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3D Compositing Guide 1

"3D Compositing Guide - part1 " by Simon Reeves

Before I start I should say that this is based on what I know and learnt from people at uni, and at work, and the internet. Other people you may learn from prefer more or less passes etc. for more control or to save on excess files - so just keep in mind there are usually different ways to do things all with their own advantages and disadvantages (and don't blame me).
In this tutorial I'll be using examples from a couple of shots from our short film ‘Diversion'. Directed by myself and Aidan Gibbons
Types of Passes
If you want very fine control in post over every element of the scene, you could render the the objects in tons of passes with a different one for every element in the render, ambient texture, different light passes, shadows, reflection, refraction, GI the list goes on.but generally thats not really necessary.
So if you like you may want to include lighting/shadows/specular/GI all in this one pass and maybe just add a couple of additional passes to this ‘Beauty' pass.
For compositing with live action in ‘Diversion', we generally used a standard set of passes on all shots, and then on some shots we needed some other specific passes to help out with mattes, masks and the like - which I'll try and mention a couple of later.
So here is an example from excitingly named ‘Shot14'. First of all, here is the background plate from the footage:

Don't think there's much to explain on that one.
? Beauty (RGBA could be named ‘Diffuse' or ‘RGB' perhaps)

This is the base for the cg elements in the shot, and it can vary a lot what is included in this pass. Our standard beauty pass included shadows cast onto the objects but not shadows cast onto the plate (separate pass for them) but as there isn't any shadows in this shot, I won't go into why now. When we light the diffuse pass, I find it helps to make it a bit brighter than you want in the final shot, especially to take into account the ‘Ambiant Occlusion' (or ‘Dirtmap') pass which adds the darker areas in this shot.
? Specular Pass (RGB, Alpha channel not needed)

This pass is for specular only, and is layered over the top of the beauty pass with 'screen' (or you could ‘add' (additive) it). Screen or Add basically remove all the black in the pass, leaving just the highlights (the specular in fact.) This is the same for a Reflection pass, and if you want to render a light in a separate pass - as again you just want to hide all the black pixels values and keep the white values.
? Reflection Pass (RGB, Alpha channel not needed)


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