MIX NODE: Doesn't handle associated alpha correctly #47117

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Troy Sobotka · 2016-01-04T01:29:38+01:00

Troy Sobotka commented

2016-01-04 01:29:38 +01:00

Create a mix node and attach an associated alpha image to the lower node input.
Add another image to the upper node such as a generated colour grid.

Mix doesn't correctly mix the results.

Test file attached.

- Create a mix node and attach an associated alpha image to the lower node input. - Add another image to the upper node such as a generated colour grid. # Mix doesn't correctly mix the results. Test file attached. ![test.exr](https://archive.blender.org/developer/F271469/test.exr)

Troy Sobotka commented

2016-01-04 01:29:38 +01:00

Changed status to: 'Open'

Troy Sobotka commented

2016-01-04 01:29:38 +01:00

Added subscriber: @troy_s

Troy Sobotka commented

2016-01-04 04:06:57 +01:00

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?

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?

Troy Sobotka commented

2016-01-04 08:14:26 +01:00

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.

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 commented

2016-01-04 10:47:20 +01:00

Added subscriber: @Sergey

Sergey Sharybin commented

2016-01-04 10:47:20 +01:00

Changed status from 'Open' to: 'Archived'

Sergey Sharybin closed this issue

2016-01-04 10:47:20 +01:00

Sergey Sharybin self-assigned this 2016-01-04 10:47:20 +01:00

Sergey Sharybin commented

2016-01-04 10:47:20 +01:00

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.

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.

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MIX NODE: Doesn't handle associated alpha correctly #47117

Mix doesn't correctly mix the results.