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Comp background showing through gradient in RGBA renders

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Mike Petersen
Comp background showing through gradient in RGBA renders
on Jul 18, 2020 at 2:28:51 pm

I'm hoping anyone out there has run across this issue with rgba renders and has a solution. Our external video editor has been having this problem with our rgba MOV renders where the red or blue gradients and glows were showing white through the colour, as if it was rendered with a white gradient in behind. We tested it in all our systems in both Ae and Premiere and we're not able to reproduce the issue with the MOVs. I'm rendering them out to Prores 4444 RGBA.

He tried it on all the machines in his studio and they were getting the same undesirable results. It shows up fine in with the preview quality set to "half" but when he changes it to "full" quality the white shows up. And of course it renders with white over top of video.

I happened to try rendering it as a TIFF sequence and opened it in Photoshop. It rendered it out flattened with the comp background colour (white) plus the alpha. When I masked it with the alpha it looked exactly like what he's getting. So I realized that somehow the editor's system is picking up the comp background white through the gradient in the MOV even though it's RGBA. (I sent him one on black and it didn't work properly)

I realize the quick solution is the change the comp background colour to the colour of the glow and gradient. But it wouldn't resolve it if I had to render more than one colour Has anyone come across this? Here are some reference images.

Original (expected result)

RGBA on White comp background

RGBA on Black comp background

RGBA on Red comp background

thanks in advance!

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Filip Vandueren
Re: Comp background showing through gradient in RGBA renders
on Jul 19, 2020 at 6:51:57 pm

Try straight alpha instead of premultiplied ?

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