Implement distance fade properties in OmniLight3D and SpotLight3D

This can be used to fade lights and their shadows in the distance,
similar to Decal nodes. This can bring significant performance
improvements, especially for lights with shadows enabled and when
using higher-than-default shadow quality settings.

While lights can be smoothly faded out over distance, shadows are
currently "all or nothing" since per-light shadow color is no longer
customizable in the Vulkan renderer. This may result in noticeable
pop-in when leaving the shadow cutoff distance, but depending on the
scene, it may not always be that noticeable.

1 files changed, 59 insertions, 4 deletions

diff --git a/servers/rendering/renderer_rd/renderer_scene_render_rd.cpp b/servers/rendering/renderer_rd/renderer_scene_render_rd.cpp
index 948340f469..92f1286d52 100644
--- a/servers/rendering/renderer_rd/renderer_scene_render_rd.cpp
+++ b/servers/rendering/renderer_rd/renderer_scene_render_rd.cpp
@@ -3440,8 +3440,22 @@ void RendererSceneRenderRD::_setup_lights(const PagedArray<RID> &p_lights, const
 					continue;
 				}
 
+				const real_t distance = camera_plane.distance_to(li->transform.origin);
+
+				if (storage->light_is_distance_fade_enabled(li->light)) {
+					const float fade_begin = storage->light_get_distance_fade_begin(li->light);
+					const float fade_length = storage->light_get_distance_fade_length(li->light);
+
+					if (distance > fade_begin) {
+						if (distance > fade_begin + fade_length) {
+							// Out of range, don't draw this light to improve performance.
+							continue;
+						}
+					}
+				}
+
 				cluster.omni_light_sort[cluster.omni_light_count].instance = li;
-				cluster.omni_light_sort[cluster.omni_light_count].depth = camera_plane.distance_to(li->transform.origin);
+				cluster.omni_light_sort[cluster.omni_light_count].depth = distance;
 				cluster.omni_light_count++;
 			} break;
 			case RS::LIGHT_SPOT: {
@@ -3449,8 +3463,22 @@ void RendererSceneRenderRD::_setup_lights(const PagedArray<RID> &p_lights, const
 					continue;
 				}
 
+				const real_t distance = camera_plane.distance_to(li->transform.origin);
+
+				if (storage->light_is_distance_fade_enabled(li->light)) {
+					const float fade_begin = storage->light_get_distance_fade_begin(li->light);
+					const float fade_length = storage->light_get_distance_fade_length(li->light);
+
+					if (distance > fade_begin) {
+						if (distance > fade_begin + fade_length) {
+							// Out of range, don't draw this light to improve performance.
+							continue;
+						}
+					}
+				}
+
 				cluster.spot_light_sort[cluster.spot_light_count].instance = li;
-				cluster.spot_light_sort[cluster.spot_light_count].depth = camera_plane.distance_to(li->transform.origin);
+				cluster.spot_light_sort[cluster.spot_light_count].depth = distance;
 				cluster.spot_light_count++;
 			} break;
 		}
@@ -3494,7 +3522,24 @@ void RendererSceneRenderRD::_setup_lights(const PagedArray<RID> &p_lights, const
 
 		light_data.attenuation = storage->light_get_param(base, RS::LIGHT_PARAM_ATTENUATION);
 
-		float energy = sign * storage->light_get_param(base, RS::LIGHT_PARAM_ENERGY) * Math_PI;
+		// Reuse fade begin, fade length and distance for shadow LOD determination later.
+		float fade_begin = 0.0;
+		float fade_length = 0.0;
+		real_t distance = 0.0;
+
+		float fade = 1.0;
+		if (storage->light_is_distance_fade_enabled(li->light)) {
+			fade_begin = storage->light_get_distance_fade_begin(li->light);
+			fade_length = storage->light_get_distance_fade_length(li->light);
+			distance = camera_plane.distance_to(li->transform.origin);
+
+			if (distance > fade_begin) {
+				// Use `smoothstep()` to make opacity changes more gradual and less noticeable to the player.
+				fade = Math::smoothstep(0.0f, 1.0f, 1.0f - float(distance - fade_begin) / fade_length);
+			}
+		}
+
+		float energy = sign * storage->light_get_param(base, RS::LIGHT_PARAM_ENERGY) * Math_PI * fade;
 
 		light_data.color[0] = linear_col.r * energy;
 		light_data.color[1] = linear_col.g * energy;
@@ -3555,7 +3600,17 @@ void RendererSceneRenderRD::_setup_lights(const PagedArray<RID> &p_lights, const
 			light_data.projector_rect[3] = 0;
 		}
 
-		if (shadow_atlas && shadow_atlas->shadow_owners.has(li->self)) {
+		const bool needs_shadow = shadow_atlas && shadow_atlas->shadow_owners.has(li->self);
+
+		bool in_shadow_range = true;
+		if (needs_shadow && storage->light_is_distance_fade_enabled(li->light)) {
+			if (distance > storage->light_get_distance_fade_shadow(li->light)) {
+				// Out of range, don't draw shadows to improve performance.
+				in_shadow_range = false;
+			}
+		}
+
+		if (needs_shadow && in_shadow_range) {
 			// fill in the shadow information
 
 			light_data.shadow_enabled = true;