Tuesday, February 24, 2009

To Premult or not to Premult

Now, I've known that 3D elements rendered out of Maya need to unpremultiplied before compositing, but I still wasn't sure when exactly I needed to do it.

I stumbled onto a document at work today that explains the rule:
  1. Unpremult before any color correction, and pre-mult after
  2. There's no need to unpremult before filtering options like blur because these operations should affect the alpha channel

Without un-premultiplying:


It's subtle, but the white 1-pixel halo around the green character's arm is gone. 

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