Add note to shadow_mesh docs to warn about improper usage

[clayjohn]

Fixes: #72071

Using a different mesh for shadow_mesh breaks rendering right now because the shadow mesh is used for the depth prepass.

If we don't use it for the depth prepass, we will hurt performance. Further, the rendering will still appear broken as we will get all kinds of self-shadowing artifacts. Therefore, we just shouldn't support using a different shaped mesh for shadows. Users should use the old workflow which is to have a child MeshInstance3D with SHADOW_CASTING_SETTING_SHADOWS_ONLY and have the source mesh use SHADOW_CASTING_SETTING_OFF. Doing so has no additional cost over using shadow_mesh, but gives the user full control over how the shadow is drawn.

[Calinou]

I guess this also invalidates godotengine/godot-proposals#2600 as a proposal? It seemed to work from my quick testing, but I didn't test on more complex meshes.

That is, unless the proposal would allow performing this automatically:

Users should use the old workflow which is to have a child MeshInstance3D with SHADOW_CASTING_SETTING_SHADOWS_ONLY and have the source mesh use SHADOW_CASTING_SETTING_OFF. Doing so has no additional cost over using shadow_mesh, but gives the user full control over how the shadow is drawn.

Performance gains can be significant with 4-split PSSM (or in scenes with lots of point lights with shadows enabled) if you have lots of complex meshes.

[clayjohn]

I guess this also invalidates godotengine/godot-proposals#2600 as a proposal? It seemed to work from my quick testing, but I didn't test on more complex meshes.

That is, unless the proposal would allow performing this automatically:

Users should use the old workflow which is to have a child MeshInstance3D with SHADOW_CASTING_SETTING_SHADOWS_ONLY and have the source mesh use SHADOW_CASTING_SETTING_OFF. Doing so has no additional cost over using shadow_mesh, but gives the user full control over how the shadow is drawn.

Performance gains can be significant with 4-split PSSM (or in scenes with lots of point lights with shadows enabled) if you have lots of complex meshes.

Using nearby LODs won't be as bad as a completely different mesh for shadows (like in the OP issue). I think godotengine/godot-proposals#2600 would be better off with an LOD bias for shadows specifically. Remember, when used in the depth prepass, using a different mesh (even a different LOD of the same mesh) will totally break rendering. However, using a lower LOD for shadows only maybe be feasible.

[akien-mga]

Thanks!