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MIX NODE: Doesn't handle associated alpha correctly
Closed, ArchivedPublic

Description

  1. Create a mix node and attach an associated alpha image to the lower node input.
  2. Add another image to the upper node such as a generated colour grid.
  3. Mix doesn't correctly mix the results.

Test file attached.

Details

Type
Bug

Event Timeline

Troy Sobotka (sobotka) set Type to Bug.
Troy Sobotka (sobotka) created this task.
Troy Sobotka (sobotka) raised the priority of this task from to Needs Triage by Developer.

While I was working on a patch for this, it strikes me that there is a zero percent chance that the "Use second image's alpha" works correctly either.

By default, I'm guessing that if an image has an alpha that the Mix node should use it?

Ok so my other bug report appeared to be correct to the best of my knowledge; the mix node in default "Mix" mode ignores the colour in the node socket if it is unattached.

In addition to this, whilst working on a patch for this node, I noticed that the "Use Alpha of Background" toggle next to the mix mode seems awfully suspect.

If we mix image A with alpha over image B with alpha, shouldn't the default behaviour be to honour both alphas? What would the logical outcome be if that were not the default? To mix and ignore one of the image's alpha? Default to the background image's alpha? Foreground's?

I'm somewhat lost here and seeking guidance as I cannot for the life of me, as an imager, see what a reasonable outcome would be other than honouring the alpha of both images by default.

Thanks for anyone's input.

Sergey Sharybin (sergey) closed this task as Archived.
Sergey Sharybin (sergey) claimed this task.

Mix node with the mix blend type does not equal to alpha over. You're just doing blending with the following formula: https://developer.blender.org/diffusion/B/browse/master/source/blender/compositor/operations/COM_MixOperation.cpp;4c37d910deb5a6e202cb9e166755780e5d0c89b1$155

If you want to do alpha over use node with that name.

Thanks for the report, but don't see issues here, it is all working as it was designed to.