spec_obj.txt

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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especially interested in the section called "Superseded statements" at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o       Superseded statements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