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Gain is the weight applied to the diffuse parameters. You may also drive this with another pattern to show things like fading or wetness (where liquid darkens a surface).
Below are examples at 0.0, 0.5, and 1.0 gain for a 50% gray material.
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Color
Color is typically where textures or patterns are connected to create color for opaque objects.
This is where a wood color texture would go, for example.
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Diffuse roughness is how you would simulate a powdery surface like dried clay or dust.
When roughness is 0.0, PxrSurface uses the Lambertian model to calculate the diffuse response.
When roughness is > 0.0, the Oren Nayar model is used instead.
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The diffuse exponent controls the diffuse falloff. It is a power exponent. A higher number gives us a sharper falloff.
For example, on the moon's surface, you can set a high exponent to produce a sharper falloff.
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If on, illuminate both sides of the surface for this diffuse lobe, that is, this will illuminate the surface whose normal is pointing away from the camera (2-dimensional objects) as well.
This is so the backface will be shaded instead of black. This is off by default.
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This only applies when Double-Sided is on. This sets the transmitted color which could be different than the diffuse or back color. This is ignored if Transmit Gain is zero. This effect is useful for thin objects like leaves or paper. Below there's a light placed in the interior of the object and some interior text can be seen as light transmits through the surface.
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RenderMan Fundamentals
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