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The various color blend mode, like Screen, Multipley, Lighten, etc Do not use the correct Alpha channel
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Authored by John Quillan (jquillan) on Feb 19 2019, 10:24 AM.

Details

Summary

I notices when using the VSE to apply some footage that was shot against a black background and meant to be blended in with Screen, had weird behavior in regards to its alpha channel, essentially clipping what was below it and showing transparency. This is a problem when you are taking that footage and transforming it, and using Screen blend mode on the transform layer.

I am not sure where I should put a screen shot that illustrates this bug?

Diff Detail

Repository
rB Blender

Event Timeline

Here is a screenshot with the bug

Here is a screenshot with the fix

@John Quillan (jquillan)

Please create Bug report with example blend file.

Also I am not familiar with this blend mode, can we agree on definition below?
https://en.wikipedia.org/wiki/Blend_modes#Screen

I created a bug report https://developer.blender.org/T61733 and attached a simple blend file with it.

https://en.wikipedia.org/wiki/Blend_modes#Screen is correct, but keep in mind this covers other blend modes that were added in commit 4b4702a (I thought I noted this, but apperantly not).

This revision is now accepted and ready to land.Feb 21 2019, 1:28 PM
This revision was automatically updated to reflect the committed changes.

I misunderstood "clipping alpha channel" so i expected problems in math...
This is OK.