Passes

[bonthan]

Hi, I just looked around at some artists' composition show reels and some of the reels have breakdown in which they showed the stage called "passes" like diffuse passes and something else I don't remember. So my questions are what are these passes and are those passes done in the composition softwares or 3-D packages?

[unxetas]

they're part of the output coming out of 3D renderers.. They are just different ways that lighting influences what something will look like, and are part of the shading process in 3D.

Usual passes would be ambient (really what it says, ambient light..), diffuse (light from direct sources), specular (highlights mostly), occlusion (where light is occluded! hehe..), etc..

It will depend on what you need to achieve and how much control you need at the compositing stage

[Seriphu]

There are countless amounts of passes you can create to aid you in the compositing stage, passes as stated earlier are just a way to give the compositor more control over the final output of the composite. An example of this is lets just say the shadow pass. You would render out nothing but the shadow cast on the ground plane of your render, this allows the compositor to tweak the brightness as needed to seamlessly blend the shadow of the CG element with the shadows of the live action plate. Each pass has its own purpose and are not always necessary which is why it helps to evaluate your scene first to see what parts of your composite you will need to have full control of.

Hope that helps a little, here are a few passes to do research on:

Diffuse, Shadow, Reflection, Specular, Beauty(full render with everything rendered used as reference), Ambient Occlusion.
Those are the basics