Screen-reading (postprocessing) shaders makes Volumetric Fog visible through walls

[Donitzo]

Godot version

4.0.1

System information

Windows 11

Issue description

When using a screen-reading shader in conjunction with environmental volumetric fog, there is a very clear difference in the way scene looks without and with the shader. The fog light emission is visible through walls.

I would assume some sort of information is lost when the scene is re-rendered through a screen-reading shader. I have tried writing the original depth in the shader as well but with the exact same result.

I understand that it is possible this isn't a bug but may actually just be a technical limitation of some kind.

Steps to reproduce

Use volumetric fog together with a passthrough screen-reading shader. Add a light behind a wall.

Minimal reproduction project

FogTest.zip

[Calinou]

Using SCREEN_TEXTURE makes the material transparent, which makes it not draw to the depth buffer. As a result, transparency sorting issues are expected to occur.

See godotengine/godot-proposals#2196.