An already rendered animation needs to have the background color-corrected. It will take too long to re-render the shot in full color. How do I turn the alpha channel of the combined layers (which are now set to render over transparent) into a black & white matte or solid color over transparent, and render quickly as just a matte pass with no lighting calculations?
Once I have the matte pass I can isolate the background and perform corrections.
I searched but could not find this exact issue in the forums.