Document global and per-instance uniforms in Shading language

[Calinou]

See #5121. This takes some parts from https://godotengine.org/article/godot-40-gets-global-and-instance-shader-uniforms (thanks reduz 🙂).

Some questions I have to further complete this documentation:

• Does using GeometryInstance3D.get_instance_shader_parameter() have a performance cost like getting global uniform values through a script? I don't get any warning when using it, but I also can't seem to get any value from it anyway.
• What's the difference between RenderingServer's global_shader_parameter_set() and global_shader_parameter_set_override()? I'm not sure what use cases each method has in practice. The former fails with an error if the uniform doesn't exist, while the latter fails silently. Only the latter fails early if the uniform's type extends Object.
  • Each uniform variable stores the value and override separately: https://github.com/godotengine/godot/blob/e3a51e53ef055a1fe94351b68173c6dfcd2cb371/servers/rendering/renderer_rd/storage_rd/material_storage.h#L123-L124
• Is a default value assigned to a global uniform ever used? Shader compilation fails if the global uniform isn't defined in the project settings.

[mhilbrunner]

cc @clayjohn :) Looks good to me and spotted nothing on a first glance, will give it a more thorough read later.

[clayjohn]

Great work! I left a few comments and suggestions

[Calinou]

I'm copying the team color bit below as it was removed from the docs (but it wasn't part of the original blog post). Still, it may be useful for others in the future.

5. Providing team colors. In team-based games, you can use global uniforms
   to provide "friendly" and "enemy" colors that can automatically be used
   everywhere relevant (player materials, map textures, special effects, GUI
   elements, ...). This can be used to let players choose team colors freely, for
   better accessibility. The "friendly" and "enemy" states themselves can be
   determined by using a :ref:per-instance uniform <doc_shading_language_per_instance_uniforms> that indicates the instance's
   current team.

@clayjohn Do you know the answer to the questions in OP? This would help flesh out the documentation 🙂

[clayjohn]

Does using GeometryInstance3D.get_instance_shader_parameter() have a performance cost like getting global uniform values through a script? I don't get any warning when using it, but I also can't seem to get any value from it anyway.

Not really, it is just reading from a hashmap

What's the difference between RenderingServer's global_shader_parameter_set() and global_shader_parameter_set_override()? I'm not sure what use cases each method has in practice. The former fails with an error if the uniform doesn't exist, while the latter fails silently. Only the latter fails early if the uniform's type extends Object.

• Each uniform variable stores the value and override separately: https://github.com/godotengine/godot/blob/e3a51e53ef055a1fe94351b68173c6dfcd2cb371/servers/rendering/renderer_rd/storage_rd/material_storage.h#L123-L124

Its for use by the ShaderGlobalOverrides node

Is a default value assigned to a global uniform ever used? Shader compilation fails if the global uniform isn't defined in the project settings.

I don't think so. Imagine if you had multiple shaders that each used a different default value, which one should be picked?