Fixed issues with blend modes in OpenGL 3 renderer

[allenwp]

Added uses_blend_alpha = true for ADD, SUBTRACT, and MULTIPLY blend modes to match the other renderers.

Fixes #76334

[clayjohn]

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)

[allenwp]

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:

godot/servers/rendering/renderer_rd/forward_mobile/scene_shader_forward_mobile.cpp

Lines 221 to 222 in b7032b5

	blend_attachment.alpha_blend_op = RD::BLEND_OP_SUBTRACT;
	blend_attachment.color_blend_op = RD::BLEND_OP_SUBTRACT;

godot/servers/rendering/renderer_rd/forward_clustered/scene_shader_forward_clustered.cpp

Lines 210 to 211 in b7032b5

	blend_attachment.alpha_blend_op = RD::BLEND_OP_SUBTRACT;
	blend_attachment.color_blend_op = RD::BLEND_OP_SUBTRACT;

godot/servers/rendering/renderer_rd/renderer_canvas_render_rd.cpp

Lines 2124 to 2125 in b7032b5

	attachment.alpha_blend_op = RD::BLEND_OP_SUBTRACT;
	attachment.color_blend_op = RD::BLEND_OP_SUBTRACT;

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

[clayjohn]

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

[clayjohn]

Thanks for taking a look at this. It is super helpful to have others working on the opengl backend

[allenwp]

Thanks for taking a look at this. It is super helpful to have others working on the opengl backend

I appreciate this! There's always some challenge with approaching a new open source project, so this feels good to hear. I also believe that one of the top 2 or maybe the top value of a game engine is portability across all devices, including old ones, so the OpenGL backend remains very important.

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉