6. Render Passes
The terms AOV or Render Passes are used interchangeably depending on the version of Maya. In Maya 2017 they are called AOVs (Arbitrary Output Variables)
AOVs or Render Passes offer advanced control in compositing and are highly recommended for any attempt to achieve realistic integration
What we usually render – without Render Passes – is the so-called beauty or RGB
Render passes decompose the scene into material or lighting attributes, i.e. specular, reflection, refraction, direct or indirect lighting, etc.
With these attributes rendered in separate images, the first step in compositing is to assemble the passes to re-compose and match the original beauty
The next step would be to tweak the passes individually, i.e. changing the color or intensity of the reflections, without affecting the diffuse base color, while aiming for a better integration
Which passes to render?
Depends on lighting and materials, and on the renderer (Mental Ray, Vray, Arnold…)
If all materials are lambert without any specularity, reflection or refraction, there’s probably no point rendering passes