Multipass Rendering and Compositing

One of the questions we get ask often is the correct way of recreating the Beauty from multiple renders such as Diffuse, Specular, Reflection, etc.

There are of course many tutorials out there which explain different ways of doing it for different renders. Unfortunately main of these differ in what they say is the correct way of working and often what they produce is not a 100% replica of the original Beauty pass.

To create an mathematically accurate rebuilding of the Beauty you must use only Plus operations, if your using Screen (which Clamps) or Multiply (which can create dark edges) you doing things wrong.

What follows is what we believe to be a 100% rebuild of the Beauty from each of the main render engines. We have tested these and believe they are 100% pixel/colour accurate.

Note, we have used the correct render pass name for each render engine.

RenderMan

SpecularDirect – SpecularDirectShadow + SpecularIndirect + SpecularEnvironment + Ambient + DiffuseDirect + Translucence – DiffuseDirectShadow + DiffuseIndirect + DiffuseEnvironment + Backscattering + Subsurface + Rim + Refraction + Incandescence = Beauty

Arnold

direct_diffuse + indirect_diffuse + direct_specular + indirect_specular + refraction + deep_scatter + mid_scatter + shallow_scatter + primary_specular + secondary_specular = Beauty

V-Ray

GI + Lighting + Specular + Reflection + Refraction + SelfIllum + SSS = Beauty

Mental Ray

Diffuse + Indirect + Specular + Reflection + Refraction + Incandescent + Scatter = Beauty


What about the Shadow and Ambient Occlusion Passes?

The question that usually follows these explainations is “What about the Shadow pass? What if I need to change the shadow?”

We would argue you should only be recolouring the shadow, nothing else, if the 3D was done correctly.

If you need to make colour adjustments the correct way to do it would be to From the Shadow pass, make any colour adjustments and then Plus is back in. To the right is an example which keeps the composite 100% mathematically sccurate and doesn’t introduce any dark edges around objects.

With regards to Ambient Occasion, our understanding is that it was a cheat only used because true Global Illumination was to expensive to render 5 years ago. Now days the hardware and renders have got much faster and doing full GI is no longer a issue so long as you optimise your render settings correctly.


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