mental ray Architectural and Design Visualization Shader Library

2.5 Detailed Description of Material Parameters 472.5.11.3 Proper CompositingDue to the redundancy available in the outputs, there are several ways to composite themtogether to yield the same result as the beauty render. Here we outline two compositingpipelines in equation form.First we have the “simple” variant, which is simply a sum of the various result parameters.This version allows only minor post production changes to the overall balance between thematerials.But it has the advantage of not needing as many files, as well as working reasonably well innon-floating-point compositing.Beauty = diffuse_result + indirect_result + spec_result +refl_result + refr_result + tran_result +add_resultThen we have the more “complex” variant which uses the various raw and level outputs, whichallows much greater control in post production.Note that the raw outputs needs to be stored and composited in floating point to maintainthe dynamic range. The level outputs always stay in a 0.0-1.0 range and does not requirefloating point storage.Beauty = diffuse_level * (diffuse_raw + (indirect_raw * ao_raw)) +spec_level * spec_raw +refl_level * refl_raw +refr_level * refr_raw +tran_level * tran_raw +add_resultPlease note that this equation contains multiplications, and postponing multiplication to astage in the rendering pipeline that lies after individual samples get filtered down to pixelshas fundamental mathematical limitations, which may be visible as artifacts on edges betweenobjects, or in any part of the image with an alpha of less than 1.0.None of these problems exist if the math is applied on a per-sample basis (by routing theseoutputs to other parts of a shade tree and performing math on them there).