If the keyword is followed by an identifier id, then it reestablishes a previous context. If the specified context was never defined, an error will result.
If the entity is given with an identifier followed by an equals sign ('='), then a new context is established, and cleared to the default material. (Note that the equals sign must be separated from other arguments by white space to be properly recognized.) If the equals sign is followed by a second identifier template, then this previously defined material will be used as a source of default values instead. This may be used to establish a material alias, or to modify an existing material and give it a new name.
The sum of the diffuse and specular reflectances and transmittances must not be greater than one (with no negative values, obviously). These values are assumed to be measured at normal incidence. If an index of refraction is given, this may modify the balance between diffuse and specular reflectance at other incident angles. If the material is one-sided (see sides entity), then it may be a dielectric interface. In this case, the specular transmittance given is that which would be measured at normal incidence for a pane of the material 5 mm thick. This is important for figuring the actual transmittance for non-planar geometries assuming a uniformly absorbing medium. (Diffuse transmittance will not be affected by thickness.) If the index of refraction has an imaginary part, then the surface is a metal and this implies other properties as well. The default index of refraction is that of a vacuum, i.e. (1,0).
# Define a blue enamel paint
m blue_enamel =
c
cxy 0.2771 0.2975
rd 0.5011
c
rs 0.0100 0.0350
# Assign blue_enamel to be the color of the south wall
m swall_mat = blue_enamel
# ...
# South wall face
m swall_mat
f sv1 sv2 sv3 sv4