Vulkan: DirectionalLight3D PSSM 2 Splits exhibits shadow acne not present in other modes, and shadows are unstable

[mrjustaguy]

Godot version

4.0 Alpha 4

System information

Windows 11, Vulkan, Nvidia GTX 1050 Ti, 511.23

Issue description

2 Split mode has severe Shadow Acne issues, unrelated to Shadow Resolution (both Orthogonal and 4 Split don't have Any issues in the same example) and the Shadows are unstable with 2 Splits (they jitter as the camera moves) again, unlike the other modes.

Steps to reproduce

Switch Between Orthogonal or 4 Split Mode and 2 Split mode and Observe the differences

Minimal reproduction project

Bug.zip

[Calinou]

I can confirm this on master fb28025, both with the Vulkan Clustered and Vulkan Mobile backends. This doesn't occur on 3.x 6073277, both with GLES3 and GLES2.

PSSM 2 Splits

PSSM 4 Splits

Orthogonal

Shadow stabilization is performed here:

godot/servers/rendering/renderer_scene_cull.cpp

Lines 2156 to 2162 in ae312b4

	// This trick here is what stabilizes the shadow (make potential jaggies to not move)
	// at the cost of some wasted resolution. Still, the quality increase is very well worth it.
	const real_t unit = (radius + soft_shadow_expand) * 2.0 / texture_size;
	x_max_cam = Math::snapped(x_vec.dot(center) + radius + soft_shadow_expand, unit);
	x_min_cam = Math::snapped(x_vec.dot(center) - radius - soft_shadow_expand, unit);
	y_max_cam = Math::snapped(y_vec.dot(center) + radius + soft_shadow_expand, unit);
	y_min_cam = Math::snapped(y_vec.dot(center) - radius - soft_shadow_expand, unit);

There doesn't appear to be a condition for the number of splits used. However, unlike Orthogonal or PSSM 4 Splits, using PSSM with 2 splits will result in a non-square shadow map texture for each split (with the first split in the top half, and the second split in the bottom half):

godot/servers/rendering/renderer_rd/renderer_scene_render_rd.cpp

Lines 5186 to 5193 in ae312b4

	} else if (storage->light_directional_get_shadow_mode(light_instance->light) == RS::LIGHT_DIRECTIONAL_SHADOW_PARALLEL_2_SPLITS) {
	atlas_rect.size.height /= 2;
	if (p_pass == 0) {
	} else {
	atlas_rect.position.y += atlas_rect.size.height;
	}
	}