Added accurate_alpha parameter to fix blending

[azagaya]

Fixes #31124

[azagaya]

Oh i forgot clang_format again

[akien-mga]

Parameters in C++ prototypes are prefixed with p_, so this should be p_accurate_alpha.

[akien-mga]

I'm not sure that the parameter's name properly conveys what it involves, but otherwise the code looks OK.

Please also add the relevant documentation to doc/classes/Image.xml.

[azagaya]

I'm not sure that the parameter's name properly conveys what it involves

p_accurate_blending would be better?

[akien-mga]

p_accurate_blending would be better?

Well, why would anyone want inaccurate blending in a function named "blend"? :)

[azagaya]

p_accurate_blending would be better?

Well, why would anyone want inaccurate blending in a function named "blend"? :)

According to the issue discussing this, this is what i understood. Usually in a game, you blend against a fully opaque background. In those cases, the result without the fix is the same as if it had the fix, because dc.a equals 1. So, in those cases, the fix would be an unnecessary loss of performance for each pixel blended.
However, you may want to export some blended images from your game, without fully opaque transparency (like the project uploaded to test the issue). In those cases, not dividing by dc.a will give you darkened areas in regions not fully opaque. So, in those cases, the blending is not being accurate.
I initially proposed something like fix_alpha, or maybe fix_alpha_darkening.
What do you think?

[bojidar-bg]

Not sure if it is applicable for this case, but maybe p_premultipled_alpha or p_straight_alpha might do?

[azagaya]

Not sure if it is applicable for this case, but maybe p_premultipled_alpha or p_straight_alpha might do?

Well, i think the output without the fix could be thought as the value in "premultiplied alpha". And as the fix divides by alpha, it could be thought that the fix is converting premultiplied to straight. So perhaps the parameter could be called "p_straight_alpha" as you suggest.

[azagaya]

Should i rename the parameter to any of those options?

[azagaya]

Should i use Color.blend() as suggested in the issue? It seems to work fine with that, and that function is already implemented.

[OverloadedOrama]

Is there any chance this will be merged before 3.2.2? It would be really helpful for Pixelorama.

[akien-mga]

Superseded by #39200.