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Sets the roughness of the refraction. The smaller the value, the smoother is the surface. Higher values can be used to emulate "ground glass" and also help emulate more complex material interiors.
Transmittance
This parameter is a more user friendly way to specify the absorption of the material. It specifies the colour that passes through the volume. Note how coloration varies with volume thickness, contrary to the refraction color parameter which is constant through the slab. This is due to an exponential falloff in such volumes.
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