path: root/doc/obj-spec.txt

Wavefront OBJ

Also Known As: Wavefront Object, OBJ

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

Type                     3D Vector
Colors                   Unlimited
Compression              Uncompressed
Maximum Image Size       Unlimited
Multiple Images Per File Yes
Numerical Format         NA
Originator               Wavefront
Platform                 UNIX
Supporting Applications  Advanced Visualizer
See Also                 Wavefront RLA

Usage
Used to store and exchange 3D data.

Comments
The Wavefront OBJ format is a useful standard for representing polygonal data
in ASCII form.

Vendor specifications are available for this format.

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

Wavefront OBJ (object) files are used by Wavefront's Advanced Visualizer
application to store geometric objects composed of lines, polygons, and
free-form curves and surfaces. Wavefront is best known for its high-end
computer graphics tools, including modeling, animation, and image compositing
tools. These programs run on powerful workstations such as those made by
Silicon Graphics, Inc.

Wavefront OBJ files are often stored with the extension ".obj" following the
UNIX convention of lowercase letters for filenames. The most recently
documented version of OBJ is v3.0, superseding the previous v2.11 release.

Contents:
File Organization
File Details
For Further Information

In Wavefront's 3D software, geometric object files may be stored in ASCII
format (using the ".obj" file extension) or in binary format (using the .MOD
extension). The binary format is proprietary and undocumented, so only the
ASCII format is described here.

The OBJ file format supports lines, polygons, and free-form curves and
surfaces. Lines and polygons are described in terms of their points, while
curves and surfaces are defined with control points and other information
depending on the type of curve. The format supports rational and non-rational
curves, including those based on Bezier, B-spline, Cardinal (Catmull-Rom
splines), and Taylor equations.

File Organization

OBJ files do not require any sort of header, although it is common to begin the
file with a comment line of some kind. Comment lines begin with a hash mark
(#). Blank space and blank lines can be freely added to the file to aid in
formatting and readability. Each non-blank line begins with a keyword and may
be followed on the same line with the data for that keyword. Lines are read and
processed until the end of the file. Lines can be logically joined with the
line continuation character ( \ ) at the end of a line.

The following keywords may be included in an OBJ file. In this list, keywords
are arranged by data type, and each is followed by a brief description.

Vertex data:

    v  Geometric vertices
    vt Texture vertices
    vn Vertex normals
    vp Parameter space vertices

Free-form curve/surface attributes:

    deg    Degree
    bmat   Basis matrix
    step   Step size
    cstype Curve or surface type

Elements:

    p     Point
    l     Line
    f     Face
    curv  Curve
    curv2 2D curve
    surf  Surface

Free-form curve/surface body statements:

    parm Parameter values
    trim Outer trimming loop
    hole Inner trimming loop
    scrv Special curve
    sp   Special point
    end  End statement

Connectivity between free-form surfaces:

    con Connect

Grouping:

    g  Group name
    s  Smoothing group
    mg Merging group
    o  Object name

Display/render attributes:

    bevel      Bevel interpolation
    c_interp   Color interpolation
    d_interp   Dissolve interpolation
    lod        Level of detail
    usemtl     Material name
    mtllib     Material library
    shadow_obj Shadow casting
    trace_obj  Ray tracing
    ctech      Curve approximation technique
    stech      Surface approximation technique

File Details

The most commonly encountered OBJ files contain only polygonal faces. To
describe a polygon, the file first describes each point with the "v" keyword,
then describes the face with the "f" keyword. The line of a face command
contains the enumerations of the points in the face, as 1-based indices into
the list of points, in the order they occurred in the file. For example, the
following describes a simple triangle:

# Simple Wavefront file
v 0.0 0.0 0.0
v 0.0 1.0 0.0
v 1.0 0.0 0.0
f 1 2 3

It is also possible to reference points using negative indices. This makes it
easy to describe the points in a face, then the face, without the need to store
a large list of points and their indexes. In this way, "v" commands and "f"
commands can be interspersed.

v -0.500000 0.000000 0.400000
v -0.500000 0.000000 -0.800000
v -0.500000 1.000000 -0.800000
v -0.500000 1.000000 0.400000
f -4 -3 -2 -1

OBJ files do not contain color definitions for faces, although they can
reference materials that are stored in a separate material library file. The
material library can be loaded using the "mtllib" keyword. The material library
contains the definitions for the RGB values for the material's diffuse,
ambient, and specular colors, along with other characteristics such as
specularity, refraction, transparency, etc.

The OBJ file references materials by name with the "usemtl" keyword. All faces
that follow are given the attributes of this material until the next "usemtl"
command is encountered.

Faces and surfaces can be assigned into named groups with the "g" keyword. This
is used to create convenient sub-objects to make it easier to edit and animate
3D models. Faces can belong to more than one group.

The following demonstrate more complicated examples of smooth surfaces of
different types, material assignment, line continuation, and grouping.

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

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
# Example of line continuation
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

Cardinal Curve

# 3.0 Cardinal curve
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

Texture-Mapped Square

# A 2 x 2 square mapped with a 1 x 1 square
# texture 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
# The first number is the point,
# then the slash,
# and the second is the texture point
f 1/1 2/2 3/3 4/4
# 1 element

For Further Information

For further information about the Wavefront OBJ format, see the specification.

You can also contact:

Wavefront Technologies
530 East Montecito Street
Santa Barbara, CA 93103
Voice: 805-962-8117
FAX: 805-963-0410
WWW: http://www.aw.sgi.com/

Wavefront also maintains a toll-free support number and a BBS for its
customers. There are many Wavefront user groups, too.

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

[txtpreva] [txtnexta] [txtupa] [txttoidx]
[btntoc] [btnglos] [btnmain] [btnfmt] [btnsoft] [btninet] [btnbook]

Copyright © 1996, 1994 O'Reilly & Associates, Inc. All Rights Reserved.