Expose more OpenGL blend modes as shader `render_mode`

[ghsoares]

Describe the project you are working on

Trying to experiment with some shader effects

Describe the problem or limitation you are having in your project

I'm trying to create a fire effect by taking a viewport that renders particles with radial gradient, procedurally calculate normal from this texture and use it as fake 3D effect in a canvas_item shader, this way it should look like multiple fire spheres overlapping each other. The problem is that I want to achieve a depth effect using max blend mode, but the current Godot's shader language API doesn't have this mode exposed

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Viewing the OpenGL Wiki page about blending (https://www.khronos.org/opengl/wiki/Blending), there's other blend modes like GL_MIN and GL_MAX, so my proposal is to expose these modes into the Godot's shader language.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

I ain't sure how it's implemented in the rasterizer and in the shader language, but it seens not too hard to expose/add those modes.

If this enhancement will not be used often, can it be worked around with a few lines of script?

Not possible, it requires modification in the engine rendering core.

Is there a reason why this should be core and not an add-on in the asset library?

Because it requires modification in the engine rendering core.

[Calinou]

Does OpenGL ES 3.0 (and ideally 2.0) expose these blending modes? If not, they'll have to be emulated in a shader, which is slower.

[ghsoares]

@Calinou OpenGL ES 3.0 does have trough the glBlendEquation https://www.khronos.org/registry/OpenGL-Refpages/es3.0/html/glBlendEquation.xhtml, but version 2.0 don't have min and max blend modes https://www.khronos.org/registry/OpenGL-Refpages/es2.0/xhtml/glBlendEquation.xml