Multipass Rendering : Quick Tutorial
by Tahl Niran.

Introduction

Most people know that you can get the best results from outputting multiple passes from your 3d app and then tweaking these in 2d. What I have put together is a simple breakdown of a setup which will allow you a lot of control in compositing without the need to re-render. I have used a bunch of customised renders out of my 3d app (in this case using mental ray out of alias� Maya). This is only a very basic look at building a look from 3d elements. Normally I would expect to use multiple versions of each pass with different treatments all layered to get the right �look�. But this tut will produce a result.

This is not a mathematically precise tut, more of a general description of shading maths. I plan to do a precise shader re-creation of a phong and a lambert shader in shake in a later tutorial so bare with me until then.



The Plates:

There are a lot passes here just to allow for more flexibility.

a pass showing diffuse shading and shadow as a grey scale. You could place the shadow on a separate pass.		a pass showing the scene reflected in the floor element
a pass showing the diffuse colour of the ball.		a pass showing the shadows cast by the character and the ball on the floor. The man�s shadow is in the red channel and the ball�s shadow is in the green.
a pass showing the specularity of the ball element		a pass showing our less than inspiring background shader. I have just done a beauty pass for this element, don�t hate me cos� I�m lazy !
a pass showing the scene reflected in the ball element		a depth map pass which was generated by using distance from camera and a ramp rather than any in-built depth utility, just so that it will anti- alias properly. However because I am stupid too it will need to be inverted in shake.
a pass showing the shadow of our character cast onto the ball. The shadow is mapped to the green channel		an ambient occlusion pass generated using the �Dirtmap� shader.
a pass showing the diffuse colour of the floor element