Automatically adjust Light3D shadow bias to account for resolution

Higher shadowmap resolutions make it possible to use lower bias
values without exhibiting shadow acne. On the other hand,
shadow bias should be increased when using lower shadowmap resolutions
(but not as much, to avoid peter-panning).

Author: Calinou

servers/rendering/renderer_rd/renderer_scene_render_rd.cpp

@@ -3272,6 +3272,11 @@ void RendererSceneRenderRD::_setup_lights(const PagedArray<RID> &p_lights, const
 
 	r_directional_light_soft_shadows = false;
 
+	ShadowAtlas *shadow_atlas = nullptr;
+	if (p_shadow_atlas.is_valid() && p_using_shadows) {
+		shadow_atlas = shadow_atlas_owner.get_or_null(p_shadow_atlas);
+	}
+
 	for (int i = 0; i < (int)p_lights.size(); i++) {
 		LightInstance *li = light_instance_owner.get_or_null(p_lights[i]);
 		if (!li) {
@@ -3419,9 +3424,19 @@ void RendererSceneRenderRD::_setup_lights(const PagedArray<RID> &p_lights, const
 						Transform3D modelview = (inverse_transform * li->shadow_transform[j].transform).inverse();
 
 						CameraMatrix shadow_mtx = rectm * bias * matrix * modelview;
+
 						light_data.shadow_split_offsets[j] = split;
-						float bias_scale = li->shadow_transform[j].bias_scale;
-						light_data.shadow_bias[j] = storage->light_get_param(base, RS::LIGHT_PARAM_SHADOW_BIAS) / 100.0 * bias_scale;
+
+						float bias_scale;
+						if (directional_shadow.size < 4096) {
+							// For shadow map sizes below the default, don't increase bias too much to avoid peter-panning.
+							bias_scale = (1 + li->shadow_transform[j].bias_scale * (4096.0 / MAX(256, directional_shadow.size))) * 0.5;
+						} else {
+							// Reduce shadow bias for shadow map sizes higher than the default.
+							bias_scale = li->shadow_transform[j].bias_scale * (4096.0 / directional_shadow.size);
+						}
+
+						light_data.shadow_bias[j] = (storage->light_get_param(base, RS::LIGHT_PARAM_SHADOW_BIAS) * bias_scale) / 100.0;
 						light_data.shadow_normal_bias[j] = storage->light_get_param(base, RS::LIGHT_PARAM_SHADOW_NORMAL_BIAS) * li->shadow_transform[j].shadow_texel_size;
 						light_data.shadow_transmittance_bias[j] = storage->light_get_transmittance_bias(base) * bias_scale;
 						light_data.shadow_z_range[j] = li->shadow_transform[j].farplane;
@@ -3499,12 +3514,6 @@ void RendererSceneRenderRD::_setup_lights(const PagedArray<RID> &p_lights, const
 		sorter.sort(cluster.spot_light_sort, cluster.spot_light_count);
 	}
 
-	ShadowAtlas *shadow_atlas = nullptr;
-
-	if (p_shadow_atlas.is_valid() && p_using_shadows) {
-		shadow_atlas = shadow_atlas_owner.get_or_null(p_shadow_atlas);
-	}
-
 	bool using_forward_ids = _uses_forward_ids();
 
 	for (uint32_t i = 0; i < (cluster.omni_light_count + cluster.spot_light_count); i++) {
@@ -3594,13 +3603,22 @@ void RendererSceneRenderRD::_setup_lights(const PagedArray<RID> &p_lights, const
 			float shadow_texel_size = light_instance_get_shadow_texel_size(li->self, p_shadow_atlas);
 			light_data.shadow_normal_bias = storage->light_get_param(base, RS::LIGHT_PARAM_SHADOW_NORMAL_BIAS) * shadow_texel_size * 10.0;
 
+			float bias_resolution_factor;
+			if (shadow_atlas->size < 4096) {
+				// For shadow map sizes below the default, don't increase bias too much to avoid peter-panning.
+				bias_resolution_factor = (1 + (4096.0 / MAX(256, shadow_atlas->size))) * 0.5;
+			} else {
+				// Reduce shadow bias for shadow map sizes higher than the default.
+				bias_resolution_factor = (4096.0 / shadow_atlas->size);
+			}
+
 			if (type == RS::LIGHT_SPOT) {
-				light_data.shadow_bias = storage->light_get_param(base, RS::LIGHT_PARAM_SHADOW_BIAS) / 100.0;
+				light_data.shadow_bias = (storage->light_get_param(base, RS::LIGHT_PARAM_SHADOW_BIAS) / 100.0) * bias_resolution_factor;
 			} else { //omni
-				light_data.shadow_bias = storage->light_get_param(base, RS::LIGHT_PARAM_SHADOW_BIAS);
+				light_data.shadow_bias = storage->light_get_param(base, RS::LIGHT_PARAM_SHADOW_BIAS) * bias_resolution_factor;
 			}
 
-			light_data.transmittance_bias = storage->light_get_transmittance_bias(base);
+			light_data.transmittance_bias = storage->light_get_transmittance_bias(base) * bias_resolution_factor;
 
 			Vector2i omni_offset;
 			Rect2 rect = light_instance_get_shadow_atlas_rect(li->self, p_shadow_atlas, omni_offset);