mental ray Architectural and Design Visualization Shader Library

2.5 Detailed Description of Material Parameters 33• When refr falloff dist is used, and refr falloff color is not used, transparency rayswill fade to black. This is like smoked glass or highly absorbent materials. Transparencywill just completely stop at a certain distance. This has the same performance advantageas using the refl falloff dist for reflections, i.e. tracing shorter rays are much faster.• However, when refr falloff color is used, it works differently. The material will thenmake physically correct absorption. Exactly at the distance given by refr falloff distwill the refractions have the color given by refr falloff color - but the rays are notlimited in reach. At twice the distance, the influence of refr falloff color is double, athalf the distance half, etc.No limit (left), fade to black (center), fade to blue (right)The leftmost cup has no fading. The center cup has refr falloff color off, and hence fadesto black, which also includes the same performance benefits of limiting the trace distance aswhen used for reflections.The rightmost cup, however, fades to a blue color. This causes proper exponential attenuationin the material, such that the thicker the material, the deeper the color. See page 49 for adiscussion about realistic colored glass.Note: To render proper shadows when using refr falloff dist one must use ray tracedshadows, and the shadow mode must be set to segment. See the mental ray manual onshadow modes.Each material can locally set a maximum trace depth using the refl depth parameter. Whenthis trace depth is reached, the material switches to tracing transparent rays. This is a newfrom mental ray 3.8, where the shader is by-design ignoring trace depth limits in refractions.This is described in detail on page 42.One can switch to the old behavior of running out of refractive trace depth (returning black)by setting this parameter to -1. If refl depth is zero, the global trace depth is used.