intuitive nature of the format, these can easily be added with a custom MTL file generator. The MTL format defines a number of formats. Basic materials A single .mtl file may define multiple materials. Materials are defined one after another in the file, each starting with the newmtl command: # define a material named 'Colored' newmtl Colored The ambient color of the material is declared using Ka. Color definitions are in RGB where each channel's value is between 0 and 1. # white Ka 1.000 1.000 1.000 Similarly, the diffuse color is declared using Kd. # white Kd 1.000 1.000 1.000 The specular color is declared using Ks, and weighted using the specular exponent Ns. # black (off) Ks 0.000 0.000 0.000 # ranges between 0 and 1000 Ns 10.000 Materials can be transparent. This is referred to as being dissolved. Unlike real transparency, the result does not depend upon the thickness of the object. A value of 1.0 for "d" is the default and means fully opaque, as does a value of 0.0 for "Tr". Dissolve works on all illumination models. # some implementations use 'd' d 0.9 # others use 'Tr' (inverted: Tr = 1 - d) Tr 0.1 Transparent materials can additionally have a Transmission Filter Color, specified with "Tf". # Transmission Filter Color (using R G B) Tf 1.0 0.5 0.5 # Transmission Filter Color (using CIEXYZ) - y and z values are optional and assumed to be equal to x if omitted Tf xyz 1.0 0.5 0.5 # Transmission Filter Color from spectral curve file (not commonly used) Tf spectral <filename>.rfl <optional factor> A material can also have an optical density for its surface. This is also known as index of refraction. # optical density Ni 1.45000 Values can range from 0.001 to 10. A value of 1.0 means that light does not bend as it passes through an object. Increasing the optical density increases the amount of bending. Glass has an index of refraction of about 1.5. Values of less than 1.0 produce bizarre results and are not recommended. Multiple illumination models are available, per material. Note that it is not required to set a transparent illumination model in order to achieve transparency with "d" or "Tr", and in modern usage illum models are often not specified, even with transparent materials. The illum models are enumerated as follows: 0. Color on and Ambient off