Render matte pass in cycles

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.

If I understood your problem…try this

1.Disable all lights from rendering in the outliner.
2.Set World strength to 0
3.Turn on film transparent
4.Set a file output for the alpha channel
5.Re-render as jpg or png (preferably RGB not RGBA) sequence

Now you have an image sequence with the background coloured black and the rest coloured white that you can use as a mask.

Hope that helps!

Thank you, SunBurn. Turning off the lights cut the render time in half. Setting world strength to 0 was also a small benefit. I don’t know how to set the file output for just the alpha channel. I currently have it to png.

What I did after the initial post was go to the Render Layers tab and use the material override so change everything in every layer to a basic white diffuse shader. I don’t see ‘shadeless’ available in cycles. That reduced render time from 3:00 minutes to :32 seconds. Your suggestions so far have reduced renders per frame to :16.9.

To cut even more the times set max and min bounces in the light paths to 0

Reduce your samples to something that doesn’t leave you with aliasing problems.

For the output first do this

then in the nodes editor do this:

I had the output path correct. I thought you might have been referring to something else. I did not think about the light bounces being reduced to 0. And, thank you for the warning about the samples.

Thank you for the help. I very much appreciate the explanations.