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===== Combining 3D-Passes =====


You can start from this formula:
  Color*Diffuse*Shadows*Ambient Occlusion+Reflection+Specular+Global Illumination+SubSurface Scattering
Where\\
  * Color contains the pure colors of textures and materials, not affected by light and shadows.
  * Diffuse shows how surfaces are illuminated by light sources and ambient light. Looks like illuminated white surfaces without textures.
  * Shadows - shadows, that are cast by surfaces to other surfaces.
  * Ambient Occlusion is diffuse shadowing by nearest surfaces.
  * Reflection - pure reflections (objects in reflections are not split into passes).
  * Specular - light sources reflected in surfaces accordind to the surfaces' specular characteristics.
  * Global Illumination - secondary illumination by light reflected from surfaces, includes coustics.
  * SubSurface Scattering - light that was scattered inside objects (below surfaces).


Use Merge, Multiply mode for multiplication and Merge, Normal mode with Alpha Gain=0 for add operation.\\
It's better to render all the passes without alpha-premultiplication and apply alpha after the passes assembling with the MatteControl tool. Otherwise you'll need to divide all the passes by alpha to avoid black edges.\\
The formula can be changed depending on particular needs and available passes.\\
Multiplication and addition in some cases may be replaced by inverted subtraction and screen respectively.\\
Sometimes, if Beauty pass isn't too ugly, it's easier to use additional passes just as masks for color correction. 

Quelle: https://www.steakunderwater.com/VFXPedia/96.0.243.189/index6107.html?title=FAQ/Combining_3D_Passes\\
Gregory Chalenko




===== 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 explanations is “What about the Shadow pass? What if I need to change the shadow?” {{ postpro:vfx:howtos:multipasslarge.png?direct&100|}}

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 accurate 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.

===== =====

Quelle: http://www.spherevfx.com/multipass-rendering-and-compositing/