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Multipass Compositing
Multipass Compositing
Deke Kincaid, updated 2005-12-18 02:35:14 UTC 65,342 views  Rating:
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Multipass Rendering : Quick Tutorial
by Tahl Niran.


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