Compatibility Renderer: LIGHT_VERTEX does not affect shadows

[hilloftheking]

Tested versions

• Reproducible in: v4.3.stable.official [77dcf97], v4.4.dev.mono.gh [bdf625b]

System information

Godot v4.3.stable - Fedora Linux 41 (Workstation Edition) - Wayland - GLES3 (Compatibility) - NVIDIA GeForce RTX 3060 Ti (nvidia; 565.77) - Intel(R) Core(TM) i7-10700F CPU @ 2.90GHz (16 Threads)

Issue description

Shadows seem to ignore LIGHT_VERTEX in the Compatibility renderer.

These screenshots and the MRP change LIGHT_VERTEX to create a 4x4 grid on a plane.

Compatibility

Forward+

Shader code:

shader_type spatial;

uniform vec3 albedo_color : source_color;

#define SURFACE_SIZE 2.0
#define TEXEL_SIZE 4.0

void fragment() {
	ALBEDO = albedo_color;

	vec2 snapped_uv = (floor(UV * TEXEL_SIZE) / TEXEL_SIZE) + vec2(0.5 / TEXEL_SIZE);
	vec2 uv_diff = snapped_uv - UV;

	LIGHT_VERTEX += TANGENT * uv_diff.x * SURFACE_SIZE;
	LIGHT_VERTEX += -BINORMAL * uv_diff.y * SURFACE_SIZE;
}

Steps to reproduce

1. Open MRP
2. View shadows in Compatibility
3. Switch to Forward+
4. View shadows in Forward+

Minimal reproduction project (MRP)

light_vertex_mrp.zip

[clayjohn]

For performance reasons the compatibility renderer does shadow calculations in the vertex shader. Therefore the LIGHT_VERTEX value isn't available when they are being calculated.

I guess to support LIGHT_VERTEX in the compatibility renderer, we need to have a slow fallback for when LIGHT_VERTEX is used