I'm not sure about this. In Godot BLEND_MODE_SUB means that the color from the pixel shader is subtracted from the color behind it. That corresponds to GL_FUNC_REVERSE_SUBTRACT. GL_FUNC_SUBTRACT subtracts the background color from the pixel shader color and returns that value.

I.e. if you want to write a white pixel to a grey background with GL_FUNC_REVERSE_SUBTRACT you get (0.5, 0.5, 0.5) - (1, 1, 1,) = (0, 0, 0) while with GL_FUNC_SUBTRACT you get (1, 1, 1) - (0.5, 0.5, 0.5) = (0.5, 0.5, 0.5)

Ah, I see that the OpenGL renderer has always used GL_FUNC_REVERSE_SUBTRACT for BLEND_MODE_SUB. Thanks for pointing this out! I will remove this from the PR.

The reason that I initially made this change was to match the behaviour of the other two renderers in Godot 4 which use BLEND_OP_SUBTRACT instead of BLEND_OP_REVERSE_SUBTRACT:

I will open a different issue sometime later regarding the inconsistency of BLEND_MODE_SUB when using different renderers in Godot 4.

Ah, good catch. So it seems the RD backends are reversed. That actually explains some things