Add an option to update shadow maps less often

This can be used to improve performance on low-end setups,
at the cost of shadows visibly lagging behind for dynamic lights/objects
when up close.

This option works best with lights that use Reverse Cull Face and a low
negative bias value.

Co-authored-by: Manuele Finocchiaro <m4nu3lf@gmail.com>

doc/classes/ProjectSettings.xml

@@ -1451,6 +1451,9 @@
 		<member name="rendering/quality/shadows/filter_mode.mobile" type="int" setter="" getter="" default="0">
 			Lower-end override for [member rendering/quality/shadows/filter_mode] on mobile devices, due to performance concerns or driver support.
 		</member>
+		<member name="rendering/quality/shadows/update_every_2_frames" type="bool" setter="" getter="" default="false">
+			If [code]true[/code], directional and point light shadows are only updated every 2 frames instead of being updated every frame. Updates are staggered across frames to avoid stuttering. This improves performance at the cost of shadows updating in a more "choppy" manner, especially at lower framerates. The difference is mainly noticeable with fast-moving lights, especially when close to the camera.
+		</member>
 		<member name="rendering/quality/skinning/force_software_skinning" type="bool" setter="" getter="" default="false">
 			Forces [MeshInstance] to always perform skinning on the CPU (applies to both GLES2 and GLES3).
 			See also [member rendering/quality/skinning/software_skinning_fallback].

servers/visual/visual_server_scene.cpp

@@ -2641,8 +2641,19 @@ void VisualServerScene::_prepare_scene(const Transform p_cam_transform, const Ca
 	RID *directional_light_ptr = &light_instance_cull_result[light_cull_count];
 	directional_light_count = 0;
 
-	// directional lights
-	{
+	if (bool(GLOBAL_GET("rendering/quality/shadows/update_every_2_frames"))) {
+		// Toggle between directional and point light shadow updating every frame.
+		// This update staggering avoids stuttering by splitting CPU/GPU load across frames.
+		if (shadow_map_update == ShadowMapUpdate::SHADOW_MAP_UPDATE_DIRECTIONAL) {
+			shadow_map_update = ShadowMapUpdate::SHADOW_MAP_UPDATE_POINT;
+		} else {
+			shadow_map_update = ShadowMapUpdate::SHADOW_MAP_UPDATE_DIRECTIONAL;
+		}
+	} else {
+		shadow_map_update = ShadowMapUpdate::SHADOW_MAP_UPDATE_POINT_AND_DIRECTIONAL;
+	}
+
+	{ // directional lights
 		Instance **lights_with_shadow = (Instance **)alloca(sizeof(Instance *) * scenario->directional_lights.size());
 		int directional_shadow_count = 0;
 
@@ -2668,17 +2679,18 @@ void VisualServerScene::_prepare_scene(const Transform p_cam_transform, const Ca
 			}
 		}
 
-		VSG::scene_render->set_directional_shadow_count(directional_shadow_count);
+		if (shadow_map_update != ShadowMapUpdate::SHADOW_MAP_UPDATE_POINT) {
+			VSG::scene_render->set_directional_shadow_count(directional_shadow_count);
 
-		for (int i = 0; i < directional_shadow_count; i++) {
-			_light_instance_update_shadow(lights_with_shadow[i], p_cam_transform, p_cam_projection, p_cam_orthogonal, p_shadow_atlas, scenario);
+			for (int i = 0; i < directional_shadow_count; i++) {
+				_light_instance_update_shadow(lights_with_shadow[i], p_cam_transform, p_cam_projection, p_cam_orthogonal, p_shadow_atlas, scenario);
+			}
 		}
 	}
 
-	{ //setup shadow maps
+	if (shadow_map_update != ShadowMapUpdate::SHADOW_MAP_UPDATE_DIRECTIONAL) {
+		// Set up point light shadow maps.
 
-		//SortArray<Instance*,_InstanceLightsort> sorter;
-		//sorter.sort(light_cull_result,light_cull_count);
 		for (int i = 0; i < light_cull_count; i++) {
 			Instance *ins = light_cull_result[i];
 

servers/visual/visual_server_scene.h

@@ -54,6 +54,12 @@ class VisualServerScene {
 		MAX_EXTERIOR_PORTALS = 128,
 	};
 
+	enum ShadowMapUpdate {
+		SHADOW_MAP_UPDATE_POINT_AND_DIRECTIONAL, // Update both point and directional light shadows for this frame (default).
+		SHADOW_MAP_UPDATE_POINT, // Update point light shadows only for this frame.
+		SHADOW_MAP_UPDATE_DIRECTIONAL, // Update directional light shadows only for this frame.
+	};
+
 	uint64_t render_pass;
 	static VisualServerScene *singleton;
 
@@ -508,6 +514,7 @@ class VisualServerScene {
 	int directional_light_count;
 	RID reflection_probe_instance_cull_result[MAX_REFLECTION_PROBES_CULLED];
 	int reflection_probe_cull_count;
+	ShadowMapUpdate shadow_map_update = ShadowMapUpdate::SHADOW_MAP_UPDATE_POINT_AND_DIRECTIONAL;
 
 	RID_Owner<Instance> instance_owner;
 

servers/visual_server.cpp

@@ -2635,6 +2635,7 @@ VisualServer::VisualServer() {
 	GLOBAL_DEF("rendering/quality/shadows/filter_mode", 1);
 	GLOBAL_DEF("rendering/quality/shadows/filter_mode.mobile", 0);
 	ProjectSettings::get_singleton()->set_custom_property_info("rendering/quality/shadows/filter_mode", PropertyInfo(Variant::INT, "rendering/quality/shadows/filter_mode", PROPERTY_HINT_ENUM, "Disabled,PCF5,PCF13"));
+	GLOBAL_DEF("rendering/quality/shadows/update_every_2_frames", false);
 
 	GLOBAL_DEF("rendering/quality/reflections/texture_array_reflections", true);
 	GLOBAL_DEF("rendering/quality/reflections/texture_array_reflections.mobile", false);