This function returns a color.
The color is the sum of the diffuse reflection for all light sources
multiplied by the co-efficient Kd and the surface color CS.
OUT = CS * Kd * diffuse(faceforward(normalize(NN),I)).
refer to page 316 in the RenderMan Companion.

Must be a float.  Defaults to 1.0.
This is the co-efficient for the diffuse function.
It controls the intensity of the returned color.
For best results Kd should range from [0,1].
refer to page 316 in the RenderMan Companion.

Must be a color.  This is the surface color.
It tints the color returned from the diffuse function.
refer to page 316 in the RenderMan Companion.

Must be a normal.  The default is the surface normal, N.
This is the normal of the surface point to be shaded, P.
If the surface point P has been changed, calculatenormal(P)
should be used to recalculate N before shading.
refer to page 316 in the RenderMan Companion.

This function returns a color.
The color is the sum of the diffuse reflection for all light sources
multiplied by the co-efficient Kd and the surface color CS.
OUT = CS * Kd * diffuse(faceforward(normalize(NN),I)).
refer to page 316 in the RenderMan Companion.