What is .mtl?

.MTL File Format Information

A .MTL file is a material definition file used in conjunction with the .OBJ 3D object file format. It describes the surface shading properties of the objects defined in the .OBJ file. Think of the .OBJ file as the blueprint of your 3D model and the .MTL file as the instructions on how to paint and texture it.

Here's a breakdown of key aspects:

• Purpose: The .MTL file defines materials (colors, textures, and other visual properties) that can be applied to the geometry defined in an associated .OBJ file. It doesn't contain the actual 3D geometry itself.

• Material Definitions: An .MTL file typically contains multiple material definitions. Each material has a unique name (e.g., newmtl RedPlastic).

• Key Properties: Within each material definition, the following properties are commonly specified:

  • Color:

    • Ka: Ambient color (color reflected from ambient light).
    • Kd: Diffuse color (base color of the material).
    • Ks: Specular color (color of the highlight).

  • Shininess:

    • Ns: Specular exponent (controls the size and sharpness of the highlight).

  • Transparency:

    • d: Dissolve factor (opacity, from 0.0 to 1.0). 1.0 is fully opaque.
    • Tr: Transparency (alternative way to specify transparency, 1.0 is fully transparent).

  • Illumination Model:

    • illum: Specifies the illumination model to be used. Different models simulate different lighting effects.

  • Texture Maps: Links to image files used as textures. These give the surface more complex visual details.

    • map_Ka: Ambient texture map.
    • map_Kd: Diffuse texture map (the most common type).
    • map_Ks: Specular texture map.
    • map_Ns: Specular highlight map.
    • map_bump: Bump map (creates the illusion of surface detail).
    • map_displacement: Displacement map (actually modifies the geometry).

• Linking to .OBJ: Within the .OBJ file, the usemtl statement specifies which material (defined in the .MTL file) should be applied to subsequent faces or objects.

• Text Format: .MTL files are plain text files, making them relatively easy to read and edit.

• Example:

  newmtl RedPlastic
  Ka 0.2 0.0 0.0
  Kd 0.8 0.0 0.0
  Ks 0.9 0.9 0.9
  Ns 20
  illum 2
  

In summary, the .MTL file complements the .OBJ file by providing the necessary surface appearance information for 3D models. Without it, the .OBJ would just represent the shape and geometry, lacking color, texture, and other visual qualities.