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After Effects doesn’t handle premultiplied images correctly

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Shao Zhang
After Effects doesn’t handle premultiplied images correctly
on Jun 16, 2020 at 1:21:44 am

I’ve been told by various people in the industry that AFX doesn’t handle premultiplied images correctly and I’ve been able to verify this myself in my own pipeline. I’m just not sure if we are all wrong or missing something or this is just the way it has always been?

I don’t use AFX at all in my pipeline for final production but I do have it to check deliveries to vendors who do use AFX. Our pipeline is all EXR/Nuke and everything works fine with my pipeline premultiplied images.

When I convert these known good images to TIFF 16bit fixed point for AFX they only look correct with straight alpha. I’m converting to straight alpha buy running an ‘unpremultiply’ operation in nuke on the source images. Both the TIFF converted premultiplied and straight files still work fine in Nuke.

However the premultiplied ones don’t look correct in AFX. There is a light colored halo around soft edges. It’s actually very hard to see this edge artifact in AFX depending on the background that is being composited onto. I usually composite onto pure red to see this bright halo. And yes I’m interpreting the premultiplied images correctly in AFX as ‘premultiplied on black’. Also setting blend colors using 1.0 gamma makes the problem worse.

I just want to check if this is just a known issue that people just use wacky work arounds for or they simply don’t even notice and thus never bring it up.

Below is a screenshot of AFX showing the artifact.



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