Full specification (c) Paul Bourke
MTL material format (c) Diane Ramey, Linda Rose, and Lisa Tyerman

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 document describes the ASCII format for object files. These files must have the extension .obj.

The following types of data may be included in an .obj file. In this list, the keyword (in parentheses) follows the data type.

• geometric vertices (v)
• texture vertices (vt)
• vertex normals (vn)

• point (p)
• line (l)
• face (f)

• group name (g)
• smoothing group (s)
• merging group (mg)
• object name (o)

• bevel interpolation (bevel)
• color interpolation (c_interp)
• dissolve interpolation (d_interp)
• level of detail (lod)
• material name (usemtl)
• material library (mtllib)
• shadow casting (shadow_obj)
• ray tracing (trace_obj)
• curve approximation technique (ctech)
• surface approximation technique (stech)

Vertex data provides coordinates for:

• geometric vertices
• texture vertices
• vertex normals

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

When vertices are loaded into the Advanced Visualizer, they are sequentially numbered, starting with 1. These reference numbers are used in element statements.

The following syntax statements are listed in order of complexity.

v x y z w

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.

vn i j k

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

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.

For polygonal geometry, the element types available in the .obj file are:

• points
• lines
• faces

All elements can be freely intermixed in the file.

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 1. 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.

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 (/).

• The first reference number is the geometric vertex.

• The second reference number is the texture vertex. It follows the first slash.

• 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

The following syntax statements are listed in order of complexity of geometry.

p  v1 v2 v3 . . .

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 . . .

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 . . .

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.

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.”

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

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

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

There are four statements in the .obj file to help you manipulate groups of elements:

• Group name statements are used to organize collections of elements and simplify data manipulation for operations in Model.

• 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.

• 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.

• 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

g group_name1 group_name2 . . .

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

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.

o object_name

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.

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

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

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

Display and render attributes describe how an object looks when displayed in Model and PreView or when rendered with Image.

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

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

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

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

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

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

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 . . .

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

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

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.

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

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

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

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

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