Alpha channel always renders "premultiplied"

Question

Hi,I'm trying to give footage with an alpha channel to someone using Avid MC, and I learned that that one only accepts direct, un-premultiplied alpha channels.Now, I rendered in AE with RGB+Alpha with a "direct" alpha channel - to DNxHD w/ alpha, - to DNxHR w/ alpha and - to 32-bit Targa sequence (i.e., also w/ alpha channel).My recipient still reported for all those files the dark halos around the objects, which indicate a premultiplied alpha.And when I then tested (= opened) my renders in Premiere Pro, I got the same dark halos.That only changed when I told Premiere to interpret the footage as having a "premultiplied" alpha instead of a "direct" one.So am I still missing a setting in the AE render settings? Or somewhere else? Or might this be a bug?Thanks in advance!

Rick Gerard · Accepted Answer

Make sure that the Output Module is set to Straight:

This applies to any format that supports an Alpha Channel.

This will give you an Alpha channel with no color added to the semi-transparent pixels. If you use the default Premultiplied then the Background Color of the Composition will be pre-multiplied with the colors of the edge pixels.

If changing interpretation the settings in Premiere Pro or After Effects to Premultiplied will appear to choke the render and color the edges based on the chosen color so it is not surprising that the halo in a pre-multiplied file will be reduced.

The industry standard is Straight - Unmatted, I do not know why Adobe always has had Premultiplied (Matted) as the default.

Run a quick test and see what you get. It is really easy to see if you have a premultiplied alpha interpreted as straight:

A straight movie interpreted as premultiplied is a little harder to see unless you have strong colors:

In both of these screenshots, the interpretation is wrong.

Roei Tzoref · Answer

when you exported from Ae did you make sure you rendered straight alpha?and when you import back did you see that it was recognized as straight?

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