newmtl

newmtl is a directive in the Wavefront MTL (material template library) file format used to begin the definition of a material. In OBJ workflows, the OBJ file can reference an external MTL file with a statement like mtllib and assign materials to faces with usemtl; the MTL file stores surface properties that shading engines use to render the material.

A material definition starts with a line containing newmtl followed by the material name. Subsequent lines describe the material’s properties until the next newmtl line or the end of the file. The most common properties are Ka (ambient color), Kd (diffuse color), and Ks (specular color), each given as three floating-point values for red, green, and blue. Ns sets the specular exponent or shininess. The d value specifies opacity (1.0 is opaque; 0 is fully transparent) and Tr is an alternative transparency value. Ni controls optical density. illum selects the illumination model used by the renderer (for example, different models for diffuse, specular, or combined shading).

Texture maps can be attached using map_Kd for the diffuse texture, map_Ks for a specular map, and map_Bump or bump for a normal or bump map. Additional maps and shading options may be supported by different tools, but the core set covers most common materials.

Example: newmtl brass Ka 0.33 0.25 0.20 Kd 0.78 0.65 0.50 Ks 0.80 0.70 0.60 Ns 20 d 1.0 illum 2 map_Kd brass_diffuse.jpg

In use, the OBJ file’s mtllib statement loads the MTL file and usemtl selects which material applies to each face.