OBJ and MTL files
See:
- https://en.wikipedia.org/wiki/Wavefront_.obj_file
- OBJ Specification
- MTL Specification
- http://www.fileformat.info/format/material/
OBJ
.OBJ files contain structural information for a 3D Model, i.e., the object mesh.
| Variable | Description |
|---|---|
v | Geometric vertices with (x, y, z [, w = 1.0]) coordinates. |
vt | Texture coordinates with (u [, v = 0, w =0]) where all values are in the inclusive range [0, 1] |
vn | Vertex normals in (x, y, z) form. They may not be unit vectors |
f | Faces defined using lists of vertices identified by the index of how they were defined in the file. Indices start at 1 inside of OBJ files. |
MTL
.MTL files contain the graphical information to color in a 3D Model, i.e., colors, texture maps, bump maps, etc.
| Variable | Description |
|---|---|
Ka | The ambient color of the material. RGB format. |
Kd | The diffuse color of the material. RGB format. |
Ks | The specular color of the material. RGB format. |
Ns | The specular exponent of the material. f32 format in range [0, 1000]. |
d | Dissolved (transparency). f32 in the inclusive range [0, 1]. 1 means fully opaque. |
Tr | The inverse of d: 1.0 - d. Transparency. |
Ni | The optical density (AKA index of refraction) of the material. This is a property of transparent surfaces. This value is likely used in physical reflection models.) |
map_Ka | The ambient texture map. |
map_Kd | The diffuse texture map (usually the same as ambient texture map). |
map_Ks | The specular texture map. |
map_Bump | The normal texture map. |
When the map_Ka, map_Kd, and map_Ks texture maps are used, after getting the RGB color using a 2D sampler, the RGB values should be component-wise multiplied against the material RGB colors for Ka, Kd, and Ks respectively.
Vertex Normals vs. Normal Maps
When the map_Bump exists and is used to calculate normal vectors, their values replace the vertex normals (and the interpolated vertex normal values per fragment).
The vertex normals are still useful for calculating the tangent space to world space transformation matrix.
See page in the Kb on normal maps for more information on this transformation matrix.