CanvasItem shader light functions receiving vec4(0.0) as LIGHT input

[conradhappeliv]

Godot version

4.0.beta9 (e780dc332) using any renderer

System information

Windows 11

Issue description

It seems like CanvasItem shaders are always receiving vec4(0.0) as their LIGHT input. Based on the docs, I would expect to get the value of the light that would be otherwise applied to the texture.

Steps to reproduce

• Create a Sprite2D that is affected by a Light2D (point or directional)
• Attach a ShaderMaterial with a canvas_item shader to the Sprite2D.
• Try writing a light() function:
  • If you don't set LIGHT, no lights are applied to the sprite at all (according to the docs, not setting LIGHT should cause the function to be a no-op)
  • If you set LIGHT = LIGHT;, no lights are applied (I would expect this to be a no-op)

Some extra info:

• the light's blend mode has no effect (same result with add/mix/sub)
• Setting a LIGHT value (e.g. LIGHT = COLOR;) seems to work fine
• I didn't expect this have any effect, but adding a BackBufferCopy node before the sprite didn't change anything

Minimal reproduction project

lightshaders.zip

[clayjohn]

The docs are out of date. They say that LIGHT contains the color of the light from the light's texture, but that is now part of LIGHT_COLOR which contains the texture color of the light multiplied by the color of the light.

LIGHT should be marked as an out variable, not an inout variable. It is only used for writing out the final contribution of the light.

Another point where the docs are out of date is that the light processor function is now used if you define it rather than if you define it and write to the LIGHT builtin. This is important as there are other builtins that you can write to.