)
o shadow casting (shadow_obj)
o ray tracing (trace_obj)
o curve approximation technique (ctech)
o surface approximation technique (stech)
The following diagram shows how these parts fit together in a typical .obj file.
Figure B1-1. Typical .obj file structure
General statement
call filename.ext arg1 arg2 . . .
Reads the contents of the specified .obj or .mod file at this
location. The call statement can be inserted into .obj files using
a text editor.
filename.ext is the name of the .obj or .mod file to be read. You
must include the extension with the filename.
arg1 arg2 . . . specifies a series of optional integer arguments
that are passed to the called file. There is no limit to the number
of nested calls that can be made.
Arguments passed to the called file are substituted in the same way
as in UNIX scripts; for example, $1 in the called file is replaced
by arg1, $2 in the called file is replaced by arg2, and so on.
If the frame number is needed in the called file for variable
substitution, "$1" must be used as the first argument in the call
statement. For example:
call filename.obj $1
Then the statement in the called file,
scmp filename.pv $1
will work as expected. For more information on the scmp statement,
see appendix C, Variable Substitution for more information.
Another method to do the same thing is:
scmp filename.pv $1
call filename.obj
Using this method, the scmp statement provides the .pv file for all
subsequently called .obj or .mod files.
csh command
csh -command
Executes the requested UNIX command. If the UNIX command returns an
error, the parser flags an error during parsing.
If a dash (-) precedes the UNIX command, the error is ignored.
command is the UNIX command.
Vertex data
Vertex data provides coordinates for:
o geometric vertices
o texture vertices
o vertex normals
For free-form objects, the vertex data also provides:
o parameter space vertices
The vertex data is represented by four vertex lists; one for each type of vertex coordinate. A right-hand coordinate system is used to specify the coordinate locations.
The following sample is a portion of an .obj file that contains the four types of vertex information.
v -5.000000 5.000000 0.000000
v -5.000000 -5.000000 0.000000
v 5.000000 -5.000000 0.000000
v 5.000000 5.000000 0.000000
vt -5.000000 5.000000 0.000000
vt -5.000000 -5.000000 0.000000
vt 5.000000 -5.000000 0.000000
vt 5.000000 5.000000 0.000000
vn 0.000000 0.000000 1.000000
vn 0.000000 0.000000 1.000000
vn 0.000000 0.000000 1.000000
vn 0.000000 0.000000 1.000000
vp 0.210000 3.590000
vp 0.000000 0.000000
vp 1.000000 0.000000
vp 0.500000 0.500000
When vertices are loaded into the Advanced Visualizer, they are sequentially numbered, starting with 1. These reference numbers are used in element statements.
Syntax
The following syntax statements are listed in order of complexity.
v x y z w
Polygonal and free-form geometry statement.
Specifies a geometric vertex and its x y z coordinates. Rational
curves and surfaces require a fourth homogeneous coordinate, also
called the weight.
x y z are the x, y, and z coordinates for the vertex. These are
floating point numbers that define the position of the vertex in
three dimensions.
w is the weight required for rational curves and surfaces. It is
not required for non-rational curves and surfaces. If you do not
specify a value for w, the default is 1.0.
NOTE: A positive weight value is recommended. Using zero or
negative values may result in an undefined point in a curve or
surface.
vp u v w
Free-form geometry statement.
Specifies a point in the parameter space of a curve or surface.
The usage determines how many coordinates are required. Special
points for curves require a 1D control point (u only) in the
parameter space of the curve. Special points for surfaces require a
2D point (u and v) in the parameter space of the surface. Control
points for non-rational trimming curves require u and v
coordinates. Control points for rational trimming curves require u,
v, and w (weight) coordinates.
u is the point in the parameter space of a curve or the first
coordinate in the parameter space of a surface.
v is the second coordinate in the parameter space of a surface.
w is the weight required for rational trimming curves. If you do
not specify a value for w, it defaults to 1.0.
NOTE: For additional information on parameter vertices, see the
curv2 and sp statements
vn i j k
Polygonal and free-form geometry statement.
Specifies a normal vector with components i, j, and k.
Vertex normals affect the smooth-shading and rendering of geometry.
For polygons, vertex normals are used in place of the actual facet
normals. For surfaces, vertex normals are interpolated over the
entire surface and replace the actual analytic surface normal.
When vertex normals are present, they supersede smoothing groups.
i j k are the i, j, and k coordinates for the vertex normal. They
are floating point numbers.
vt u v w
Vertex statement for both polygonal and free-form geometry.
Specifies a texture vertex and its coordinates. A 1D texture
requires only u texture coordinates, a 2D texture requires both u
and v texture coordinates, and a 3D texture requires all three
coordinates.
u is the value for the horizontal direction of the texture.
v is an optional argument.
v is the value for the vertical direction of the texture. The
default is 0.
w is an optional argument.
w is a value for the depth of the texture. The default is 0.
Specifying free-form curves/surfaces
There are three steps involved in specifying a free-form curve or surface element.
o Specify the type of curve or surface (basis matrix, Bezier,
B-spline, Cardinal, or Taylor) using free-form curve/surface
attributes.
o Describe the curve or surface with element statements.
o Supply additional information, using free-form curve/surface
body statements
The next three sections of this appendix provide detailed information on each of these steps.
Data requirements for curves and surfaces
All curves and surfaces require a certain set of data. This consists of the following:
Free-form curve/surface attributes
o All curves and surfaces require type data, which is given with
the cstype statement.
o All curves and surfaces require degree data, which is given
with the deg statement.
o Basis matrix curves or surfaces require a bmat statement.
o Basis matrix curves or surfaces also require a step size, which
is given with the step statement.
Elements
o All curves and surfaces require control points, which are
referenced in the curv, curv2, or surf statements.
o 3D curves and surfaces require a parameter range, which is
given in the curv and surf statements, respectively.
Free-form curve/surface body statements
o All curves and surfaces require a set of global parameters or a
knot vector, both of which are given with the parm statement.
o All curves and surfaces body statements require an explicit end
statement.
Error checks
The above set of data starts out empty with no default values when reading of an .obj file begins. While the file is being read, statements are encountered, information is accumulated, and some errors may be reported.
When the end statement is encountered, the following error checks, which involve consistency between various statements, are performed:
o All required information is present.
o The number of control points, number of parameter values
(knots), and degree are consistent with the curve or surface
type. If the type is bmatrix, the step size is also consistent.
(For more information, refer to the parameter vector equations
in the section, "Mathematics of free-form curves/ surfaces" at
the end of appendix B1.)
o If the type is bmatrix and the degree is n, the size of the
basis matrix is (n + 1) x (n + 1).
Note that any information given by the state-setting statements remains in effect from one curve or surface to the next. Information given within a curve or surface body is only effective for the curve or surface it is given with.
Free-form curve/surface attributes
Five types of free-form geometry are available in the .obj file format:
o Bezier
o basis matrix
o B-spline
o Cardinal
o Taylor
You can apply these types only to curves and surfaces. Each of these five types can be rational or non-rational.
In addition to specifying the type, you must define the degree for the curve or surface. For basis matrix curve and surface elements, you must also specify the basis matrix and step size.
All free-form curve and surface attribute statements are state-setting. This means that once an attribute statement is set, it applies to all elements that follow until it is reset to a different value.
Syntax
The following syntax statements are listed in order of use.
cstype rat type
Free-form geometry statement.
Specifies the type of curve or surface and indicates a rational or
non-rational form.
rat is an optional argument.
rat specifies a rational form for the curve or surface type. If rat
is not included, the curve or surface is non-rational
type specifies the curve or surface type. Allowed types are:
bmatrix basis matrix
bezier Bezier
bspline B-spline
cardinal Cardinal
taylor Taylor
There is no default. A value must be supplied.
deg degu degv
Free-form geometry statement.
Sets the polynomial degree for curves and surfaces.
degu is the degree in the u direction. It is required for both
curves and surfaces.
degv is the degree in the v direction. It is required only for
surfaces. For Bezier, B-spline, Taylor, and basis matrix, there is
no default; a value must be supplied. For Cardinal, the degree is
always 3. If some other value is given for Cardinal, it will be
ignored.
bmat u matrix
bmat v matrix
Free-form geometry statement.
Sets the basis matrices used for basis matrix curves and surfaces.
The u and v values must be specified in separate bmat statements.
NOTE: The deg statement must be given before the bmat statements
and the size of the matrix must be appropriate for the degree.
u specifies that the basis matrix is applied in the u direction.
v specifies that the basis matrix is applied in the v direction.
matrix lists the contents of the basis matrix with column subscript
j varying the fastest. If n is the degree in the given u or v
direction, the matrix (i,j) should be of size (n + 1) x (n + 1).
There is no default. A value must be supplied.
NOTE: The arrangement of the matrix is different from that commonly
found in other references. For more information, see the examples
at the end of this section and also the section, "Mathematics for
free-form curves and surfaces."
step stepu stepv
Free-form geometry statement.
Sets the step size for curves and surfaces that use a basis
matrix.
stepu is the step size in the u direction. It is required for both
curves and surfaces that use a basis matrix.
stepv is the step size in the v direction. It is required only for
surfaces that use a basis matrix. There is no default. A value must
be supplied.
When a curve or surface is being evaluated and a transition from
one segment or patch to the next occurs, the set of control points
used is incremented by the step size. The appropriate step size
depends on the representation type, which is expressed through the
basis matrix, and on the degree.
That is, suppose we are given a curve with k control points:
{v , ... v }
1 k
If the curve is of degree n, then n + 1 control points are needed
for each polynomial segment. If the step size is given as s, then
the ith polynomial segment, where i = 0 is the first segment, will
use the control points:
{v ,...,v }
is+1 is+n+1
For example, for Bezier curves, s = n .
For surfaces, the above description applies independently to each
parametric direction.
When you create a file which uses the basis matrix type, be sure to
specify a step size appropriate for the current curve or surface
representation.
Examples
Cubic Bezier surface made with a basis matrix
To create a cubic Bezier surface:
cstype bmatrix deg 3 3 step 3 3 bmat u 1 -3 3 -1
0 3 -6 3 \ 0 0 3 -3 \ 0 0 0 1bmat v 1 -3 3 -1
0 3 -6 3 \ 0 0 3 -3 \ 0 0 0 1Hermite curve made with a basis matrix
To create a Hermite curve:
cstype bmatrix deg 3 step 2 bmat u 1 0 -3 2 0 0 3 -2
0 1 -2 1 0 0 -1 1Bezier in u direction with B-spline in v direction; made with a basis matrix
To create a surface with a cubic Bezier in the u direction and cubic uniform B-spline in the v direction:
cstype bmatrix deg 3 3 step 3 1 bmat u 1 -3 3 -1
0 3 -6 3 \ 0 0 3 -3 \ 0 0 0 1bmat v 0.16666 -0.50000 0.50000 -0.16666
0.66666 0.00000 -1.00000 0.50000 \ 0.16666 0.50000 0.50000 -0.50000 \ 0.00000 0.00000 0.00000 0.16666
Elements
For polygonal geometry, the element types available in the .obj file are:
o points
o lines
o faces
For free-form geometry, the element types available in the .obj file are:
o curve
o 2D curve on a surface
o surface
All elements can be freely intermixed in the file.
Referencing vertex data
For all elements, reference numbers are used to identify geometric vertices, texture vertices, vertex normals, and parameter space vertices.
Each of these types of vertices is numbered separately, starting with
- This means that the first geometric vertex in the file is 1, the second is 2, and so on. The first texture vertex in the file is 1, the second is 2, and so on. The numbering continues sequentially throughout the entire file. Frequently, files have multiple lists of vertex data. This numbering sequence continues even when vertex data is separated by other data.
In addition to counting vertices down from the top of the first list in the file, you can also count vertices back up the list from an element's position in the file. When you count up the list from an element, the reference numbers are negative. A reference number of -1 indicates the vertex immediately above the element. A reference number of -2 indicates two references above and so on.
Referencing groups of vertices
Some elements, such as faces and surfaces, may have a triplet of numbers that reference vertex data.These numbers are the reference numbers for a geometric vertex, a texture vertex, and a vertex normal.
Each triplet of numbers specifies a geometric vertex, texture vertex, and vertex normal. The reference numbers must be in order and must separated by slashes (/).
o The first reference number is the geometric vertex.
o The second reference number is the texture vertex. It follows
the first slash.
o The third reference number is the vertex normal. It follows the
second slash.
There is no space between numbers and the slashes. There may be more than one series of geometric vertex/texture vertex/vertex normal numbers on a line.
The following is a portion of a sample file for a four-sided face element:
f 1/1/1 2/2/2 3/3/3 4/4/4
Using v, vt, and vn to represent geometric vertices, texture vertices, and vertex normals, the statement would read:
f v/vt/vn v/vt/vn v/vt/vn v/vt/vn
If there are only vertices and vertex normals for a face element (no texture vertices), you would enter two slashes (//). For example, to specify only the vertex and vertex normal reference numbers, you would enter:
f 1//1 2//2 3//3 4//4
When you are using a series of triplets, you must be consistent in the way you reference the vertex data. For example, it is illegal to give vertex normals for some vertices, but not all.
The following is an example of an illegal statement.
f 1/1/1 2/2/2 3//3 4//4
Syntax
The following syntax statements are listed in order of complexity of geometry.
p v1 v2 v3 . . .
Polygonal geometry statement.
Specifies a point element and its vertex. You can specify multiple
points with this statement. Although points cannot be shaded or
rendered, they are used by other Advanced Visualizer programs.
v is the vertex reference number for a point element. Each point
element requires one vertex. Positive values indicate absolute
vertex numbers. Negative values indicate relative vertex numbers.
l v1/vt1 v2/vt2 v3/vt3 . . .
Polygonal geometry statement.
Specifies a line and its vertex reference numbers. You can
optionally include the texture vertex reference numbers. Although
lines cannot be shaded or rendered, they are used by other Advanced
Visualizer programs.
The reference numbers for the vertices and texture vertices must be
separated by a slash (/). There is no space between the number and
the slash.
v is a reference number for a vertex on the line. A minimum of two
vertex numbers are required. There is no limit on the maximum.
Positive values indicate absolute vertex numbers. Negative values
indicate relative vertex numbers.
vt is an optional argument.
vt is the reference number for a texture vertex in the line
element. It must always follow the first slash.
f v1/vt1/vn1 v2/vt2/vn2 v3/vt3/vn3 . . .
Polygonal geometry statement.
Specifies a face element and its vertex reference number. You can
optionally include the texture vertex and vertex normal reference
numbers.
The reference numbers for the vertices, texture vertices, and
vertex normals must be separated by slashes (/). There is no space
between the number and the slash.
v is the reference number for a vertex in the face element. A
minimum of three vertices are required.
vt is an optional argument.
vt is the reference number for a texture vertex in the face
element. It always follows the first slash.
vn is an optional argument.
vn is the reference number for a vertex normal in the face element.
It must always follow the second slash.
Face elements use surface normals to indicate their orientation. If
vertices are ordered counterclockwise around the face, both the
face and the normal will point toward the viewer. If the vertex
ordering is clockwise, both will point away from the viewer. If
vertex normals are assigned, they should point in the general
direction of the surface normal, otherwise unpredictable results
may occur.
If a face has a texture map assigned to it and no texture vertices
are assigned in the f statement, the texture map is ignored when
the element is rendered.
NOTE: Any references to fo (face outline) are no longer valid as of
version 2.11. You can use f (face) to get the same results.
References to fo in existing .obj files will still be read,
however, they will be written out as f when the file is saved.
curv u0 u1 v1 v2 . . .
Element statement for free-form geometry.
Specifies a curve, its parameter range, and its control vertices.
Although curves cannot be shaded or rendered, they are used by
other Advanced Visualizer programs.
u0 is the starting parameter value for the curve. This is a
floating point number.
u1 is the ending parameter value for the curve. This is a floating
point number.
v is the vertex reference number for a control point. You can
specify multiple control points. A minimum of two control points
are required for a curve.
For a non-rational curve, the control points must be 3D. For a
rational curve, the control points are 3D or 4D. The fourth
coordinate (weight) defaults to 1.0 if omitted.
curv2 vp1 vp2 vp3. . .
Free-form geometry statement.
Specifies a 2D curve on a surface and its control points. A 2D
curve is used as an outer or inner trimming curve, as a special
curve, or for connectivity.
vp is the parameter vertex reference number for the control point.
You can specify multiple control points. A minimum of two control
points is required for a 2D curve.
The control points are parameter vertices because the curve must
lie in the parameter space of some surface. For a non-rational
curve, the control vertices can be 2D. For a rational curve, the
control vertices can be 2D or 3D. The third coordinate (weight)
defaults to 1.0 if omitted.
surf s0 s1 t0 t1 v1/vt1/vn1 v2/vt2/vn2 . . .
Element statement for free-form geometry.
Specifies a surface, its parameter range, and its control vertices.
The surface is evaluated within the global parameter range from s0
to s1 in the u direction and t0 to t1 in the v direction.
s0 is the starting parameter value for the surface in the u
direction.
s1 is the ending parameter value for the surface in the u
direction.
t0 is the starting parameter value for the surface in the v
direction.
t1 is the ending parameter value for the surface in the v
direction.
v is the reference number for a control vertex in the surface.
vt is an optional argument.
vt is the reference number for a texture vertex in the surface. It
must always follow the first slash.
vn is an optional argument.
vn is the reference number for a vertex normal in the surface. It
must always follow the second slash.
For a non-rational surface, the control vertices are 3D. For a
rational surface the control vertices can be 3D or 4D. The fourth
coordinate (weight) defaults to 1.0 if ommitted.
NOTE: For more information on the ordering of control points for
survaces, refer to the section on surfaces and control points in
"mathematics of free-form curves/surfaces" at the end of this
appendix.
Examples
These are examples for polygonal geometry.
For examples using free-form geometry, see the examples at the end of the next section, "Free-form curve/surface body statements."
- Square
This example shows a square that measures two units on each side and faces in the positive direction (toward the camera). Note that the ordering of the vertices is counterclockwise. This ordering determines that the square is facing forward.
v 0.000000 2.000000 0.000000
v 0.000000 0.000000 0.000000
v 2.000000 0.000000 0.000000
v 2.000000 2.000000 0.000000
f 1 2 3 4
- Cube
This is a cube that measures two units on each side. Each vertex is shared by three different faces.
v 0.000000 2.000000 2.000000
v 0.000000 0.000000 2.000000
v 2.000000 0.000000 2.000000
v 2.000000 2.000000 2.000000
v 0.000000 2.000000 0.000000
v 0.000000 0.000000 0.000000
v 2.000000 0.000000 0.000000
v 2.000000 2.000000 0.000000
f 1 2 3 4
f 8 7 6 5
f 4 3 7 8
f 5 1 4 8
f 5 6 2 1
f 2 6 7 3
- Cube with negative reference numbers
This is a cube with negative vertex reference numbers. Each element references the vertices stored immediately above it in the file. Note that vertices are not shared.
v 0.000000 2.000000 2.000000 v 0.000000 0.000000 2.000000 v 2.000000 0.000000 2.000000 v 2.000000 2.000000 2.000000 f -4 -3 -2 -1
v 2.000000 2.000000 0.000000 v 2.000000 0.000000 0.000000 v 0.000000 0.000000 0.000000 v 0.000000 2.000000 0.000000 f -4 -3 -2 -1
v 2.000000 2.000000 2.000000 v 2.000000 0.000000 2.000000 v 2.000000 0.000000 0.000000 v 2.000000 2.000000 0.000000 f -4 -3 -2 -1
v 0.000000 2.000000 0.000000 v 0.000000 2.000000 2.000000 v 2.000000 2.000000 2.000000 v 2.000000 2.000000 0.000000 f -4 -3 -2 -1
v 0.000000 2.000000 0.000000 v 0.000000 0.000000 0.000000 v 0.000000 0.000000 2.000000 v 0.000000 2.000000 2.000000 f -4 -3 -2 -1
v 0.000000 0.000000 2.000000 v 0.000000 0.000000 0.000000 v 2.000000 0.000000 0.000000 v 2.000000 0.000000 2.000000 f -4 -3 -2 -1
Free-form curve/surface body statements
You can specify additional information for free-form curve and surface elements using a series of statements called body statements. The series is concluded by an end statement.
Body statements are valid only when they appear between the free-form element statement (curv, curv2, surf) and the end statement. If they are anywhere else in the .obj file, they do not have any effect.
You can use body statements to specify the following values:
o parameter
o knot vector
o trimming loop
o hole
o special curve
o special point
You cannot use any other statements between the free-form curve or surface statement and the end statement. Using any other of type of statement may cause unpredictable results.
This portion of a sample file shows the knot vector values for a rational B-spline surface with a trimming loop. Notice the end statement to conclude the body statements.
cstype rat bspline
deg 2 2
surf -1.0 2.5 -2.0 2.0 -9 -8 -7 -6 -5 -4 -3 -2 -1
parm u -1.00 -1.00 -1.00 2.50 2.50 2.50
parm v -2.00 -2.00 -2.00 -2.00 -2.00 -2.00
trim 0.0 2.0 1
end
Parameter values and knot vectors
All curve and surface elements require a set of parameter values.
For polynomial curves and surfaces, this specifies global parameter values. For B-spline curves and surfaces, this specifies the knot vectors.
For surfaces, the parameter values must be specified for both the u and v directions. For curves, the parameter values must be specified for only the u direction.
If multiple parameter value statements for the same parametric direction are used inside a single curve or surface body, the last statement is used.
Trimming loops and holes
The trimming loop statement builds a single outer trimming loop as a sequence of curves which lie on a given surface.
The hole statement builds a single inner trimming loop as a sequence of curves which lie on a given surface. The inner loop creates a hole.
The curves are referenced by number in the same way vertices are referenced by face elements.
The individual curves must lie end-to-end to form a closed loop which does not intersect itself and which lies within the parameter range specified for the surface. The loop as a whole may be oriented in either direction (clockwise or counterclockwise).
To cut one or more holes in a region, use a trim statement followed by one or more hole statements. To introduce another trimmed region in the same surface, use another trim statement followed by one or more hole statements. The ordering that associates holes and the regions they cut is important and must be maintained.
If the first trim statement in the sequence is omitted, the enclosing outer trimming loop is taken to be the parameter range of the surface. If no trim or hole statements are specified, then the surface is trimmed at its parameter range.
This portion of a sample file shows a non-rational Bezier surface with two regions, each with a single hole:
cstype bezier
deg 1 1
surf 0.0 2.0 0.0 2.0 1 2 3 4
parm u 0.00 2.00
parm v 0.00 2.00
trim 0.0 4.0 1
hole 0.0 4.0 2
trim 0.0 4.0 3
hole 0.0 4.0 4
end
Special curve
A special curve statement builds a single special curve as a sequence of curves which lie on a given surface.
The curves are referenced by number in the same way vertices are referenced by face elements.
A special curve is guaranteed to be included in any triangulation of the surface. This means that the line formed by approximating the special curve with a sequence of straight line segments will actually appear as a sequence of triangle edges in the final triangulation.
Special point
A special point statement specifies that special geometric points are to be associated with a curve or surface. For space curves and trimming curves, the parameter vertices must be 1D. For surfaces, the parameter vertices must be 2D.
These special points will be included in any linear approximation of the curve or surface.
For space curves, this means that the point corresponding to the given curve parameter is included as one of the vertices in an approximation consisting of a sequence of line segments.
For surfaces, this means that the point corresponding to the given surface parameters is included as a triangle vertex in the triangulation.
For trimming curves, the treatment is slightly different: a special point on a trimming curve is essentially the same as a special point on the surface it trims.
The following portion of a sample files shows special points for a rational Bezier 2D curve on a surface.
vp -0.675 1.850 3.000
vp 0.915 1.930
vp 2.485 0.470 2.000
vp 2.485 -1.030
vp 1.605 -1.890 10.700
vp -0.745 -0.654 0.500
cstype rat bezier
curv2 -6 -5 -4 -3 -2 -1 -6
parm u 0.00 1.00 2.00
sp 2 3
end
Syntax
The following syntax statement are listed in order of normal use.
parm u p1 p2 p3. . .
parm v p1 p2 p3 . . .
Body statement for free-form geometry.
Specifies global parameter values. For B-spline curves and
surfaces, this specifies the knot vectors.
u is the u direction for the parameter values.
v is the v direction for the parameter values.
To set u and v values, use separate command lines.
p is the global parameter or knot value. You can specify multiple
values. A minimum of two parameter values are required. Parameter
values must increase monotonically. The type of surface and the
degree dictate the number of values required.
trim u0 u1 curv2d u0 u1 curv2d . . .
Body statement for free-form geometry.
Specifies a sequence of curves to build a single outer trimming
loop.
u0 is the starting parameter value for the trimming curve curv2d.
u1 is the ending parameter value for the trimming curve curv2d.
curv2d is the index of the trimming curve lying in the parameter
space of the surface. This curve must have been previously defined
with the curv2 statement.
hole u0 u1 curv2d u0 u1 curv2d . . .
Body statement for free-form geometry.
Specifies a sequence of curves to build a single inner trimming
loop (hole).
u0 is the starting parameter value for the trimming curve curv2d.
u1 is the ending parameter value for the trimming curve curv2d.
curv2d is the index of the trimming curve lying in the parameter
space of the surface. This curve must have been previously defined
with the curv2 statement.
scrv u0 u1 curv2d u0 u1 curv2d . . .
Body statement for free-form geometry.
Specifies a sequence of curves which lie on the given surface to
build a single special curve.
u0 is the starting parameter value for the special curve curv2d.
u1 is the ending parameter value for the special curve curv2d.
curv2d is the index of the special curve lying in the parameter
space of the surface. This curve must have been previously defined
with the curv2 statement.
sp vp1 vp. . .
Body statement for free-form geometry.
Specifies special geometric points to be associated with a curve or
surface. For space curves and trimming curves, the parameter
vertices must be 1D. For surfaces, the parameter vertices must be
2D.
vp is the reference number for the parameter vertex of a special
point to be associated with the parameter space point of the curve
or surface.
end
Body statement for free-form geometry.
Specifies the end of a curve or surface body begun by a curv,
curv2, or surf statement.
Examples
Taylor curve
For creating a single-segment Taylor polynomial curve of the form:
2 3 4x = 3.00 + 2.30t + 7.98t + 8.30t + 6.34t
2 3 4y = 1.00 - 10.10t + 5.40t - 4.70t + 2.03t
2 3 4z = -2.50 + 0.50t - 7.00t + 18.10t + 0.08t
and evaluated between the global parameters 0.5 and 1.6:
v 3.000 1.000 -2.500
v 2.300 -10.100 0.500
v 7.980 5.400 -7.000
v 8.300 -4.700 18.100
v 6.340 2.030 0.080
cstype taylor
deg 4
curv 0.500 1.600 1 2 3 4 5
parm u 0.000 2.000
end
- Bezier curve
This example shows a non-rational Bezier curve with 13 control points.
v -2.300000 1.950000 0.000000
v -2.200000 0.790000 0.000000
v -2.340000 -1.510000 0.000000
v -1.530000 -1.490000 0.000000
v -0.720000 -1.470000 0.000000
v -0.780000 0.230000 0.000000
v 0.070000 0.250000 0.000000
v 0.920000 0.270000 0.000000
v 0.800000 -1.610000 0.000000
v 1.620000 -1.590000 0.000000
v 2.440000 -1.570000 0.000000
v 2.690000 0.670000 0.000000
v 2.900000 1.980000 0.000000
# 13 vertices
cstype bezier
ctech cparm 1.000000
deg 3
curv 0.000000 4.000000 1 2 3 4 5 6 7 8 9 10 \
11 12 13
parm u 0.000000 1.000000 2.000000 3.000000 \
4.000000
end
# 1 element
- B-spline surface
This is an example of a cubic B-spline surface.
g bspatch
v -5.000000 -5.000000 -7.808327
v -5.000000 -1.666667 -7.808327
v -5.000000 1.666667 -7.808327
v -5.000000 5.000000 -7.808327
v -1.666667 -5.000000 -7.808327
v -1.666667 -1.666667 11.977780
v -1.666667 1.666667 11.977780
v -1.666667 5.000000 -7.808327
v 1.666667 -5.000000 -7.808327
v 1.666667 -1.666667 11.977780
v 1.666667 1.666667 11.977780
v 1.666667 5.000000 -7.808327
v 5.000000 -5.000000 -7.808327
v 5.000000 -1.666667 -7.808327
v 5.000000 1.666667 -7.808327
v 5.000000 5.000000 -7.808327
# 16 vertices
cstype bspline
stech curv 0.5 10.000000
deg 3 3
8surf 0.000000 1.000000 0.000000 1.000000 13 14 \ 15 16 9 10 11 12 5 6
7 8 1 2 3 4
parm u -3.000000 -2.000000 -1.000000 0.000000 \
1.000000 2.000000 3.000000 4.000000
parm v -3.000000 -2.000000 -1.000000 0.000000 \
1.000000 2.000000 3.000000 4.000000
end
# 1 element
- Cardinal surface
This example shows a Cardinal surface.
v -5.000000 -5.000000 0.000000
v -5.000000 -1.666667 0.000000
v -5.000000 1.666667 0.000000
v -5.000000 5.000000 0.000000
v -1.666667 -5.000000 0.000000
v -1.666667 -1.666667 0.000000
v -1.666667 1.666667 0.000000
v -1.666667 5.000000 0.000000
v 1.666667 -5.000000 0.000000
v 1.666667 -1.666667 0.000000
v 1.666667 1.666667 0.000000
v 1.666667 5.000000 0.000000
v 5.000000 -5.000000 0.000000
v 5.000000 -1.666667 0.000000
v 5.000000 1.666667 0.000000
v 5.000000 5.000000 0.000000
# 16 vertices
cstype cardinal
stech cparma 1.000000 1.000000
deg 3 3
surf 0.000000 1.000000 0.000000 1.000000 13 14 \
15 16 9 10 11 12 5 6 7 8 1 2 3 4
parm u 0.000000 1.000000
parm v 0.000000 1.000000
end
# 1 element
- Rational B-spline surface
This example creates a second-degree, rational B-spline surface using open, uniform knot vectors. A texture map is applied to the surface.
v -1.3 -1.0 0.0
v 0.1 -1.0 0.4 7.6
v 1.4 -1.0 0.0 2.3
v -1.4 0.0 0.2
v 0.1 0.0 0.9 0.5
v 1.3 0.0 0.4 1.5
v -1.4 1.0 0.0 2.3
v 0.1 1.0 0.3 6.1
v 1.1 1.0 0.0 3.3
vt 0.0 0.0
vt 0.5 0.0
vt 1.0 0.0
vt 0.0 0.5
vt 0.5 0.5
vt 1.0 0.5
vt 0.0 1.0
vt 0.5 1.0
vt 1.0 1.0
cstype rat bspline
deg 2 2
surf 0.0 1.0 0.0 1.0 1/1 2/2 3/3 4/4 5/5 6/6 \
7/7 8/8 9/9
parm u 0.0 0.0 0.0 1.0 1.0 1.0
parm v 0.0 0.0 0.0 1.0 1.0 1.0
end
- Trimmed NURB surface
This is a complete example of a file containing a trimmed NURB surface with negative reference numbers for vertices.
# trimming curve
vp -0.675 1.850 3.000
vp 0.915 1.930
vp 2.485 0.470 2.000
vp 2.485 -1.030
vp 1.605 -1.890 10.700
vp -0.745 -0.654 0.500
cstype rat bezier
deg 3
curv2 -6 -5 -4 -3 -2 -1 -6
parm u 0.00 1.00 2.00
end
# surface
v -1.350 -1.030 0.000
v 0.130 -1.030 0.432 7.600
v 1.480 -1.030 0.000 2.300
v -1.460 0.060 0.201
v 0.120 0.060 0.915 0.500
v 1.380 0.060 0.454 1.500
v -1.480 1.030 0.000 2.300
v 0.120 1.030 0.394 6.100
v 1.170 1.030 0.000 3.300
cstype rat bspline
deg 2 2
surf -1.0 2.5 -2.0 2.0 -9 -8 -7 -6 -5 -4 -3 -2 -1
parm u -1.00 -1.00 -1.00 2.50 2.50 2.50
parm v -2.00 -2.00 -2.00 -2.00 -2.00 -2.00
trim 0.0 2.0 1
end
- Two trimming regions with a hole
This example shows a Bezier surface with two trimming regions, each with a hole in them.
# outer loop of first region
deg 1
cstype bezier
vp 0.100 0.100
vp 0.900 0.100
vp 0.900 0.900
vp 0.100 0.900
curv2 1 2 3 4 1
parm u 0.00 1.00 2.00 3.00 4.00
end
# hole in first region
vp 0.300 0.300
vp 0.700 0.300
vp 0.700 0.700
vp 0.300 0.700
curv2 5 6 7 8 5
parm u 0.00 1.00 2.00 3.00 4.00
end
# outer loop of second region
vp 1.100 1.100
vp 1.900 1.100
vp 1.900 1.900
vp 1.100 1.900
curv2 9 10 11 12 9
parm u 0.00 1.00 2.00 3.00 4.00
end
# hole in second region
vp 1.300 1.300
vp 1.700 1.300
vp 1.700 1.700
vp 1.300 1.700
curv2 13 14 15 16 13
parm u 0.00 1.00 2.00 3.00 4.00
end
# surface
v 0.000 0.000 0.000
v 1.000 0.000 0.000
v 0.000 1.000 0.000
v 1.000 1.000 0.000
deg 1 1
cstype bezier
surf 0.0 2.0 0.0 2.0 1 2 3 4
parm u 0.00 2.00
parm v 0.00 2.00
trim 0.0 4.0 1
hole 0.0 4.0 2
trim 0.0 4.0 3
hole 0.0 4.0 4
end
Trimming with a special curve This example is similar to the trimmed NURB surface example (6), except there is a special curve on the surface. This example uses negative vertex numbers.
trimming curve
vp -0.675 1.850 3.000 vp 0.915 1.930 vp 2.485 0.470 2.000 vp 2.485 -1.030 vp 1.605 -1.890 10.700 vp -0.745 -0.654 0.500 cstype rat bezier deg 3 curv2 -6 -5 -4 -3 -2 -1 -6 parm u 0.00 1.00 2.00 end
special curve
vp -0.185 0.322 vp 0.214 0.818 vp 1.652 0.207 vp 1.652 -0.455 curv2 -4 -3 -2 -1 parm u 2.00 10.00 end
surface
v -1.350 -1.030 0.000 v 0.130 -1.030 0.432 7.600 v 1.480 -1.030 0.000 2.300 v -1.460 0.060 0.201 v 0.120 0.060 0.915 0.500 v 1.380 0.060 0.454 1.500 v -1.480 1.030 0.000 2.300 v 0.120 1.030 0.394 6.100 v 1.170 1.030 0.000 3.300 cstype rat bspline deg 2 2 surf -1.0 2.5 -2.0 2.0 -9 -8 -7 -6 -5 -4 -3 -2 -1 parm u -1.00 -1.00 -1.00 2.50 2.50 2.50 parm v -2.00 -2.00 -2.00 2.00 2.00 2.00 trim 0.0 2.0 1 scrv 4.2 9.7 2 end
Trimming with special points
This example extends the trimmed NURB surface example (6) to include special points on both the trimming curve and surface. A space curve with a special point is also included. This example uses negative vertex numbers.
# special point and space curve data
vp 0.500
vp 0.700
vp 1.100
vp 0.200 0.950
v 0.300 1.500 0.100
v 0.000 0.000 0.000
v 1.000 1.000 0.000
v 2.000 1.000 0.000
v 3.000 0.000 0.000
cstype bezier
deg 3
curv 0.2 0.9 -4 -3 -2 -1
sp 1
parm u 0.00 1.00
end
# trimming curve
vp -0.675 1.850 3.000
vp 0.915 1.930
vp 2.485 0.470 2.000
vp 2.485 -1.030
vp 1.605 -1.890 10.700
vp -0.745 -0.654 0.500
cstype rat bezier
curv2 -6 -5 -4 -3 -2 -1 -6
parm u 0.00 1.00 2.00
sp 2 3
end
# surface
v -1.350 -1.030 0.000
v 0.130 -1.030 0.432 7.600
v 1.480 -1.030 0.000 2.300
v -1.460 0.060 0.201
v 0.120 0.060 0.915 0.500
v 1.380 0.060 0.454 1.500
v -1.480 1.030 0.000 2.300
v 0.120 1.030 0.394 6.100
v 1.170 1.030 0.000 3.300
cstype rat bspline
deg 2 2
surf -1.0 2.5 -2.0 2.0 -9 -8 -7 -6 -5 -4 -3 -2 -1
parm u -1.00 -1.00 -1.00 2.50 2.50 2.50
parm v -2.00 -2.00 -2.00 2.00 2.00 2.00
trim 0.0 2.0 1
sp 4
end
Connectivity between free-form surfaces
Connectivity connects two surfaces along their trimming curves.
The con statement specifies the first surface with its trimming curve and the second surface with its trimming curve. This information is useful for edge merging. Without this surface and curve data, connectivity must be determined numerically at greater expense and with reduced accuracy using the mg statement.
Connectivity between surfaces in different merging groups is ignored. Also, although connectivity which crosses points of C1discontinuity in trimming curves is legal, it is not recommended. Instead, use two connectivity statements which meet at the point of discontinuity.
The two curves and their starting and ending parameters should all map to the same curve and starting and ending points in object space.
Syntax
con surf_1 q0_1 q1_1 curv2d_1 surf_2 q0_2 q1_2 curv2d_2
Free-form geometry statement.
Specifies connectivity between two surfaces.
surf_1 is the index of the first surface.
q0_1 is the starting parameter for the curve referenced by
curv2d_1.
q1_1 is the ending parameter for the curve referenced by curv2d_1.
curv2d_1 is the index of a curve on the first surface. This curve
must have been previously defined with the curv2 statement.
surf_2 is the index of the second surface.
q0_2 is the starting parameter for the curve referenced by
curv2d_2.
q1_2 is the ending parameter for the curve referenced by curv2d_2.
curv2d_2 is the index of a curve on the second surface. This curve
must have been previously defined with the curv2 statement.
Example
- Connectivity between two surfaces
This example shows the connectivity between two surfaces with trimming curves.
cstype bezier
deg 1 1
v 0 0 0
v 1 0 0
v 0 1 0
v 1 1 0
vp 0 0
vp 1 0
vp 1 1
vp 0 1
curv2 1 2 3 4 1
parm u 0.0 1.0 2.0 3.0 4.0
end
surf 0.0 1.0 0.0 1.0 1 2 3 4
parm u 0.0 1.0
parm v 0.0 1.0
trim 0.0 4.0 1
end
v 1 0 0
v 2 0 0
v 1 1 0
v 2 1 0
surf 0.0 1.0 0.0 1.0 5 6 7 8
parm u 0.0 1.0
parm v 0.0 1.0
trim 0.0 4.0 1
end
con 1 2.0 2.0 1 2 4.0 3.0 1
Grouping
There are four statements in the .obj file to help you manipulate groups of elements:
o Gropu name statements are used to organize collections of
elements and simplify data manipulation for operations in
Model.
o Smoothing group statements let you identify elements over which
normals are to be interpolated to give those elements a smooth,
non-faceted appearance. This is a quick way to specify vertex
normals.
o Merging group statements are used to ideneify free-form elements
that should be inspected for adjacency detection. You can also
use merging groups to exclude surfaces which are close enough to
be considered adjacent but should not be merged.
o Object name statements let you assign a name to an entire object
in a single file.
All grouping statements are state-setting. This means that once a group statement is set, it alpplies to all elements that follow until the next group statement.
This portion of a sample file shows a single element which belongs to three groups. The smoothing group is turned off.
g square thing all
s off
f 1 2 3 4
This example shows two surfaces in merging group 1 with a merge resolution of 0.5.
mg 1 .5
surf 0.0 1.0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
surf 0.0 1.0 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
Syntax
g group_name1 group_name2 . . .
Polygonal and free-form geometry statement.
Specifies the group name for the elements that follow it. You can
have multiple group names. If there are multiple groups on one
line, the data that follows belong to all groups. Group information
is optional.
group_name is the name for the group. Letters, numbers, and
combinations of letters and numbers are accepted for group names.
The default group name is default.
s group_number
Polygonal and free-form geometry statement.
Sets the smoothing group for the elements that follow it. If you do
not want to use a smoothing group, specify off or a value of 0.
To display with smooth shading in Model and PreView, you must
create vertex normals after you have assigned the smoothing groups.
You can create vertex normals with the vn statement or with the
Model program.
To smooth polygonal geometry for rendering with Image, it is
sufficient to put elements in some smoothing group. However, vertex
normals override smoothing information for Image.
group_number is the smoothing group number. To turn off smoothing
groups, use a value of 0 or off. Polygonal elements use group
numbers to put elements in different smoothing groups. For
free-form surfaces, smoothing groups are either turned on or off;
there is no difference between values greater than 0.
mg group_number res
Free-form geometry statement.
Sets the merging group and merge resolution for the free-form
surfaces that follow it. If you do not want to use a merging group,
specify off or a value of 0.
Adjacency detection is performed only within groups, never between
groups. Connectivity between surfaces in different merging groups
is not allowed. Surfaces in the same merging group are merged
together along edges that are within the distance res apart.
NOTE: Adjacency detection is an expensive numerical comparison
process. It is best to restrict this process to as small a domain
as possible by using small merging groups.
group_number is the merging group number. To turn off adjacency
detection, use a value of 0 or off.
res is the maximum distance between two surfaces that will be
merged together. The resolution must be a value greater than 0.
This is a required argument only when using merging groups.
o object_name
Polygonal and free-form geometry statement.
Optional statement; it is not processed by any Wavefront programs.
It specifies a user-defined object name for the elements defined
after this statement.
object_name is the user-defined object name. There is no default.
Examples
- Cube with group names
The following example is a cube with each of its faces placed in a separate group. In addition, all elements belong to the group cube.
v 0.000000 2.000000 2.000000
v 0.000000 0.000000 2.000000
v 2.000000 0.000000 2.000000
v 2.000000 2.000000 2.000000
v 0.000000 2.000000 0.000000
v 0.000000 0.000000 0.000000
v 2.000000 0.000000 0.000000
v 2.000000 2.000000 0.000000
# 8 vertices
g front cube
f 1 2 3 4
g back cube
f 8 7 6 5
g right cube
f 4 3 7 8
g top cube
f 5 1 4 8
g left cube
f 5 6 2 1
g bottom cube
f 2 6 7 3
# 6 elements
- Two adjoining squares with a smoothing group
This example shows two adjoining squares that share a common edge. The squares are placed in a smoothing group to ensure that their common edge will be smoothed when rendered with Image.
v 0.000000 2.000000 0.000000
v 0.000000 0.000000 0.000000
v 2.000000 0.000000 0.000000
v 2.000000 2.000000 0.000000
v 4.000000 0.000000 -1.255298
v 4.000000 2.000000 -1.255298
# 6 vertices
g all
s 1
f 1 2 3 4
f 4 3 5 6
# 2 elements
- Two adjoining squares with vertex normals
This example also shows two squares that share a common edge. Vertex normals have been added to the corners of each square to ensure that their common edge will be smoothed during display in Model and PreView and when rendered with Image.
v 0.000000 2.000000 0.000000
v 0.000000 0.000000 0.000000
v 2.000000 0.000000 0.000000
v 2.000000 2.000000 0.000000
v 4.000000 0.000000 -1.255298
v 4.000000 2.000000 -1.255298
vn 0.000000 0.000000 1.000000
vn 0.000000 0.000000 1.000000
vn 0.276597 0.000000 0.960986
vn 0.276597 0.000000 0.960986
vn 0.531611 0.000000 0.846988
vn 0.531611 0.000000 0.846988
# 6 vertices
# 6 normals
g all
s 1
f 1//1 2//2 3//3 4//4
f 4//4 3//3 5//5 6//6
# 2 elements
- Merging group
This example shows two Bezier surfaces that meet at a common edge. They have both been placed in the same merging group to ensure continuity at the edge where they meet. This prevents "cracks" from appearing along the seam between the two surfaces during rendering. Merging groups will be ignored during flat-shading, smooth-shading, and material shading of the surface.
v -4.949854 -5.000000 0.000000
v -4.949854 -1.666667 0.000000
v -4.949854 1.666667 0.000000
v -4.949854 5.000000 0.000000
v -1.616521 -5.000000 0.000000
v -1.616521 -1.666667 0.000000
v -1.616521 1.666667 0.000000
v -1.616521 5.000000 0.000000
v 1.716813 -5.000000 0.000000
v 1.716813 -1.666667 0.000000
v 1.716813 1.666667 0.000000
v 1.716813 5.000000 0.000000
v 5.050146 -5.000000 0.000000
v 5.050146 -1.666667 0.000000
v 5.050146 1.666667 0.000000
v 5.050146 5.000000 0.000000
v -15.015566 -4.974991 0.000000
v -15.015566 -1.641658 0.000000
v -15.015566 1.691675 0.000000
v -15.015566 5.025009 0.000000
v -11.682233 -4.974991 0.000000
v -11.682233 -1.641658 0.000000
v -11.682233 1.691675 0.000000
v -11.682233 5.025009 0.000000
v -8.348900 -4.974991 0.000000
v -8.348900 -1.641658 0.000000
v -8.348900 1.691675 0.000000
v -8.348900 5.025009 0.000000
v -5.015566 -4.974991 0.000000
v -5.015566 -1.641658 0.000000
v -5.015566 1.691675 0.000000
v -5.015566 5.025009 0.000000
mg 1 0.500000
cstype bezier
deg 3 3
surf 0.000000 1.000000 0.000000 1.000000 13 14 \
15 16 9 10 11 12 5 6 7 8 1 2 3 4
parm u 0.000000 1.000000
parm v 0.000000 1.000000
end
surf 0.000000 1.000000 0.000000 1.000000 29 30 31 32 25 26 27 28 21 22 \
23 24 17 18 19 20
parm u 0.000000 1.000000
parm v 0.000000 1.000000
end
Display/render attributes
Display and render attributes describe how an object looks when displayed in Model and PreView or when rendered with Image.
Some attributes apply to both free-form and polygonal geometry, such as material name and library, ray tracing, and shadow casting. Interpolation attributes apply only to polygonal geometry. Curve and surface resolutions are used for only free-form geometry.
The following chart shows the display and render statements available for polygonal and free-form geometry.
Table B1-1. Display and render attributes
polygonal only polygonal or free-form free-form only
bevel lod ctech c_interp usemtl stech d_interp mtllib
shadow_obj
trace_obj
All display and render attribute statements are state-setting. This means that once an attribute statement is set, it applies to all elements that follow until it is reset to a different value.
The following sample shows rendering and display statements for a face element.:
s 1
usemtl blue
usemap marble
f 1 2 3 4
Syntax
The following syntax statements are listed by the type of geometry. First are statements for polygonal geometry. Second are statements for both free-form and polygonal geometry. Third are statements for free-form geometry only.
bevel on/off
Polygonal geometry statement.
Sets bevel interpolation on or off. It works only with beveled
objects, that is, objects with sides separated by beveled faces.
Bevel interpolation uses normal vector interpolation to give an
illusion of roundness to a flat bevel. It does not affect the
smoothing of non-bevelled faces.
Bevel interpolation does not alter the geometry of the original
object.
on turns on bevel interpolation.
off turns off bevel interpolation. The default is off.
NOTE: Image cannot render bevel-interpolated elements that have
vertex normals.
c_interp on/off
Polygonal geometry statement.
Sets color interpolation on or off.
Color interpolation creates a blend across the surface of a polygon
between the materials assigned to its vertices. This creates a
blending of colors across a face element.
To support color interpolation, materials must be assigned per
vertex, not per element. The illumination models for all materials
of vertices attached to the polygon must be the same. Color
interpolation applies to the values for ambient (Ka), diffuse (Kd),
specular (Ks), and specular highlight (Ns) material properties.
on turns on color interpolation.
off turns off color interpolation. The default is off.
d_interp on/off
Polygonal geometry statement.
Sets dissolve interpolation on or off.
Dissolve interpolation creates an interpolation or blend across a
polygon between the dissolve (d) values of the materials assigned
to its vertices. This feature is used to create effects exhibiting
varying degrees of apparent transparency, as in glass or clouds.
To support dissolve interpolation, materials must be assigned per
vertex, not per element. All the materials assigned to the vertices
involved in the dissolve interpolation must contain a dissolve
factor command to specify a dissolve.
on turns on dissolve interpolation.
off turns off dissolve interpolation. The default is off.
lod level
Polygonal and free-form geometry statement.
Sets the level of detail to be displayed in a PreView animation.
The level of detail feature lets you control which elements of an
object are displayed while working in PreView.
level is the level of detail to be displayed. When you set the
level of detail to 0 or omit the lod statement, all elements are
displayed. Specifying an integer between 1 and 100 sets the level
of detail to be displayed when reading the .obj file.
maplib filename1 filename2 . . .
This is a rendering identifier that specifies the map library file
for the texture map definitions set with the usemap identifier. You
can specify multiple filenames with maplib. If multiple filenames
are specified, the first file listed is searched first for the map
definition, the second file is searched next, and so on.
When you assign a map library using the Model program, Model allows
only one map library per .obj file. You can assign multiple
libraries using a text editor.
filename is the name of the library file where the texture maps are
defined. There is no default.
usemap map_name/off
This is a rendering identifier that specifies the texture map name
for the element following it. To turn off texture mapping, specify
off instead of the map name.
If you specify texture mapping for a face without texture vertices,
the texture map will be ignored.
map_name is the name of the texture map.
off turns off texture mapping. The default is off.
usemtl material_name
Polygonal and free-form geometry statement.
Specifies the material name for the element following it. Once a
material is assigned, it cannot be turned off; it can only be
changed.
material_name is the name of the material. If a material name is
not specified, a white material is used.
mtllib filename1 filename2 . . .
Polygonal and free-form geometry statement.
Specifies the material library file for the material definitions
set with the usemtl statement. You can specify multiple filenames
with mtllib. If multiple filenames are specified, the first file
listed is searched first for the material definition, the second
file is searched next, and so on.
When you assign a material library using the Model program, only
one map library per .obj file is allowed. You can assign multiple
libraries using a text editor.
filename is the name of the library file that defines the
materials. There is no default.
shadow_obj filename
Polygonal and free-form geometry statement.
Specifies the shadow object filename. This object is used to cast
shadows for the current object. Shadows are only visible in a
rendered image; they cannot be seen using hardware shading. The
shadow object is invisible except for its shadow.
An object will cast shadows only if it has a shadow object. You can
use an object as its own shadow object. However, a simplified
version of the original object is usually preferable for shadow
objects, since shadow casting can greatly increase rendering time.
filename is the filename for the shadow object. You can enter any
valid object filename for the shadow object. The object file can be
an .obj or .mod file. If a filename is given without an extension,
an extension of .obj is assumed.
Only one shadow object can be stored in a file. If more than one
shadow object is specified, the last one specified will be used.
trace_obj filename
Polygonal and free-form geometry statement.
Specifies the ray tracing object filename. This object will be used
in generating reflections of the current object on reflective
surfaces. Reflections are only visible in a rendered image; they
cannot be seen using hardware shading.
An object will appear in reflections only if it has a trace object.
You can use an object as its own trace object. However, a
simplified version of the original object is usually preferable for
trace objects, since ray tracing can greatly increase rendering
time.
filename is the filename for the ray tracing object. You can enter
any valid object filename for the trace object. You can enter any
valid object filename for the shadow object. The object file can be
an .obj or .mod file. If a filename is given without an extension,
an extension of .obj is assumed.
Only one trace object can be stored in a file. If more than one is
specified, the last one is used.
ctech technique resolution
Free-form geometry statement.
Specifies a curve approximation technique. The arguments specify
the technique and resolution for the curve.
You must select from one of the following three techniques.
ctech cparm res
Specifies a curve with constant parametric subdivision using
one resolution parameter. Each polynomial segment of the curve
is subdivided n times in parameter space, where n is the
resolution parameter multiplied by the degree of the curve.
res is the resolution parameter. The larger the value, the
finer the resolution. If res has a value of 0, each polynomial
curve segment is represented by a single line segment.
ctech cspace maxlength
Specifies a curve with constant spatial subdivision. The curve
is approximated by a series of line segments whose lengths in
real space are less than or equal to the maxlength.
maxlength is the maximum length of the line segments. The
smaller the value, the finer the resolution.
ctech curv maxdist maxangle
Specifies curvature-dependent subdivision using separate
resolution parameters for the maximum distance and the maximum
angle.
The curve is approximated by a series of line segments in which
1) the distance in object space between a line segment and the
actual curve must be less than the maxdist parameter and 2) the
angle in degrees between tangent vectors at the ends of a line
segment must be less than the maxangle parameter.
maxdist is the distance in real space between a line segment
and the actual curve.
maxangle is the angle (in degrees) between tangent vectors at
the ends of a line segment.
The smaller the values for maxdist and maxangle, the finer the
resolution.
NOTE: Approximation information for trimming, hole, and special
curves is stored in the corresponding surface. The ctech statement
for the surface is used, not the ctech statement applied to the
curv2 statement. Although untrimmed surfaces have no explicit
trimming loop, a loop is constructed which bounds the legal
parameter range. This implicit loop follows the same rules as any
other loop and is approximated according to the ctech information
for the surface.
stech technique resolution
Free-form geometry statement.
Specifies a surface approximation technique. The arguments specify
the technique and resolution for the surface.
You must select from one of the following techniques:
stech cparma ures vres
Specifies a surface with constant parametric subdivision using
separate resolution parameters for the u and v directions. Each
patch of the surface is subdivided n times in parameter space,
where n is the resolution parameter multiplied by the degree of
the surface.
ures is the resolution parameter for the u direction.
vres is the resolution parameter for the v direction.
The larger the values for ures and vres, the finer the
resolution. If you enter a value of 0 for both ures and vres,
each patch is approximated by two triangles.
stech cparmb uvres
Specifies a surface with constant parametric subdivision, with
refinement using one resolution parameter for both the u and v
directions.
An initial triangulation is performed using only the points on
the trimming curves. This triangulation is then refined until
all edges are of an appropriate length. The resulting triangles
are not oriented along isoparametric lines as they are in the
cparma technique.
uvres is the resolution parameter for both the u and v
directions. The larger the value, the finer the resolution.
stech cspace maxlength
Specifies a surface with constant spatial subdivision.
The surface is subdivided in rectangular regions until the
length in real space of any rectangle edge is less than the
maxlength. These rectangular regions are then triangulated.
maxlength is the length in real space of any rectangle edge.
The smaller the value, the finer the resolution.
stech curv maxdist maxangle
Specifies a surface with curvature-dependent subdivision using
separate resolution parameters for the maximum distance and the
maximum angle.
The surface is subdivided in rectangular regions until 1) the
distance in real space between the approximating rectangle and
the actual surface is less than the maxdist (approximately) and
2) the angle in degrees between surface normals at the corners
of the rectangle is less than the maxangle. Following
subdivision, the regions are triangulated.
maxdist is the distance in real space between the approximating
rectangle and the actual surface.
maxangle is the angle in degrees between surface normals at the
corners of the rectangle.
The smaller the values for maxdist and maxangle, the finer the
resolution.
Examples
- Cube with materials
This cube has a different material applied to each of its faces.
mtllib master.mtl
v 0.000000 2.000000 2.000000
v 0.000000 0.000000 2.000000
v 2.000000 0.000000 2.000000
v 2.000000 2.000000 2.000000
v 0.000000 2.000000 0.000000
v 0.000000 0.000000 0.000000
v 2.000000 0.000000 0.000000
v 2.000000 2.000000 0.000000
# 8 vertices
g front
usemtl red
f 1 2 3 4
g back
usemtl blue
f 8 7 6 5
g right
usemtl green
f 4 3 7 8
g top
usemtl gold
f 5 1 4 8
g left
usemtl orange
f 5 6 2 1
g bottom
usemtl purple
f 2 6 7 3
# 6 elements
- Cube casting a shadow
In this example, the cube casts a shadow on the other objects when it is rendered with Image. The cube, which is stored in the file cube.obj, references itself as the shadow object.
mtllib master.mtl
shadow_obj cube.obj
v 0.000000 2.000000 2.000000
v 0.000000 0.000000 2.000000
v 2.000000 0.000000 2.000000
v 2.000000 2.000000 2.000000
v 0.000000 2.000000 0.000000
v 0.000000 0.000000 0.000000
v 2.000000 0.000000 0.000000
v 2.000000 2.000000 0.000000
# 8 vertices
g front
usemtl red
f 1 2 3 4
g back
usemtl blue
f 8 7 6 5
g right
usemtl green
f 4 3 7 8
g top
usemtl gold
f 5 1 4 8
g left
usemtl orange
f 5 6 2 1
g bottom
usemtl purple
f 2 6 7 3
# 6 elements
- Cube casting a reflection
This cube casts its reflection on any reflective objects when it is rendered with Image. The cube, which is stored in the file cube.obj, references itself as the trace object.
mtllib master.mtl
trace_obj cube.obj
v 0.000000 2.000000 2.000000
v 0.000000 0.000000 2.000000
v 2.000000 0.000000 2.000000
v 2.000000 2.000000 2.000000
v 0.000000 2.000000 0.000000
v 0.000000 0.000000 0.000000
v 2.000000 0.000000 0.000000
v 2.000000 2.000000 0.000000
# 8 vertices
g front
usemtl red
f 1 2 3 4
g back
usemtl blue
f 8 7 6 5
g right
usemtl green
f 4 3 7 8
g top
usemtl gold
f 5 1 4 8
g left
usemtl orange
f 5 6 2 1
g bottom
usemtl purple
f 2 6 7 3
# 6 elements
- Texture-mapped square
This example describes a 2 x 2 square. It is mapped with a 1 x 1 square texture. The texture is stretched to fit the square exactly.
mtllib master.mtl
v 0.000000 2.000000 0.000000 v 0.000000 0.000000 0.000000 v 2.000000 0.000000 0.000000 v 2.000000 2.000000 0.000000 vt 0.000000 1.000000 0.000000 vt 0.000000 0.000000 0.000000 vt 1.000000 0.000000 0.000000 vt 1.000000 1.000000 0.000000
4 vertices
usemtl wood f 1/1 2/2 3/3 4/4
1 element
- Approximation technique for a surface
This example shows a B-spline surface which will be approximated using curvature-dependent subdivision specified by the stech command.
g bspatch
v -5.000000 -5.000000 -7.808327
v -5.000000 -1.666667 -7.808327
v -5.000000 1.666667 -7.808327
v -5.000000 5.000000 -7.808327
v -1.666667 -5.000000 -7.808327
v -1.666667 -1.666667 11.977780
v -1.666667 1.666667 11.977780
v -1.666667 5.000000 -7.808327
v 1.666667 -5.000000 -7.808327
v 1.666667 -1.666667 11.977780
v 1.666667 1.666667 11.977780
v 1.666667 5.000000 -7.808327
v 5.000000 -5.000000 -7.808327
v 5.000000 -1.666667 -7.808327
v 5.000000 1.666667 -7.808327
v 5.000000 5.000000 -7.808327
# 16 vertices
g bspatch
cstype bspline
stech curv 0.5 10.000000
deg 3 3
surf 0.000000 1.000000 0.000000 1.000000 13 14 \ 15 16 9 10 11 12 5 6 7
8 1 2 3 4
parm u -3.000000 -2.000000 -1.000000 0.000000 \
1.000000 2.000000 3.000000 4.000000
parm v -3.000000 -2.000000 -1.000000 0.000000 \
1.000000 2.000000 3.000000 4.000000
end
# 1 element
- Approximation technique for a curve
This example shows a Bezier curve which will be approximated using constant parametric subdivision specified by the ctech command.
v -2.300000 1.950000 0.000000
v -2.200000 0.790000 0.000000
v -2.340000 -1.510000 0.000000
v -1.530000 -1.490000 0.000000
v -0.720000 -1.470000 0.000000
v -0.780000 0.230000 0.000000
v 0.070000 0.250000 0.000000
v 0.920000 0.270000 0.000000
v 0.800000 -1.610000 0.000000
v 1.620000 -1.590000 0.000000
v 2.440000 -1.570000 0.000000
v 2.690000 0.670000 0.000000
v 2.900000 1.980000 0.000000
# 13 vertices
g default
cstype bezier
ctech cparm 1.000000
deg 3
curv 0.000000 4.000000 1 2 3 4 5 6 7 8 9 10 \
11 12 13
parm u 0.000000 1.000000 2.000000 3.000000 \
4.000000
end
# 1 element
Comments
Comments can appear anywhere in an .obj file. They are used to annotate the file; they are not processed.
Here is an example:
# this is a comment
The Model program automatically inserts comments when it creates .obj files. For example, it reports the number of geometric vertices, texture vertices, and vertex normals in a file.
# 4 vertices
# 4 texture vertices
# 4 normals
Mathematics for free-form curves/surfaces
[I apologize but this section will make absolutely no sense whatsoever without the equations and diagrams and there was just no easy way to include them in a pure ASCII document. You should probably just skip ahead to the section "Superseded statements." -Jim]
General forms
Rational and non-rational curves and surfaces
In general, any non-rational curve segment may be written as:
where
K + 1 is the number of control points
di are the control points
n is the degree of the curve
Ni,n(t) are the degree n basis functions
Extending this to the bivariate case, any non-rational surface patch may be written as:
where:
K1 + 1 is the number of control points in the u direction
K2 + 1 is the number of control points in the v direction
di,j are the control points
m is the degree of the surface in the u direction
n is the degree of the surface in the v direction
Ni,m(u) are the degree m basis functions in the u direction
Nj,n(v) are the degree n basis functions in the v direction
NOTE: The front of the surface is defined as the side where the u parameter increases to the right and the v parameter increases upward.
We may extend this curve to the rational case as:
where wi are the weights associated with the control points di. Similarly, a rational surface may be expressed as:
where wi,j are the weights associated with the control points di,j.
NOTE: If a curve or surface in an .obj file is rational, it must use the rat option with the cstype statement and it requires some weight values for each control point.
The weights for the rational form are given as a third control point coordinate (for trimming curves) or fourth coordinate (for space curves and surfaces). These weights are optional and default to 1.0 if not given.
This default weight is only reasonable for curves and surfaces whose basis functions sum to 1.0, such as Bezier, Cardinal, and NURB. It does not make sense for Taylor and may or may not make sense for a representation given in basis-matrix form.
For all forms other than B-spline, the final curve or surface is constructed by piecing together the individual curve segments or surface patches. A global parameter space is then defined over the entire composite curve or surface using the parameter vector given with the parm statement.
The parameter vector for a curve is a list of p global parameter values {t1, . . . , tp}. If t1 t < ti+1 is a point in global parameter space, then:
is the corresponding point in local parameter space for the ith polynomial segment. It is this t which is used when evaluating a given segment of the piecewise curve. For surfaces, this mapping from global to local parameter space is applied independently in both the u and v parametric directions.
B-splines require a knot vector rather than a parameter vector, although this is also given with the parm statement. Refer to the description of B-splines below.
The following discussion of each type is expressed in terms of the above definitions.
NOTE: The maximum degree for all curve and surface types is currently set at 20, which is high enough for most purposes.
Free-form curve and surface types
B-spline
Type bspline specifies arbitrary degree non-uniform B-splines which are commonly referred to as NURBs in their rational form. The basis functions are defined by the Cox-deBoor recursion formulas as:
and:
where, by convention, 0/0 = 0.
The xi {x0, . . . ,xq} form a set known as the knot vector which is given by the parm statement. It is required that
xi xi + 1,
x0 < xn + 1,
xq -n -1 < xq,
xi < xi + n for 0 < i < q - n - 1,
xn t min < tmax xK+ 1, where [tmin, tmax] is the parameter over which the B-spline is to be evaluated, and
K = q - n - 1.
A knot is said to be of multiplicity r if its value is repeated r times in the knot vector. The second through fourth conditions above restrict knots to be of at most multiplicity n + 1 at the ends of the vector and at most n everywhere else.
The last condition requires that the number of control points is equal to one less than the number of knots minus the degree. For surfaces, all of the above conditions apply independently for the u and v parametric directions.
Bezier
Type bezier specifies arbitrary degree Bezier curves and surfaces. This basis function is defined as:
where:
When using type bezier, the number of global parameter values given with the parm statement must be K/n + 1, where K is the number of control points. For surfaces, this requirement applies independently for the u and v parametric directions.
Cardinal
Type cardinal specifies a cubic, first derivative, continuous curve or surface. For curves, this interpolates all but the first and last control points. For surfaces, all but the first and last row and column of control points are interpolated.
Cardinal splines, also known as Catmull-Rom splines, are best understood by considering the conversion from Cardinal to Bezier control points for a single curve segment:
Here, the ci variables are the Cardinal control points and the bi variables are the Bezier control points. We see that the second and third Cardinal points are the beginning and ending points for the segment, respectively. Also, the beginning tangent lies along the vector from the first to the third point, and the ending tangent along the vector from the second to the last point.
If we let Bi(t) be the cubic Bezier basis functions (i.e. what was given above for Bezier as Ni,n(t) with n = 3), then we may write the Cardinal basis functions as:
Note that Cardinal splines are only defined for the cubic case.
When using type cardinal, the number of global parameter values given with the parm statement must be K - n + 2, where K is the number of control points. For surfaces, this requirement applies independently for the u and v parametric directions.
Taylor
Type taylor specifies arbitrary degree Taylor polynomial curves and surfaces. The basis function is simply:
NOTE: The control points in this case are the polynomial coefficients and have no obvious geometric significance.
When using type taylor, the number of global parameter values given with the parm statement must be (K + 1)/(n + 1) + 1, where K is the number of control points. For surfaces, this requirement applies independently for the u and v parametric directions.
Basis matrix
Type bmatrix specifies general, arbitrary-degree curves defined through the use of a basis matrix rather than an explicit type such as Bezier. The basis functions are defined as:
where the basis matrix is the bi,j. In order to make the matrix nature of this more obvious, we may also write:
When constructing basis matrices, you should keep this definition in mind, as different authors write this in different ways. A more common matrix representation is:
To use such matrices in the .obj file, simply transpose the matrix and reverse the column ordering.
When using type basis, the number of global parameter values given with the parm statement must be (K - n)/s + 2, where K is the number of control points and s is the step size given with the step statement. For surfaces, this requirement applies independently for the u and v parametric directions.
Surface vertex data
Control points
The control points for a surface consisting of a single patch are listed in the order i = 0 to K1 for j = 0, followed by i = 0 to K1 for j = 1, and so on until j = K2.
For surfaces made up of many patches, which is the usual case, the control points are ordered as if the surface were a single large patch. For example, the control points for a bicubic Bezier surface consisting of four patches would be arranged as follows:
where (m, n) is the global parameter space of the surface and the numbers indicate the ordering of the vertex indices in the surf statement.
Texture vertices and texture mapping
When texture vertices are not supplied, the original surface parameterization is used for texture mapping. However, if texture vertices are supplied, they are interpreted as additional information to be interpolated or approximated separately from, but using the same interpolation functions as the control vertices.
That is, whereas the surface itself, in the non-rational case, was given in the section "Rational and non-rational curves and surfaces" as:
the texture vertices are interpolated or approximated by:
where ti,j are the texture vertices and the basis functions are the same as for S(u,v). It is T(u,v), rather than the surface parameterization (u,v), which is used when a texture map is applied.
Vertex normals and normal mapping
Vertex normals are treated exactly like texture vertices. When vertex normals are not supplied, the true surface normals are used. If vertex normals are supplied, they are calculated as:
where qi,j are the vertex normals and the basis functions are the same as for S(u,v) and T(u,v).
NOTE: Vertex normals do not affect the shape of the surface; they are simply associated with the triangle vertices in the final triangulation. As with faces, supplying vertex normals only affects lighting calculations for the surface.
The treatment of both texture vertices and vertex normals in the case of rational surfaces is identical. It is important to notice that even when the surface S(u,v) is rational, the texture and normal surfaces, T(u,v) and Q(u,v), are not rational. This is because the control points (the texture vertices and vertex normals) are never rational.
Curve and surface operations
Special points
The following equations give a more precise description of special points for space curves and discuss the extension to trimming curves and surfaces.
Let C(t) be a space curve with the global parameter t. We can approximate this curve by a set of k-1 line segments which connect the points:
for some set of k global parameter values {t1,...,tk}
Given a special point ts in the parameter space of the curve (referenced by vp), we guarantee that ts {t1, . . . ,tk}. More specifically, we approximate the curve by:
where, at the point i where ts is inserted, we have ti ts < ti+1.
Special curves
The following equations give a more precise description of a special curve.
Let T(t) be a special curve with the global parameter t. We have:
where (m,n) is a point in the global parameter space of a surface. We can approximate this curve by a set of k-1 line segments which connect the points:
for some set of k global parameter values.
Let S(m,n) be a surface with the global parameters m and n. We can approximate this surface by a triangulation of a set of p points.
which lie on the surface. We further define E as the set of all edges such that ei,j E implies that S(mi,ni) and S(mj,nj) are connected in the triangulation. Finally, we guarantee that there exists some subset of E:
such that the points:
are connected in the triangulation.
Connectivity
Recall that the syntax of the con statement is:
con surf_1 q0_1 q1_1 curv2d_1 surf_2 q0_2 q1_2 curv2d_2
If we let:
T1(t1) be the curve referenced by curv2d_1
S1(m1, n1) be the surface referenced by surf1 on which T1(t1) lies
T2(t2) be the curve referenced by curv2d_2
S2(m2, n2) be the surface referenced by surf2 on which T2(t2) lies
then S1(T1(t1)), S2(T2(t2)) must be identical up to reparameterization. Moreover, it must be the case that:
S1(T1(q0_1)) = S2(T2(q0_2))
and:
S1(T1(q1_1)) = S2(T2(q1_2))
It is along the curve S1(T1(t1)) between t1 = q0_1 and t1 = q1_1, and the curve S2(T2(t2)) between t2 = q0_2 and t2 = q1_2 that the surface S1(m1, n1) is connected to the surface S2(m2, n2).
Superseded statements
The new .obj file format has eliminated the need for several patch and curve statements. These statements have been replaced by free-form geometry statements.
In the 3.0 release, the following keywords have been superseded:
o bsp
o bzp
o cdc
o cdp
o res
You can still read these statements in this version 3.0, however, the system will no longer write files in this format.
This release is the last release that will read these statements. If you want to save any data from this format, read in the file and write it out. The system will convert the data to the new .obj format.
For more information on the new syntax statements, see "Specifying free-form curves and surfaces."
Syntax
The following syntax statements are for the superseded keywords.
bsp v1 v2 . . . v16
Specifies a B-spline patch. B-spline patches have sixteen control
points, defined as vertices. Only four of the control points are
distributed over the surface of the patch; the remainder are
distributed around the perimeter of the patch.
Patches must be tessellated in Model before they can be correctly
shaded or rendered.
v is the vertex number for a control point. Sixteen vertex numbers
are required. Positive values indicate absolute vertex numbers.
Negative values indicate relative vertex numbers.
bzp v1 v2 . . . v16
Specifies a Bezier patch. Bezier patches have sixteen control
points, defined as vertices. The control points are distributed
uniformly over its surface.
Patches must be tessellated in Model before they can be correctly
shaded or rendered.
v is the vertex number for a control point. Sixteen vertex numbers
are required. Positive values indicate absolute vertex numbers.
Negative values indicate relative vertex numbers.
cdc v1 v2 v3 v4 v5 . . .
Specifies a Cardinal curve. Cardinal curves have a minimum of four
control points, defined as vertices.
Cardinal curves cannot be correctly shaded or rendered. They can be
tessellated and then extruded in Model to create 3D shapes.
v is the vertex number for a control point. A minimum of four
vertex numbers are required. There is no limit on the maximum.
Positive values indicate absolute vertex numbers. Negative values
indicate relative vertex numbers.
cdp v1 v2 v3 . . . v16
Specifies a Cardinal patch. Cardinal patches have sixteen control
points, defined as vertices. Four of the control points are
attached to the corners of the patch.
Patches must be tessellated in Model before they can be correctly
shaded or rendered.
v is the vertex number for a control point. Sixteen vertex numbers
are required. Positive values indicate absolute vertex numbers.
Negative values indicate relative vertex numbers.
res useg vseg
Reference and display statement.
Sets the number of segments for Bezier, B-spline and Cardinal
patches that follow it.
useg is the number of segments in the u direction (horizontal or x
direction). The minimum setting is 3 and the maximum setting is
120. The default is 4.
vseg is the number of segments in the v direction (vertical or y
direction). The minimum setting is 3 and the maximum setting is
120. The default is 4.
Comparison of 2.11 and 3.0 syntax
Cardinal curve
The following example shows the 2.11 syntax and the 3.0 syntax for the same Cardinal curve.
2.11 Cardinal curve
# 2.11 Cardinal Curve
v 2.570000 1.280000 0.000000
v 0.940000 1.340000 0.000000
v -0.670000 0.820000 0.000000
v -0.770000 -0.940000 0.000000
v 1.030000 -1.350000 0.000000
v 3.070000 -1.310000 0.000000
# 6 vertices
cdc 1 2 3 4 5 6
3.0 Cardinal curve
# 3.0 Cardinal curve
v 2.570000 1.280000 0.000000
v 0.940000 1.340000 0.000000
v -0.670000 0.820000 0.000000
v -0.770000 -0.940000 0.000000
v 1.030000 -1.350000 0.000000
v 3.070000 -1.310000 0.000000
# 6 vertices
cstype cardinal
deg 3
curv 0.000000 3.000000 1 2 3 4 5 6
parm u 0.000000 1.000000 2.000000 3.000000
end
# 1 element
Bezier patch
The following example shows the 2.11 syntax and the 3.0 syntax for the same Bezier patch.
2.11 Bezier patch
# 2.11 Bezier Patch
v -5.000000 -5.000000 0.000000
v -5.000000 -1.666667 0.000000
v -5.000000 1.666667 0.000000
v -5.000000 5.000000 0.000000
v -1.666667 -5.000000 0.000000
v -1.666667 -1.666667 0.000000
v -1.666667 1.666667 0.000000
v -1.666667 5.000000 0.000000
v 1.666667 -5.000000 0.000000
v 1.666667 -1.666667 0.000000
v 1.666667 1.666667 0.000000
v 1.666667 5.000000 0.000000
v 5.000000 -5.000000 0.000000
v 5.000000 -1.666667 0.000000
v 5.000000 1.666667 0.000000
v 5.000000 5.000000 0.000000
# 16 vertices
bzp 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
# 1 element
3.0 Bezier patch
# 3.0 Bezier patch
v -5.000000 -5.000000 0.000000
v -5.000000 -1.666667 0.000000
v -5.000000 1.666667 0.000000
v -5.000000 5.000000 0.000000
v -1.666667 -5.000000 0.000000
v -1.666667 -1.666667 0.000000
v -1.666667 1.666667 0.000000
v -1.666667 5.000000 0.000000
v 1.666667 -5.000000 0.000000
v 1.666667 -1.666667 0.000000
v 1.666667 1.666667 0.000000
v 1.666667 5.000000 0.000000
v 5.000000 -5.000000 0.000000
v 5.000000 -1.666667 0.000000
v 5.000000 1.666667 0.000000
v 5.000000 5.000000 0.000000
# 16 vertices
cstype bezier
deg 3 3
surf 0.000000 1.000000 0.000000 1.000000 13 14 \
15 16 9 10 11 12 5 6 7 8 1 2 3 4
parm u 0.000000 1.000000
parm v 0.000000 1.000000
end
# 1 element
- Material Library File (.mtl)
Material library files contain one or more material definitions, each of which includes the color, texture, and reflection map of individual materials. These are applied to the surfaces and vertices of objects. Material files are stored in ASCII format and have the .mtl extension.
An .mtl file differs from other Alias|Wavefront property files, such as light and atmosphere files, in that it can contain more than one material definition (other files contain the definition of only one item).
An .mtl file is typically organized as shown below.
newmtl my_red Material color & illumination statements
texture map statements
reflection map statement
newmtl my_blue Material color & illumination statements
texture map statements
reflection map statement
newmtl my_green Material color & illumination statements
texture map statements
reflection map statement
Figure 5-1. Typical organization of .mtl file
Each material description in an .mtl file consists of the newmtl statement, which assigns a name to the material and designates the start of a material description. This statement is followed by the material color, texture map, and reflection map statements that describe the material. An .mtl file map contain many different material descriptions.
After you specify a new material with the "newmtl" statement, you can enter the statements that describe the materials in any order. However, when the Property Editor writes an .mtl file, it puts the statements in a system-assigned order. In this chapter, the statements are described in the system-assigned order.
Format
The following is a sample format for a material definition in an .mtl file:
Material name statement: newmtl my_mtl
Material color and illumination statements: Ka 0.0435 0.0435 0.0435 Kd 0.1086 0.1086 0.1086 Ks 0.0000 0.0000 0.0000 Tf 0.9885 0.9885 0.9885 illum 6 d -halo 0.6600 Ns 10.0000 sharpness 60 Ni 1.19713
Texture map statements: map_Ka -s 1 1 1 -o 0 0 0 -mm 0 1 chrome.mpc map_Kd -s 1 1 1 -o 0 0 0 -mm 0 1 chrome.mpc map_Ks -s 1 1 1 -o 0 0 0 -mm 0 1 chrome.mpc map_Ns -s 1 1 1 -o 0 0 0 -mm 0 1 wisp.mps map_d -s 1 1 1 -o 0 0 0 -mm 0 1 wisp.mps disp -s 1 1 .5 wisp.mps decal -s 1 1 1 -o 0 0 0 -mm 0 1 sand.mps bump -s 1 1 1 -o 0 0 0 -bm 1 sand.mpb
Reflection map statement: refl -type sphere -mm 0 1 clouds.mpc
Material Name
The material name statement assigns a name to the material description.
Syntax The folowing syntax describes the material name statement.
newmtl name
Specifies the start of a material description and assigns a name to the material. An .mtl file must have one newmtl statement at the start of each material description.
"name" is the name of the material. Names may be any length but cannot include blanks. Underscores may be used in material names.
Material color and illumination
The statements in this section specify color, transparency, and reflectivity values.
Syntax The following syntax describes the material color and illumination statements that apply to all .mtl files.
Ka r g b Ka spectral file.rfl factor Ka xyz x y z
To specify the ambient reflectivity of the current material, you can use the "Ka" statement, the "Ka spectral" statement, or the "Ka xyz" statement.
Tip These statements are mutually exclusive. They cannot be used concurrently in the same material.
Ka r g b
The Ka statement specifies the ambient reflectivity using RGB values.
"r g b" are the values for the red, green, and blue components of the color. The g and b arguments are optional. If only r is specified, then g, and b are assumed to be equal to r. The r g b values are normally in the range of 0.0 to 1.0. Values outside this range increase or decrease the relectivity accordingly.
Ka spectral file.rfl factor
The "Ka spectral" statement specifies the ambient reflectivity using a spectral curve.
"file.rfl" is the name of the .rfl file. "factor" is an optional argument. "factor" is a multiplier for the values in the .rfl file and defaults to 1.0, if not specified.
Ka xyz x y z
The "Ka xyz" statement specifies the ambient reflectivity using CIEXYZ values.
"x y z" are the values of the CIEXYZ color space. The y and z arguments are optional. If only x is specified, then y and z are assumed to be equal to x. The x y z values are normally in the range of 0 to 1. Values outside this range increase or decrease the reflectivity accordingly.
Kd r g b Kd spectral file.rfl factor Kd xyz x y z
To specify the diffuse reflectivity of the current material, you can use the "Kd" statement, the "Kd spectral" statement, or the "Kd xyz" statement.
Tip These statements are mutually exclusive. They cannot be used concurrently in the same material.
Kd r g b
The Kd statement specifies the diffuse reflectivity using RGB values.
"r g b" are the values for the red, green, and blue components of the atmosphere. The g and b arguments are optional. If only r is specified, then g, and b are assumed to be equal to r. The r g b values are normally in the range of 0.0 to 1.0. Values outside this range increase or decrease the relectivity accordingly.
Kd spectral file.rfl factor
The "Kd spectral" statement specifies the diffuse reflectivity using a spectral curve.
"file.rfl" is the name of the .rfl file. "factor" is an optional argument. "factor" is a multiplier for the values in the .rfl file and defaults to 1.0, if not specified.
Kd xyz x y z
The "Kd xyz" statement specifies the diffuse reflectivity using CIEXYZ values.
"x y z" are the values of the CIEXYZ color space. The y and z arguments are optional. If only x is specified, then y and z are assumed to be equal to x. The x y z values are normally in the range of 0 to 1. Values outside this range increase or decrease the reflectivity accordingly.
Ks r g b Ks spectral file.rfl factor Ks xyz x y z
To specify the specular reflectivity of the current material, you can use the "Ks" statement, the "Ks spectral" statement, or the "Ks xyz" statement.
Tip These statements are mutually exclusive. They cannot be used concurrently in the same material.
Ks r g b
The Ks statement specifies the specular reflectivity using RGB values.
"r g b" are the values for the red, green, and blue components of the atmosphere. The g and b arguments are optional. If only r is specified, then g, and b are assumed to be equal to r. The r g b values are normally in the range of 0.0 to 1.0. Values outside this range increase or decrease the relectivity accordingly.
Ks spectral file.rfl factor
The "Ks spectral" statement specifies the specular reflectivity using a spectral curve.
"file.rfl" is the name of the .rfl file. "factor" is an optional argument. "factor" is a multiplier for the values in the .rfl file and defaults to 1.0, if not specified.
Ks xyz x y z
The "Ks xyz" statement specifies the specular reflectivity using CIEXYZ values.
"x y z" are the values of the CIEXYZ color space. The y and z arguments are optional. If only x is specified, then y and z are assumed to be equal to x. The x y z values are normally in the range of 0 to 1. Values outside this range increase or decrease the reflectivity accordingly.
Tf r g b Tf spectral file.rfl factor Tf xyz x y z
To specify the transmission filter of the current material, you can use the "Tf" statement, the "Tf spectral" statement, or the "Tf xyz" statement.
Any light passing through the object is filtered by the transmission filter, which only allows the specifiec colors to pass through. For example, Tf 0 1 0 allows all the green to pass through and filters out all the red and blue.
Tip These statements are mutually exclusive. They cannot be used concurrently in the same material.
Tf r g b
The Tf statement specifies the transmission filter using RGB values.
"r g b" are the values for the red, green, and blue components of the atmosphere. The g and b arguments are optional. If only r is specified, then g, and b are assumed to be equal to r. The r g b values are normally in the range of 0.0 to 1.0. Values outside this range increase or decrease the relectivity accordingly.
Tf spectral file.rfl factor
The "Tf spectral" statement specifies the transmission filterusing a spectral curve.
"file.rfl" is the name of the .rfl file. "factor" is an optional argument. "factor" is a multiplier for the values in the .rfl file and defaults to 1.0, if not specified.
Tf xyz x y z
The "Ks xyz" statement specifies the specular reflectivity using CIEXYZ values.
"x y z" are the values of the CIEXYZ color space. The y and z arguments are optional. If only x is specified, then y and z are assumed to be equal to x. The x y z values are normally in the range of 0 to 1. Values outside this range will increase or decrease the intensity of the light transmission accordingly.
illum illum_#
The "illum" statement specifies the illumination model to use in the material. Illumination models are mathematical equations that represent various material lighting and shading effects.
"illum_#"can be a number from 0 to 10. The illumination models are summarized below; for complete descriptions see "Illumination models" on page 5-30.
Illumination Properties that are turned on in the model Property Editor
0 Color on and Ambient off 1 Color on and Ambient on 2 Highlight on 3 Reflection on and Ray trace on 4 Transparency: Glass on Reflection: Ray trace on 5 Reflection: Fresnel on and Ray trace on 6 Transparency: Refraction on Reflection: Fresnel off and Ray trace on 7 Transparency: Refraction on Reflection: Fresnel on and Ray trace on 8 Reflection on and Ray trace off 9 Transparency: Glass on Reflection: Ray trace off 10 Casts shadows onto invisible surfaces
d factor
Specifies the dissolve for the current material.
"factor" is the amount this material dissolves into the background. A factor of 1.0 is fully opaque. This is the default when a new material is created. A factor of 0.0 is fully dissolved (completely transparent).
Unlike a real transparent material, the dissolve does not depend upon material thickness nor does it have any spectral character. Dissolve works on all illumination models.
d -halo factor
Specifies that a dissolve is dependent on the surface orientation relative to the viewer. For example, a sphere with the following dissolve, d -halo 0.0, will be fully dissolved at its center and will appear gradually more opaque toward its edge.
"factor" is the minimum amount of dissolve applied to the material. The amount of dissolve will vary between 1.0 (fully opaque) and the specified "factor". The formula is:
dissolve = 1.0 - (N*v)(1.0-factor)
For a definition of terms, see "Illumination models" on page 5-30.
Ns exponent
Specifies the specular exponent for the current material. This defines the focus of the specular highlight.
"exponent" is the value for the specular exponent. A high exponent results in a tight, concentrated highlight. Ns values normally range from 0 to 1000.
sharpness value
Specifies the sharpness of the reflections from the local reflection map. If a material does not have a local reflection map defined in its material definition, sharpness will apply to the global reflection map defined in PreView.
"value" can be a number from 0 to 1000. The default is 60. A high value results in a clear reflection of objects in the reflection map.
Tip Sharpness values greater than 100 map introduce aliasing effects in flat surfaces that are viewed at a sharp angle
Ni optical_density
Specifies the optical density for the surface. This is also known as index of refraction.
"optical_density" is the value for the optical density. The values can range from 0.001 to 10. A value of 1.0 means that light does not bend as it passes through an object. Increasing the optical_density increases the amount of bending. Glass has an index of refraction of about 1.5. Values of less than 1.0 produce bizarre results and are not recommended.
Material texture map
Texture map statements modify the material parameters of a surface by associating an image or texture file with material parameters that can be mapped. By modifying existing parameters instead of replacing them, texture maps provide great flexibility in changing the appearance of an object's surface.
Image files and texture files can be used interchangeably. If you use an image file, that file is converted to a texture in memory and is discarded after rendering.
Tip Using images instead of textures saves disk space and setup time, however, it introduces a small computational cost at the beginning of a render.
The material parameters that can be modified by a texture map are:
- Ka (color)
- Kd (color)
- Ks (color)
- Ns (scalar)
- d (scalar)
In addition to the material parameters, the surface normal can be modified.
Image file types
You can link any image file type that is currently supported. Supported image file types are listed in the chapter "About Image" in the "Advanced Visualizer User's Guide". You can also use the "im_info -a" command to list Image file types, among other things.
Texture file types
The texture file types you can use are:
- mip-mapped texture files (.mpc, .mps, .mpb)
- compiled procedural texture files (.cxc, .cxs, .cxb)
Mip-mapped texture files
Mip-mapped texture files are created from images using the Create Textures panel in the Director or the "texture2D" program. There are three types of texture files:
- color texture files (.mpc)
- scalar texture files (.mps)
- bump texture files (.mpb)
Color textures. Color texture files are designated by an extension of ".mpc" in the filename, such as "chrome.mpc". Color textures modify the material color as follows:
- Ka - material ambient is multiplied by the texture value
- Kd - material diffuse is multiplied by the texture value
- Ks - material specular is multiplied by the texture value
Scalar textures. Scalar texture files are designated by an extension of ".mps" in the filename, such as "wisp.mps". Scalar textures modify the material scalar values as follows:
- Ns - material specular exponent is multiplied by the texture value
- d - material dissolve is multiplied by the texture value
- decal - uses a scalar value to deform the surface of an object to create surface roughness
Bump textures. Bump texture files are designated by an extension of ".mpb" in the filename, such as "sand.mpb". Bump textures modify surface normals. The image used for a bump texture represents the topology or height of the surface relative to the average surface. Dark areas are depressions and light areas are high points. The effect is like embossing the surface with the texture.
Procedural texture files
Procedural texture files use mathematical formulas to calculate sample values of the texture. The procedural texture file is compiled, stored, and accessed by the Image program when rendering. for more information see chapter 9, "Procedural Texture Files (.cxc, .cxb. and .cxs)".
Syntax
The following syntax describes the texture map statements that apply to .mtl files. These statements can be used alone or with any combination of options. The options and their arguments are inserted between the keyword and the "filename".
map_Ka -options args filename
Specifies that a color texture file or a color procedural texture file is applied to the ambient reflectivity of the material. During rendering, the "map_Ka" value is multiplied by the "Ka" value.
"filename" is the name of a color texture file (.mpc), a color procedural texture file (.cxc), or an image file.
Tip To make sure that the texture retains its original look, use the .rfl file "ident" as the underlying material. This applies to the "map_Ka", "map_Kd", and "map_Ks" statements. For more information on .rfl files, see chapter 8, "Spectral Curve File (.rfl)".
The options for the "map_Ka" statement are listed below. These options are described in detail in "Options for texture map statements" on page 5-18.
-blendu on | off -blendv on | off -cc on | off -clamp on | off -mm base gain -o u v w -s u v w -t u v w -texres value
map_Kd -options args filename
Specifies that a color texture file or color procedural texture file is linked to the diffuse reflectivity of the material. During rendering, the map_Kd value is multiplied by the Kd value.
"filename" is the name of a color texture file (.mpc), a color procedural texture file (.cxc), or an image file.
The options for the map_Kd statement are listed below. These options are described in detail in "Options for texture map statements" on page 5-18.
-blendu on | off -blendv on | off -cc on | off -clamp on | off -mm base gain -o u v w -s u v w -t u v w -texres value
map_Ks -options args filename
Specifies that a color texture file or color procedural texture file is linked to the specular reflectivity of the material. During rendering, the map_Ks value is multiplied by the Ks value.
"filename" is the name of a color texture file (.mpc), a color procedural texture file (.cxc), or an image file.
The options for the map_Ks statement are listed below. These options are described in detail in "Options for texture map statements" on page 5-18.
-blendu on | off -blendv on | off -cc on | off -clamp on | off -mm base gain -o u v w -s u v w -t u v w -texres value
map_Ns -options args filename
Specifies that a scalar texture file or scalar procedural texture file is linked to the specular exponent of the material. During rendering, the map_Ns value is multiplied by the Ns value.
"filename" is the name of a scalar texture file (.mps), a scalar procedural texture file (.cxs), or an image file.
The options for the map_Ns statement are listed below. These options are described in detail in "Options for texture map statements" on page 5-18.
-blendu on | off -blendv on | off -clamp on | off -imfchan r | g | b | m | l | z -mm base gain -o u v w -s u v w -t u v w -texres value
map_d -options args filename
Specifies that a scalar texture file or scalar procedural texture file is linked to the dissolve of the material. During rendering, the map_d value is multiplied by the d value.
"filename" is the name of a scalar texture file (.mps), a scalar procedural texture file (.cxs), or an image file.
The options for the map_d statement are listed below. These options are described in detail in "Options for texture map statements" on page 5-18.
-blendu on | off -blendv on | off -clamp on | off -imfchan r | g | b | m | l | z -mm base gain -o u v w -s u v w -t u v w -texres value
map_aat on
Turns on anti-aliasing of textures in this material without anti-aliasing all textures in the scene.
If you wish to selectively anti-alias textures, first insert this statement in the material file. Then, when rendering with the Image panel, choose the anti-alias settings: "shadows", "reflections polygons", or "polygons only". If using Image from the command line, use the -aa or -os options. Do not use the -aat option.
Image will anti-alias all textures in materials with the map_aat on statement, using the oversampling level you choose when you run Image. Textures in other materials will not be oversampled.
You cannot set a different oversampling level individually for each material, nor can you anti-alias some textures in a material and not others. To anti-alias all textures in all materials, use the -aat option from the Image command line. If a material with "map_aat on" includes a reflection map, all textures in that reflection map will be anti-aliased as well.
You will not see the effects of map_aat in the Property Editor.
Tip Some .mpc textures map exhibit undesirable effects around the edges of smoothed objects. The "map_aat" statement will correct this.
decal -options args filename
Specifies that a scalar texture file or a scalar procedural texture file is used to selectively replace the material color with the texture color.
"filename" is the name of a scalar texture file (.mps), a scalar procedural texture file (.cxs), or an image file.
During rendering, the Ka, Kd, and Ks values and the map_Ka, map_Kd, and map_Ks values are blended according to the following formula:
result_color=tex_color(tv)decal(tv)+mtl_color(1.0-decal(tv))
where tv is the texture vertex.
"result_color" is the blended Ka, Kd, and Ks values.
The options for the decal statement are listed below. These options are described in detail in "Options for texture map statements" on page 5-18.
-blendu on | off -blendv on | off -clamp on | off -imfchan r | g | b | m | l | z -mm base gain -o u v w -s u v w -t u v w -texres value
disp -options args filename
Specifies that a scalar texture is used to deform the surface of an object, creating surface roughness.
"filename" is the name of a scalar texture file (.mps), a bump procedural texture file (.cxb), or an image file.
The options for the disp statement are listed below. These options are described in detail in "Options for texture map statements" on page 5-18.
-blendu on | off -blendv on | off -clamp on | off -imfchan r | g | b | m | l | z -mm base gain -o u v w -s u v w -t u v w -texres value
bump -options args filename
Specifies that a bump texture file or a bump procedural texture file is linked to the material.
"filename" is the name of a bump texture file (.mpb), a bump procedural texture file (.cxb), or an image file.
The options for the bump statement are listed below. These options are described in detail in "Options for texture map statements" on page 5-18.
-bm mult -clamp on | off -blendu on | off -blendv on | off -imfchan r | g | b | m | l | z -mm base gain -o u v w -s u v w -t u v w -texres value
Options for texture map statements
The following options and arguments can be used to modify the texture map statements.
-blenu on | off
The -blendu option turns texture blending in the horizontal direction (u direction) on or off. The default is on.
-blenv on | off
The -blendv option turns texture blending in the vertical direction (v direction) on or off. The default is on.
-bm mult
The -bm option specifies a bump multiplier. You can use it only with the "bump" statement. Values stored with the texture or procedural texture file are multiplied by this value before they are applied to the surface.
"mult" is the value for the bump multiplier. It can be positive or negative. Extreme bump multipliers may cause odd visual results because only the surface normal is perturbed and the surface position does not change. For best results, use values between 0 and 1.
-boost value
The -boost option increases the sharpness, or clarity, of mip-mapped texture files -- that is, color (.mpc), scalar (.mps), and bump (.mpb) files. If you render animations with boost, you may experience some texture crawling. The effects of boost are seen when you render in Image or test render in Model or PreView; they aren't as noticeable in Property Editor.
"value" is any non-negative floating point value representing the degree of increased clarity; the greater the value, the greater the clarity. You should start with a boost value of no more than 1 or 2 and increase the value as needed. Note that larger values have more potential to introduce texture crawling when animated.
-cc on | off
The -cc option turns on color correction for the texture. You can use it only with the color map statements: map_Ka, map_Kd, and map_Ks.
-clamp on | off
The -clamp option turns clamping on or off. When clamping is on, textures are restricted to 0-1 in the uvw range. The default is off.
When clamping is turned on, one copy of the texture is mapped onto the surface, rather than repeating copies of the original texture across the surface of a polygon, which is the default. Outside of the origin texture, the underlying material is unchanged.
A postage stamp on an envelope or a label on a can of soup is an example of a texture with clamping turned on. A tile floor or a sidewalk is an example of a texture with clamping turned off.
Two-dimensional textures are clamped in the u and v dimensions; 3D procedural textures are clamped in the u, v, and w dimensions.
-imfchan r | g | b | m | l | z
The -imfchan option specifies the channel used to create a scalar or bump texture. Scalar textures are applied to:
transparency specular exponent decal displacement
The channel choices are:
r specifies the red channel. g specifies the green channel. b specifies the blue channel. m specifies the matte channel. l specifies the luminance channel. z specifies the z-depth channel.
The default for bump and scalar textures is "l" (luminance), unless you are building a decal. In that case, the default is "m" (matte).
-mm base gain
The -mm option modifies the range over which scalar or color texture values may vary. This has an effect only during rendering and does not change the file.
"base" adds a base value to the texture values. A positive value makes everything brighter; a negative value makes everything dimmer. The default is 0; the range is unlimited.
"gain" expands the range of the texture values. Increasing the number increases the contrast. The default is 1; the range is unlimited.
-o u v w
The -o option offsets the position of the texture map on the surface by shifting the position of the map origin. The default is 0, 0, 0.
"u" is the value for the horizontal direction of the texture
"v" is an optional argument. "v" is the value for the vertical direction of the texture.
"w" is an optional argument. "w" is the value used for the depth of a 3D texture.
-s u v w
The -s option scales the size of the texture pattern on the textured surface by expanding or shrinking the pattern. The default is 1, 1, 1.
"u" is the value for the horizontal direction of the texture
"v" is an optional argument. "v" is the value for the vertical direction of the texture.
"w" is an optional argument. "w" is a value used for the depth of a 3D texture. "w" is a value used for the amount of tessellation of the displacement map.
-t u v w
The -t option turns on turbulence for textures. Adding turbulence to a texture along a specified direction adds variance to the original image and allows a simple image to be repeated over a larger area without noticeable tiling effects.
turbulence also lets you use a 2D image as if it were a solid texture, similar to 3D procedural textures like marble and granite.
"u" is the value for the horizontal direction of the texture turbulence.
"v" is an optional argument. "v" is the value for the vertical direction of the texture turbulence.
"w" is an optional argument. "w" is a value used for the depth of the texture turbulence.
By default, the turbulence for every texture map used in a material is uvw = (0,0,0). This means that no turbulence will be applied and the 2D texture will behave normally.
Only when you raise the turbulence values above zero will you see the effects of turbulence.
-texres resolution
The -texres option specifies the resolution of texture created when an image is used. The default texture size is the largest power of two that does not exceed the original image size.
If the source image is an exact power of 2, the texture cannot be built any larger. If the source image size is not an exact power of 2, you can specify that the texture be built at the next power of 2 greater than the source image size.
The original image should be square, otherwise, it will be scaled to fit the closest square size that is not larger than the original. Scaling reduces sharpness.
Material reflection map
A reflection map is an environment that simulates reflections in specified objects. The environment is represented by a color texture file or procedural texture file that is mapped on the inside of an infinitely large, space. Reflection maps can be spherical or cubic. A spherical reflection map requires only one texture or image file, while a cubic reflection map requires six.
Each material description can contain one reflection map statement that specifies a color texture file or a color procedural texture file to represent the environment. The material itself must be assigned an illumination model of 3 or greater.
The reflection map statement in the .mtl file defines a local reflection map. That is, each material assigned to an object in a scene can have an individual reflection map. In PreView, you can assign a global reflection map to an object and specify the orientation of the reflection map. Rotating the reflection map creates the effect of animating reflections independently of object motion. When you replace a global reflection map with a local reflection map, the local reflection map inherits the transformation of the global reflection map.
Syntax
The following syntax statements describe the reflection map statement for .mtl files.
refl -type sphere -options -args filename
Specifies an infinitely large sphere that casts reflections onto the material. You specify one texture file.
"filename" is the color texture file, color procedural texture file, or image file that will be mapped onto the inside of the shape.
refl -type cube_side -options -args filenames
Specifies an infinitely large sphere that casts reflections onto the material. You can specify different texture files for the "top", "bottom", "front", "back", "left", and "right" with the following statements:
refl -type cube_top refl -type cube_bottom refl -type cube_front refl -type cube_back refl -type cube_left refl -type cube_right
"filenames" are the color texture files, color procedural texture files, or image files that will be mapped onto the inside of the shape.
The "refl" statements for sphere and cube can be used alone or with any combination of the following options. The options and their arguments are inserted between "refl" and "filename".
-blendu on | off -blendv on | off -cc on | off -clamp on | off -mm base gain -o u v w -s u v w -t u v w -texres value
The options for the reflection map statement are described in detail in "Options for texture map statements" on page 18.
Examples
1 Neon green
This is a bright green material. When applied to an object, it will remain bright green regardless of any lighting in the scene.
newmtl neon_green Kd 0.0000 1.0000 0.0000 illum 0
2 Flat green
This is a flat green material.
newmtl flat_green Ka 0.0000 1.0000 0.0000 Kd 0.0000 1.0000 0.0000 illum 1
3 Dissolved green
This is a flat green, partially dissolved material.
newmtl diss_green Ka 0.0000 1.0000 0.0000 Kd 0.0000 1.0000 0.0000 d 0.8000 illum 1
4 Shiny green
This is a shiny green material. When applied to an object, it shows a white specular highlight.
newmtl shiny_green Ka 0.0000 1.0000 0.0000 Kd 0.0000 1.0000 0.0000 Ks 1.0000 1.0000 1.0000 Ns 200.0000 illum 1
5 Green mirror
This is a reflective green material. When applied to an object, it reflects other objects in the same scene.
newmtl green_mirror Ka 0.0000 1.0000 0.0000 Kd 0.0000 1.0000 0.0000 Ks 0.0000 1.0000 0.0000 Ns 200.0000 illum 3
6 Fake windshield
This material approximates a glass surface. Is it almost completely transparent, but it shows reflections of other objects in the scene. It will not distort the image of objects seen through the material.
newmtl fake_windsh Ka 0.0000 0.0000 0.0000 Kd 0.0000 0.0000 0.0000 Ks 0.9000 0.9000 0.9000 d 0.1000 Ns 200 illum 4
7 Fresnel blue
This material exhibits an effect known as Fresnel reflection. When applied to an object, white fringes may appear where the object's surface is viewed at a glancing angle.
newmtl fresnel_blu Ka 0.0000 0.0000 0.0000 Kd 0.0000 0.0000 0.0000 Ks 0.6180 0.8760 0.1430 Ns 200 illum 5
8 Real windshield
This material accurately represents a glass surface. It filters of colorizes objects that are seen through it. Filtering is done according to the transmission color of the material. The material also distorts the image of objects according to its optical density. Note that the material is not dissolved and that its ambient, diffuse, and specular reflective colors have been set to black. Only the transmission color is non-black.
newmtl real_windsh Ka 0.0000 0.0000 0.0000 Kd 0.0000 0.0000 0.0000 Ks 0.0000 0.0000 0.0000 Tf 1.0000 1.0000 1.0000 Ns 200 Ni 1.2000 illum 6
9 Fresnel windshield
This material combines the effects in examples 7 and 8.
newmtl fresnel_win Ka 0.0000 0.0000 1.0000 Kd 0.0000 0.0000 1.0000 Ks 0.6180 0.8760 0.1430 Tf 1.0000 1.0000 1.0000 Ns 200 Ni 1.2000 illum 7
10 Tin
This material is based on spectral reflectance samples taken from an actual piece of tin. These samples are stored in a separate .rfl file that is referred to by name in the material. Spectral sample files (.rfl) can be used in any type of material as an alternative to RGB values.
newmtl tin Ka spectral tin.rfl Kd spectral tin.rfl Ks spectral tin.rfl Ns 200 illum 3
11 Pine Wood
This material includes a texture map of a pine pattern. The material color is set to "ident" to preserve the texture's true color. When applied to an object, this texture map will affect only the ambient and diffuse regions of that object's surface.
The color information for the texture is stored in a separate .mpc file that is referred to in the material by its name, "pine.mpc". If you use different .mpc files for ambient and diffuse, you will get unrealistic results.
newmtl pine_wood Ka spectral ident.rfl 1 Kd spectral ident.rfl 1 illum 1 map_Ka pine.mpc map_Kd pine.mpc
12 Bumpy leather
This material includes a texture map of a leather pattern. The material color is set to "ident" to preserve the texture's true color. When applied to an object, it affects both the color of the object's surface and its apparent bumpiness.
The color information for the texture is stored in a separate .mpc file that is referred to in the material by its name, "brown.mpc". The bump information is stored in a separate .mpb file that is referred to in the material by its name, "leath.mpb". The -bm option is used to raise the apparent height of the leather bumps.
newmtl bumpy_leath Ka spectral ident.rfl 1 Kd spectral ident.rfl 1 Ks spectral ident.rfl 1 illum 2 map_Ka brown.mpc map_Kd brown.mpc map_Ks brown.mpc bump -bm 2.000 leath.mpb
13 Frosted window
This material includes a texture map used to alter the opacity of an object's surface. The material color is set to "ident" to preserve the texture's true color. When applied to an object, the object becomes transparent in certain areas and opaque in others.
The variation between opaque and transparent regions is controlled by scalar information stored in a separate .mps file that is referred to in the material by its name, "window.mps". The "-mm" option is used to shift and compress the range of opacity.
newmtl frost_wind Ka 0.2 0.2 0.2 Kd 0.6 0.6 0.6 Ks 0.1 0.1 0.1 d 1 Ns 200 illum 2 map_d -mm 0.200 0.800 window.mps
14 Shifted logo
This material includes a texture map which illustrates how a texture's origin may be shifted left/right (the "u" direction) or up/down (the "v" direction). The material color is set to "ident" to preserve the texture's true color.
In this example, the original image of the logo is off-center to the left. To compensate, the texture's origin is shifted back to the right (the positive "u" direction) using the "-o" option to modify the origin.
Ka spectral ident.rfl 1 Kd spectral ident.rfl 1 Ks spectral ident.rfl 1 illum 2 map_Ka -o 0.200 0.000 0.000 logo.mpc map_Kd -o 0.200 0.000 0.000 logo.mpc map_Ks -o 0.200 0.000 0.000 logo.mpc
15 Scaled logo
This material includes a texture map showing how a texture may be scaled left or right (in the "u" direction) or up and down (in the "v" direction). The material color is set to "ident" to preserve the texture's true color.
In this example, the original image of the logo is too small. To compensate, the texture is scaled slightly to the right (in the positive "u" direction) and up (in the positive "v" direction) using the "-s" option to modify the scale.
Ka spectral ident.rfl 1 Kd spectral ident.rfl 1 Ks spectral ident.rfl 1 illum 2 map_Ka -s 1.200 1.200 0.000 logo.mpc map_Kd -s 1.200 1.200 0.000 logo.mpc map_Ks -s 1.200 1.200 0.000 logo.mpc
16 Chrome with spherical reflection map
This illustrates a common use for local reflection maps (defined in a material).
this material is highly reflective with no diffuse or ambient contribution. Its reflection map is an image with silver streaks that yields a chrome appearance when viewed as a reflection.
ka 0 0 0 kd 0 0 0 ks .7 .7 .7 illum 1 refl -type sphere chrome.rla
Illumination models
The following list defines the terms and vectors that are used in the illumination model equations:
Term Definition
Ft Fresnel reflectance Ft Fresnel transmittance Ia ambient light I light intensity Ir intensity from reflected direction (reflection map and/or ray tracing) It intensity from transmitted direction Ka ambient reflectance Kd diffuse reflectance Ks specular reflectance Tf transmission filter
Vector Definition
H unit vector bisector between L and V L unit light vector N unit surface normal V unit view vector
The illumination models are:
0 This is a constant color illumination model. The color is the specified Kd for the material. The formula is:
color = Kd
1 This is a diffuse illumination model using Lambertian shading. The color includes an ambient constant term and a diffuse shading term for each light source. The formula is
color = KaIa + Kd { SUM j=1..ls, (N * Lj)Ij }
2 This is a diffuse and specular illumination model using Lambertian shading and Blinn's interpretation of Phong's specular illumination model (BLIN77). The color includes an ambient constant term, and a diffuse and specular shading term for each light source. The formula is:
color = KaIa
- Kd { SUM j=1..ls, (N*Lj)Ij }
- Ks { SUM j=1..ls, ((H*Hj)^Ns)Ij }
3 This is a diffuse and specular illumination model with reflection using Lambertian shading, Blinn's interpretation of Phong's specular illumination model (BLIN77), and a reflection term similar to that in Whitted's illumination model (WHIT80). The color includes an ambient constant term and a diffuse and specular shading term for each light source. The formula is:
color = KaIa
- Kd { SUM j=1..ls, (N*Lj)Ij }
- Ks ({ SUM j=1..ls, ((H*Hj)^Ns)Ij } + Ir)
Ir = (intensity of reflection map) + (ray trace)
4 The diffuse and specular illumination model used to simulate glass is the same as illumination model 3. When using a very low dissolve (approximately 0.1), specular highlights from lights or reflections become imperceptible.
Simulating glass requires an almost transparent object that still reflects strong highlights. The maximum of the average intensity of highlights and reflected lights is used to adjust the dissolve factor. The formula is:
color = KaIa
- Kd { SUM j=1..ls, (N*Lj)Ij }
- Ks ({ SUM j=1..ls, ((H*Hj)^Ns)Ij } + Ir)
5 This is a diffuse and specular shading models similar to illumination model 3, except that reflection due to Fresnel effects is introduced into the equation. Fresnel reflection results from light striking a diffuse surface at a grazing or glancing angle. When light reflects at a grazing angle, the Ks value approaches 1.0 for all color samples. The formula is:
color = KaIa
- Kd { SUM j=1..ls, (N*Lj)Ij }
- Ks ({ SUM j=1..ls, ((H*Hj)^Ns)Ij Fr(Lj*Hj,Ks,Ns)Ij} + Fr(N*V,Ks,Ns)Ir})
6 This is a diffuse and specular illumination model similar to that used by Whitted (WHIT80) that allows rays to refract through a surface. The amount of refraction is based on optical density (Ni). The intensity of light that refracts is equal to 1.0 minus the value of Ks, and the resulting light is filtered by Tf (transmission filter) as it passes through the object. The formula is:
color = KaIa
- Kd { SUM j=1..ls, (N*Lj)Ij }
- Ks ({ SUM j=1..ls, ((H*Hj)^Ns)Ij } + Ir)
- (1.0 - Ks) TfIt
7 This illumination model is similar to illumination model 6, except that reflection and transmission due to Fresnel effects has been introduced to the equation. At grazing angles, more light is reflected and less light is refracted through the object. The formula is:
color = KaIa
- Kd { SUM j=1..ls, (N*Lj)Ij }
Ks ({ SUM j=1..ls, ((H*Hj)^Ns)Ij Fr(Lj*Hj,Ks,Ns)Ij} + Fr(N*V,Ks,Ns)Ir})
(1.0 - Kx)Ft (N*V,(1.0-Ks),Ns)TfIt
8 This illumination model is similar to illumination model 3 without ray tracing. The formula is:
color = KaIa
- Kd { SUM j=1..ls, (N*Lj)Ij }
- Ks ({ SUM j=1..ls, ((H*Hj)^Ns)Ij } + Ir)
Ir = (intensity of reflection map)
9 This illumination model is similar to illumination model 4without ray tracing. The formula is:
color = KaIa
- Kd { SUM j=1..ls, (N*Lj)Ij }
- Ks ({ SUM j=1..ls, ((H*Hj)^Ns)Ij } + Ir)
Ir = (intensity of reflection map)
10 This illumination model is used to cast shadows onto an invisible surface. This is most useful when compositing computer-generated imagery onto live action, since it allows shadows from rendered objects to be composited directly on top of video-grabbed images. The equation for computation of a shadowmatte is formulated as follows.
color = Pixel color. The pixel color of a shadowmatte material is always black.
color = black
M = Matte channel value. This is the image channel which typically represents the opacity of the point on the surface. To store the shadow in the matte channel of the image, it is calculated as:
M = 1 - W / P
where:
P = Unweighted sum. This is the sum of all S values for each light:
P = S1 + S2 + S3 + .....
W = Weighted sum. This is the sum of all S values, each weighted by the visibility factor (Q) for the light:
W = (S1 * Q1) + (S2 * Q2) + .....
Q = Visibility factor. This is the amount of light from a particular light source that reaches the point to be shaded, after traveling through all shadow objects between the light and the point on the surface. Q = 0 means no light reached the point to be shaded; it was blocked by shadow objects, thus casting a shadow. Q = 1 means that nothing blocked the light, and no shadow was cast. 0 < Q < 1 means that the light was partially blocked by objects that were partially dissolved.
S = Summed brightness. This is the sum of the spectral sample intensities for a particular light. The samples are variable, but the default is 3:
S = samp1 + samp2 + samp3.
Excerpt from FILE FORMATS, Version 4.2 October 1995
Documentation created by: Diane Ramey, Linda Rose, and Lisa Tyerman