Implement blend_disabled render mode for Spatial shaders

[Jummit]

Describe the project you are working on

A 3D painting application.

Describe the problem or limitation you are having in your project

While porting a 2D shader to 3D, I found that spatial shaders don't have a way of removing opacity other than blend_sub, which doesn't work in my case.

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

The spatial blend mode blend_disabled would work like in 2D, replacing both the albedo and alpha.

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

shader_type spatial;
render_mode blend_disabled;

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

It can be worked around by using two shaders, one for adding and one for subtracting, but that gets messy very fast.

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

The shader language is core.

[OhiraKyou]

Just ran into this problem while trying to render floats to RGBA (8 bits of the 32-bit float per channel) using a SubViewport. I need to write to the alpha channel without blending.

[Calinou]

There are several PRs aiming to address this:

• Add disable_alpha render mode to shaders godot#51711
• [3.x] Add BLEND_MODE_DISABLED for spatial shaders. godot#56279
• [3.x] Add disable_alpha render mode to shaders godot#51844

However, none are currently in a mergeable state.

[Yelnats321]

Is there a way to help contribute to getting the PRs merged? I would love this feature to make it into 4.2 or 4.3.

[Calinou]

Is there a way to help contribute to getting the PRs merged? I would love this feature to make it into 4.2 or 4.3.

4.2 is in feature freeze, so the earliest version this feature can be integrated in is 4.3 now.

Rendering code has changed significantly since I made godotengine/godot#51711, so that PR would need to be redone from scratch at this point. That said, there are concerns that features like #7916 may make this feature obsolete (as disabling alpha rendering is mostly a hack to use spatial shaders as buffers).