Automatically increase Light3D shadow bias when shadow blur is above 1.0

This makes it possible to adjust shadow blur without having to tweak
bias to avoid shadow acne. The formula automatically increases
shadow bias, normal bias and transmittance bias when shadow blur
is above 1.0:

    shadow_blur_bias_factor = max(1, 0.5 + shadow_blur * 0.5);

At the highest shadow blur value allowed in the inspector (8),
the shadow bias and normal bias will each be multiplied by 5.5.

Automatic adjustment is not performed for PCSS-style shadow blur
(light size > 0) yet.

Author: Calinou

servers/rendering/renderer_rd/renderer_scene_render_rd.cpp

@@ -3367,9 +3367,15 @@ void RendererSceneRenderRD::_setup_lights(const PagedArray<RID> &p_lights, const
 
 						CameraMatrix shadow_mtx = rectm * bias * matrix * modelview;
 						light_data.shadow_split_offsets[j] = split;
-						float bias_scale = li->shadow_transform[j].bias_scale;
+
+						// If shadow blur is set above 1.0, scale the bias scale and normal bias with shadow blur to avoid shadow acne.
+						// At shadow blur = 8 (the highest value exposed in the inspector), the blur factor will be 5.5.
+						// This may still require further adjustments by the user, but this provides a good automatic baseline.
+						const float shadow_blur_bias_factor = MAX(1, 0.5 + light_storage->light_get_param(base, RS::LIGHT_PARAM_SHADOW_BLUR) * 0.5);
+
+						const float bias_scale = li->shadow_transform[j].bias_scale * shadow_blur_bias_factor;
 						light_data.shadow_bias[j] = light_storage->light_get_param(base, RS::LIGHT_PARAM_SHADOW_BIAS) / 100.0 * bias_scale;
-						light_data.shadow_normal_bias[j] = light_storage->light_get_param(base, RS::LIGHT_PARAM_SHADOW_NORMAL_BIAS) * li->shadow_transform[j].shadow_texel_size;
+						light_data.shadow_normal_bias[j] = light_storage->light_get_param(base, RS::LIGHT_PARAM_SHADOW_NORMAL_BIAS) * li->shadow_transform[j].shadow_texel_size * shadow_blur_bias_factor;
 						light_data.shadow_transmittance_bias[j] = light_storage->light_get_transmittance_bias(base) * bias_scale;
 						light_data.shadow_z_range[j] = li->shadow_transform[j].farplane;
 						light_data.shadow_range_begin[j] = li->shadow_transform[j].range_begin;
@@ -3593,16 +3599,22 @@ void RendererSceneRenderRD::_setup_lights(const PagedArray<RID> &p_lights, const
 
 			light_data.shadow_enabled = true;
 
-			float shadow_texel_size = light_instance_get_shadow_texel_size(li->self, p_shadow_atlas);
-			light_data.shadow_normal_bias = light_storage->light_get_param(base, RS::LIGHT_PARAM_SHADOW_NORMAL_BIAS) * shadow_texel_size * 10.0;
+			const float shadow_texel_size = light_instance_get_shadow_texel_size(li->self, p_shadow_atlas);
+
+			// If shadow blur is set above 1.0, scale the bias scale and normal bias with shadow blur to avoid shadow acne.
+			// At shadow blur = 8 (the highest value exposed in the inspector), the blur factor will be 5.5.
+			// This may still require further adjustments by the user, but this provides a good automatic baseline.
+			const float shadow_blur_bias_factor = MAX(1, 0.5 + light_storage->light_get_param(base, RS::LIGHT_PARAM_SHADOW_BLUR) * 0.5);
+
+			light_data.shadow_normal_bias = light_storage->light_get_param(base, RS::LIGHT_PARAM_SHADOW_NORMAL_BIAS) * shadow_texel_size * 10.0 * shadow_blur_bias_factor;
 
 			if (type == RS::LIGHT_SPOT) {
-				light_data.shadow_bias = light_storage->light_get_param(base, RS::LIGHT_PARAM_SHADOW_BIAS) / 100.0;
-			} else { //omni
-				light_data.shadow_bias = light_storage->light_get_param(base, RS::LIGHT_PARAM_SHADOW_BIAS);
+				light_data.shadow_bias = (light_storage->light_get_param(base, RS::LIGHT_PARAM_SHADOW_BIAS) / 100.0) * shadow_blur_bias_factor;
+			} else { // OmniLight3D
+				light_data.shadow_bias = light_storage->light_get_param(base, RS::LIGHT_PARAM_SHADOW_BIAS) * shadow_blur_bias_factor;
 			}
 
-			light_data.transmittance_bias = light_storage->light_get_transmittance_bias(base);
+			light_data.transmittance_bias = light_storage->light_get_transmittance_bias(base) * shadow_blur_bias_factor;
 
 			Vector2i omni_offset;
 			Rect2 rect = light_instance_get_shadow_atlas_rect(li->self, p_shadow_atlas, omni_offset);