Alec sent me a few articles when looking at my compositing- including the one on AOVs below.
Basically, it talked about how different tutorials show different ways of recreating the Beauty from the different render passes (diffuse, specular etc). Many tutorials explain different ways of doing this for different renders, but these have different ways in creating the 100% replica of the 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.
Below is a list compiled of the 100% rebuild of the beauty from each of the main render engines- believed to be 100% pixel/ colour accurate.
SpecularDirect – SpecularDirectShadow + SpecularIndirect + SpecularEnvironment + Ambient + DiffuseDirect + Translucence – DiffuseDirectShadow + DiffuseIndirect + DiffuseEnvironment + Backscattering + Subsurface + Rim + Refraction + Incandescence = Beauty
direct_diffuse + indirect_diffuse + direct_specular + indirect_specular + refraction + deep_scatter + mid_scatter + shallow_scatter + primary_specular + secondary_specular = Beauty
GI + Lighting + Specular + Reflection + Refraction + SelfIllum + SSS = Beauty
Diffuse + Indirect + Specular + Reflection + Refraction + Incandescent + Scatter = Beauty
What about the Shadow and Ambient Occlusion Passes?
Additional conciderations were addressed when mentioning the shadow pass- what if the shadow was needed to be changed. As I have mentioned previously, you should only be recolouring the shadow, nothing else, if the 3D was done correctly. If colour adjustments need to be done, simply make the adjustment and plus it back in.
When regarding the Ambient Occlusion pass, this is used to cheat using Global Illumination, a expensive to render pass. Nowadays, 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.