Backport DirectionalLight fade_start property to 3.x

- Implement shadow fading when using the Orthogonal shadow mode
  (like in `master`).

This allows customizing the distance at which directional shadows
start to fade away. Shadow fading will also always start at the same
distance now, regardless of the current shadow mode in use.

This is useful for enclosed levels to prevent shadows from fading
at all with a well-tuned maximum distance.

The default fade start value (0.8) results in fading happening later
in the distance compared to the previous behavior, where fading started
from the last shadow split distance (0.6 in PSSM 4 Splits and
0.1 in PSSM 2 Splits).

doc/classes/DirectionalLight.xml

@@ -21,6 +21,9 @@
 		<member name="directional_shadow_depth_range" type="int" setter="set_shadow_depth_range" getter="get_shadow_depth_range" enum="DirectionalLight.ShadowDepthRange" default="0">
 			Optimizes shadow rendering for detail versus movement. See [enum ShadowDepthRange].
 		</member>
+		<member name="directional_shadow_fade_start" type="float" setter="set_param" getter="get_param" default="0.8">
+			Proportion of [member directional_shadow_max_distance] at which point the shadow starts to fade. At [member directional_shadow_max_distance], the shadow will disappear. The default value is a balance between smooth fading and distant shadow visibility. If the camera moves fast and the [member directional_shadow_max_distance] is low, consider lowering [member directional_shadow_fade_start] below [code]0.8[/code] to make shadow transitions less noticeable. On the other hand, if you tuned [member directional_shadow_max_distance] to cover the entire scene, you can set [member directional_shadow_fade_start] to [code]1.0[/code] to prevent the shadow from fading in the distance (it will suddenly cut off instead).
+		</member>
 		<member name="directional_shadow_max_distance" type="float" setter="set_param" getter="get_param" default="100.0">
 			The maximum distance for shadow splits. Increasing this value will make directional shadows visible from further away, at the cost of lower overall shadow detail and performance (since more objects need to be included in the directional shadow rendering).
 		</member>

doc/classes/Light.xml

@@ -121,6 +121,9 @@
 		<constant name="PARAM_SHADOW_BIAS_SPLIT_SCALE" value="15" enum="Param">
 			Constant for accessing [member DirectionalLight.directional_shadow_bias_split_scale].
 		</constant>
+		<constant name="PARAM_SHADOW_FADE_START" value="16" enum="Param">
+			Constant for accessing [member DirectionalLight.directional_shadow_fade_start].
+		</constant>
 		<constant name="PARAM_MAX" value="17" enum="Param">
 			Represents the size of the [enum Param] enum.
 		</constant>

doc/classes/RenderingServer.xml

@@ -3542,6 +3542,9 @@
 		<constant name="LIGHT_PARAM_SHADOW_BIAS_SPLIT_SCALE" value="15" enum="LightParam">
 			Increases bias on further splits to fix self-shadowing that only occurs far away from the camera.
 		</constant>
+		<constant name="LIGHT_PARAM_SHADOW_FADE_START" value="16" enum="LightParam">
+			Proportion of [constant LIGHT_PARAM_SHADOW_MAX_DISTANCE] at which point the shadow starts to fade. At [constant LIGHT_PARAM_SHADOW_MAX_DISTANCE], the shadow will disappear. The default value is a balance between smooth fading and distant shadow visibility. If the camera moves fast and the [constant LIGHT_PARAM_SHADOW_MAX_DISTANCE] is low, consider lowering [constant LIGHT_PARAM_SHADOW_FADE_START] below [code]0.8[/code] to make shadow transitions less noticeable. On the other hand, if you tuned [constant LIGHT_PARAM_SHADOW_MAX_DISTANCE] to cover the entire scene, you can set [constant LIGHT_PARAM_SHADOW_FADE_START] to [code]1.0[/code] to prevent the shadow from fading in the distance (it will suddenly cut off instead).
+		</constant>
 		<constant name="LIGHT_PARAM_MAX" value="17" enum="LightParam">
 			Represents the size of the [enum LightParam] enum.
 		</constant>

drivers/gles2/rasterizer_scene_gles2.cpp

@@ -2079,6 +2079,12 @@ void RasterizerSceneGLES2::_setup_light(LightInstance *light, ShadowAtlas *shado
 				//	state.scene_shader.set_uniform(SceneShaderGLES2::LIGHT_CLAMP, light_clamp);
 				state.scene_shader.set_uniform(SceneShaderGLES2::SHADOW_PIXEL_SIZE, Size2(1.0 / directional_shadow.size, 1.0 / directional_shadow.size));
 				state.scene_shader.set_uniform(SceneShaderGLES2::LIGHT_SPLIT_OFFSETS, split_offsets);
+
+				const float fade_start = light_ptr->param[RS::LIGHT_PARAM_SHADOW_FADE_START];
+				// Using 1.0 would break `smoothstep()` in the shader.
+				state.scene_shader.set_uniform(SceneShaderGLES2::FADE_FROM, -split_offsets[shadow_count - 1] * MIN(fade_start, 0.999));
+				state.scene_shader.set_uniform(SceneShaderGLES2::FADE_TO, -split_offsets[shadow_count - 1]);
+
 				state.scene_shader.set_uniform(SceneShaderGLES2::LIGHT_SHADOW_MATRIX, matrices[0]);
 				state.scene_shader.set_uniform(SceneShaderGLES2::LIGHT_SHADOW_MATRIX2, matrices[1]);
 				state.scene_shader.set_uniform(SceneShaderGLES2::LIGHT_SHADOW_MATRIX3, matrices[2]);

drivers/gles2/rasterizer_storage_gles2.cpp

@@ -4244,6 +4244,7 @@ RID RasterizerStorageGLES2::light_create(RS::LightType p_type) {
 	light->param[RS::LIGHT_PARAM_SHADOW_SPLIT_3_OFFSET] = 0.6;
 	light->param[RS::LIGHT_PARAM_SHADOW_NORMAL_BIAS] = 0.1;
 	light->param[RS::LIGHT_PARAM_SHADOW_BIAS_SPLIT_SCALE] = 0.1;
+	light->param[RS::LIGHT_PARAM_SHADOW_FADE_START] = 0.8;
 
 	light->color = Color(1, 1, 1, 1);
 	light->shadow = false;
@@ -4281,7 +4282,8 @@ void RasterizerStorageGLES2::light_set_param(RID p_light, RS::LightParam p_param
 		case RS::LIGHT_PARAM_SHADOW_SPLIT_2_OFFSET:
 		case RS::LIGHT_PARAM_SHADOW_SPLIT_3_OFFSET:
 		case RS::LIGHT_PARAM_SHADOW_NORMAL_BIAS:
-		case RS::LIGHT_PARAM_SHADOW_BIAS: {
+		case RS::LIGHT_PARAM_SHADOW_BIAS:
+		case RS::LIGHT_PARAM_SHADOW_FADE_START: {
 			light->version++;
 			light->instance_change_notify(true, false);
 		} break;

drivers/gles2/shaders/scene.glsl

@@ -1103,6 +1103,8 @@ uniform highp sampler2D light_shadow_atlas; //texunit:-3
 #ifdef LIGHT_MODE_DIRECTIONAL
 uniform highp sampler2D light_directional_shadow; // texunit:-3
 uniform highp vec4 light_split_offsets;
+uniform mediump float fade_from;
+uniform mediump float fade_to;
 #endif
 
 varying highp vec4 shadow_coord;
@@ -1983,7 +1985,6 @@ FRAGMENT_SHADER_CODE
 	float shadow4 = sample_shadow(light_directional_shadow, shadow_coord4);
 
 	if (depth_z < light_split_offsets.w) {
-		float pssm_fade = 0.0;
 		float shadow_att = 1.0;
 #ifdef LIGHT_USE_PSSM_BLEND
 		float shadow_att2 = 1.0;
@@ -2019,7 +2020,6 @@ FRAGMENT_SHADER_CODE
 
 			} else {
 				shadow_att = shadow4;
-				pssm_fade = smoothstep(light_split_offsets.z, light_split_offsets.w, depth_z);
 
 #if defined(LIGHT_USE_PSSM_BLEND)
 				use_blend = false;
@@ -2043,7 +2043,6 @@ FRAGMENT_SHADER_CODE
 	float shadow3 = sample_shadow(light_directional_shadow, shadow_coord3);
 
 	if (depth_z < light_split_offsets.z) {
-		float pssm_fade = 0.0;
 		float shadow_att = 1.0;
 #ifdef LIGHT_USE_PSSM_BLEND
 		float shadow_att2 = 1.0;
@@ -2093,15 +2092,13 @@ FRAGMENT_SHADER_CODE
 
 	if (depth_z < light_split_offsets.y) {
 		float shadow_att = 1.0;
-		float pssm_fade = 0.0;
 
 #ifdef LIGHT_USE_PSSM_BLEND
 		float shadow_att2 = 1.0;
 		float pssm_blend = 0.0;
 		bool use_blend = true;
 #endif
 		if (depth_z < light_split_offsets.x) {
-			float pssm_fade = 0.0;
 			shadow_att = shadow1;
 
 #ifdef LIGHT_USE_PSSM_BLEND
@@ -2110,7 +2107,6 @@ FRAGMENT_SHADER_CODE
 #endif
 		} else {
 			shadow_att = shadow2;
-			pssm_fade = smoothstep(light_split_offsets.x, light_split_offsets.y, depth_z);
 #ifdef LIGHT_USE_PSSM_BLEND
 			use_blend = false;
 #endif
@@ -2144,7 +2140,6 @@ FRAGMENT_SHADER_CODE
 #endif //pssm2
 
 			highp vec4 pssm_coord;
-			float pssm_fade = 0.0;
 
 #ifdef LIGHT_USE_PSSM_BLEND
 			float pssm_blend;
@@ -2183,7 +2178,6 @@ FRAGMENT_SHADER_CODE
 
 				} else {
 					pssm_coord = shadow_coord4;
-					pssm_fade = smoothstep(light_split_offsets.z, light_split_offsets.w, depth_z);
 
 #if defined(LIGHT_USE_PSSM_BLEND)
 					use_blend = false;
@@ -2213,7 +2207,6 @@ FRAGMENT_SHADER_CODE
 				}
 			} else {
 				pssm_coord = shadow_coord3;
-				pssm_fade = smoothstep(light_split_offsets.y, light_split_offsets.z, depth_z);
 
 #if defined(LIGHT_USE_PSSM_BLEND)
 				use_blend = false;
@@ -2232,7 +2225,6 @@ FRAGMENT_SHADER_CODE
 #endif
 			} else {
 				pssm_coord = shadow_coord2;
-				pssm_fade = smoothstep(light_split_offsets.x, light_split_offsets.y, depth_z);
 #ifdef LIGHT_USE_PSSM_BLEND
 				use_blend = false;
 #endif
@@ -2254,7 +2246,8 @@ FRAGMENT_SHADER_CODE
 			}
 #endif
 
-			light_att *= mix(shadow_color.rgb, vec3(1.0), shadow);
+			float pssm_fade = smoothstep(fade_from, fade_to, vertex.z);
+			light_att *= mix(mix(shadow_color.rgb, vec3(1.0), shadow), vec3(1.0), pssm_fade);
 		}
 	}
 #endif //use vertex lighting

scene/3d/light.cpp

@@ -268,6 +268,7 @@ void Light::_bind_methods() {
 	BIND_ENUM_CONSTANT(PARAM_SHADOW_NORMAL_BIAS);
 	BIND_ENUM_CONSTANT(PARAM_SHADOW_BIAS);
 	BIND_ENUM_CONSTANT(PARAM_SHADOW_BIAS_SPLIT_SCALE);
+	BIND_ENUM_CONSTANT(PARAM_SHADOW_FADE_START);
 	BIND_ENUM_CONSTANT(PARAM_MAX);
 
 	BIND_ENUM_CONSTANT(BAKE_DISABLED);
@@ -315,6 +316,7 @@ Light::Light(RenderingServer::LightType p_type) {
 	set_param(PARAM_SHADOW_SPLIT_1_OFFSET, 0.1);
 	set_param(PARAM_SHADOW_SPLIT_2_OFFSET, 0.2);
 	set_param(PARAM_SHADOW_SPLIT_3_OFFSET, 0.5);
+	set_param(PARAM_SHADOW_FADE_START, 0.8);
 	set_param(PARAM_SHADOW_NORMAL_BIAS, 0.0);
 	set_param(PARAM_SHADOW_BIAS, 0.15);
 	set_disable_scale(true);
@@ -392,6 +394,7 @@ void DirectionalLight::_bind_methods() {
 	ADD_PROPERTYI(PropertyInfo(Variant::REAL, "directional_shadow_split_2", PROPERTY_HINT_RANGE, "0,1,0.001"), "set_param", "get_param", PARAM_SHADOW_SPLIT_2_OFFSET);
 	ADD_PROPERTYI(PropertyInfo(Variant::REAL, "directional_shadow_split_3", PROPERTY_HINT_RANGE, "0,1,0.001"), "set_param", "get_param", PARAM_SHADOW_SPLIT_3_OFFSET);
 	ADD_PROPERTY(PropertyInfo(Variant::BOOL, "directional_shadow_blend_splits"), "set_blend_splits", "is_blend_splits_enabled");
+	ADD_PROPERTYI(PropertyInfo(Variant::REAL, "directional_shadow_fade_start", PROPERTY_HINT_RANGE, "0,1,0.01"), "set_param", "get_param", PARAM_SHADOW_FADE_START);
 	ADD_PROPERTYI(PropertyInfo(Variant::REAL, "directional_shadow_normal_bias", PROPERTY_HINT_RANGE, "0,10,0.001"), "set_param", "get_param", PARAM_SHADOW_NORMAL_BIAS);
 	ADD_PROPERTYI(PropertyInfo(Variant::REAL, "directional_shadow_bias_split_scale", PROPERTY_HINT_RANGE, "0,1,0.001"), "set_param", "get_param", PARAM_SHADOW_BIAS_SPLIT_SCALE);
 	ADD_PROPERTY(PropertyInfo(Variant::INT, "directional_shadow_depth_range", PROPERTY_HINT_ENUM, "Stable,Optimized"), "set_shadow_depth_range", "get_shadow_depth_range");
@@ -412,6 +415,7 @@ DirectionalLight::DirectionalLight() :
 	set_param(PARAM_SHADOW_BIAS, 0.1);
 	set_param(PARAM_SHADOW_MAX_DISTANCE, 100);
 	set_param(PARAM_SHADOW_BIAS_SPLIT_SCALE, 0.25);
+	set_param(PARAM_SHADOW_FADE_START, 0.8);
 	set_shadow_mode(SHADOW_PARALLEL_4_SPLITS);
 	set_shadow_depth_range(SHADOW_DEPTH_RANGE_STABLE);
 

scene/3d/light.h

@@ -57,6 +57,7 @@ class Light : public VisualInstance {
 		PARAM_SHADOW_NORMAL_BIAS = RS::LIGHT_PARAM_SHADOW_NORMAL_BIAS,
 		PARAM_SHADOW_BIAS = RS::LIGHT_PARAM_SHADOW_BIAS,
 		PARAM_SHADOW_BIAS_SPLIT_SCALE = RS::LIGHT_PARAM_SHADOW_BIAS_SPLIT_SCALE,
+		PARAM_SHADOW_FADE_START = RS::LIGHT_PARAM_SHADOW_FADE_START,
 		PARAM_MAX = RS::LIGHT_PARAM_MAX
 	};
 

servers/rendering_server.cpp

@@ -2405,6 +2405,7 @@ void RenderingServer::_bind_methods() {
 	BIND_ENUM_CONSTANT(LIGHT_PARAM_SHADOW_NORMAL_BIAS);
 	BIND_ENUM_CONSTANT(LIGHT_PARAM_SHADOW_BIAS);
 	BIND_ENUM_CONSTANT(LIGHT_PARAM_SHADOW_BIAS_SPLIT_SCALE);
+	BIND_ENUM_CONSTANT(LIGHT_PARAM_SHADOW_FADE_START);
 	BIND_ENUM_CONSTANT(LIGHT_PARAM_MAX);
 
 	BIND_ENUM_CONSTANT(LIGHT_BAKE_DISABLED);