Add disable_alpha render mode to shaders

[Calinou]

master version of #51844. Follow-up to #51709.

This can be used by multipass shaders to write to a texture's alpha channel without causing the material to become transparent.

Testing project: test_shader_disable_alpha_master.zip

Preview

Default render mode

disable_alpha render mode

[clayjohn]

I'm not sure I understand the use case for this. What effect does this setting allow you to achieve that you could not otherwise?

[Calinou]

I'm not sure I understand the use case for this. What effect does this setting allow you to achieve that you could not otherwise?

The use case is described in godotengine/godot-proposals#2935.

[Lauson1ex]

I'm not sure I understand the use case for this. What effect does this setting allow you to achieve that you could not otherwise?

@clayjohn It's to store and transfer data between render targets. I'll give you one real world use case applied on the Godot engine itself: on Godot 4.0's bokeh DOF shader, the size of the circle of confusion is stored on the alpha component, and accessed by later passes of the shader. On Material Maker, they have to store depth in a separate buffer first, and then color on a second pass, essentially rendering the screen twice always. With this, they could just store depth on the alpha component, everything in a single pass. Having 4 components is so useful.

[Lauson1ex]

@Calinou Although disable_alpha is indeed preventing the geometry from going through the transparent pipeline, it seems that ALPHA is not being written to the render target. Trying to retrieve alpha from the render target yields 1.0 always. Even when transparent_bg is enabled. I've attached a sample project.


disable_alpha_test.zip

[Calinou]

Trying to retrieve alpha from the render target yields 1.0 always. Even when transparent_bg is enabled. I've attached a sample project.

transparent_bg is not reimplemented yet in Vulkan. I'd try the 3.x version of this PR.

[Lauson1ex]

Trying to retrieve alpha from the render target yields 1.0 always. Even when transparent_bg is enabled. I've attached a sample project.

transparent_bg is not reimplemented yet in Vulkan. I'd try the 3.x version of this PR.

Okay. Thank you. I'll be right into it 🙂

[Lauson1ex]

I'd try the 3.x version of this PR.

@Calinou I'm afraid I didn't find a 3.x version of this PR. #51708 doesn't feature disable_alpha, so I couldn't test it there.

[Calinou]

@Calinou I'm afraid I didn't find a 3.x version of this PR. #51708 doesn't feature disable_alpha, so I couldn't test it there.

I forgot to open a pull request for it. Here it is: #51844