OpenGL: Blend modes other than Mix don't work unless Depth Draw Mode is set to Never

[decacis]

Godot version

v4.1.dev1.official [db13026]

System information

Windows 10, Compatibility, NVIDIA GeForce GTX 1050 Ti (528.24)

Issue description

Setting the blend mode to something besides mix behaves the same as mix. This is the same whether is a ShaderMaterial or a StandarMaterial3D.

I think this should have been fixed by #75968 unless I misunderstood the PR, and it does seem to be included in this version (4.1 dev1). It does work if you set the Depth Draw Mode to Never though.

Steps to reproduce

• Add a MeshInstance3D with a mesh of any type
• Add a StandarMaterial3D
• Set the blend mode to add (or any other than mix)
• It won't work unless you set the depth draw mode to Never

Minimal reproduction project

BlendAdd.zip

[clayjohn]

Tagged as "good first issue" for clarity, this is a good first issue for someone with a little bit of OpenGL experience

[allenwp]

This is a good first issue :) I was able to determine the problem by comparing the OpenGL renderer with the forward mobile and forward clustered renderers. It seems that the following was omitted in OpenGL's set_code(const String &p_code) function for BLEND_MODE_ADD, BLEND_MODE_SUB, and BLEND_MODE_MUL:

uses_blend_alpha = true; //force alpha used because of blend

I'll have a PR to fix this issue soon, but will need to go over the contribution guidelines first.

[allenwp]

I have noticed that the OpenGL Subtract blend mode doesn't work no matter what other draw modes are selected. This appears to be because it was using GL_FUNC_REVERSE_SUBTRACT instead of GL_FUNC_SUBTRACT. I will include this fix in my PR as well to address all of the "Blend modes other than Mix don't work" cases.

The OpenGL renderer has always used GL_FUNC_REVERSE_SUBTRACT, but it looks like the other new renderers in Godot 4 use BLEND_OP_SUBTRACT instead of BLEND_OP_REVERSE_SUBTRACT. I'll open a different issue to discuss this inconsistency between renderers.