Cascaded shadow maps. (#7064)

Co-authored-by: Robert Swain <robert.swain@gmail.com>

# Objective

Implements cascaded shadow maps for directional lights, which produces better quality shadows without needing excessively large shadow maps.

Fixes #3629

Before
![image](https://user-images.githubusercontent.com/1222141/210061203-bbd965a4-8d11-4cec-9a88-67fc59d0819f.png)

After
![image](https://user-images.githubusercontent.com/1222141/210061334-2ff15334-e6d7-4a31-9314-f34a7805cac6.png)


## Solution

Rather than rendering a single shadow map for directional light, the view frustum is divided into a series of cascades, each of which gets its own shadow map. The correct cascade is then sampled for shadow determination.

---

## Changelog

Directional lights now use cascaded shadow maps for improved shadow quality.


## Migration Guide

You no longer have to manually specify a `shadow_projection` for a directional light, and these settings should be removed. If customization of how cascaded shadow maps work is desired, modify the `CascadeShadowConfig` component instead.

crates/bevy_core_pipeline/src/core_2d/camera_2d.rs

@@ -61,7 +61,7 @@ impl Camera2dBundle {
         let transform = Transform::from_xyz(0.0, 0.0, far - 0.1);
         let view_projection =
             projection.get_projection_matrix() * transform.compute_matrix().inverse();
-        let frustum = Frustum::from_view_projection(
+        let frustum = Frustum::from_view_projection_custom_far(
             &view_projection,
             &transform.translation,
             &transform.back(),

crates/bevy_pbr/src/bundle.rs

@@ -1,13 +1,17 @@
-use crate::{DirectionalLight, Material, PointLight, SpotLight, StandardMaterial};
+use crate::{
+    CascadeShadowConfig, Cascades, DirectionalLight, Material, PointLight, SpotLight,
+    StandardMaterial,
+};
 use bevy_asset::Handle;
-use bevy_ecs::{bundle::Bundle, component::Component, reflect::ReflectComponent};
+use bevy_ecs::{bundle::Bundle, component::Component, prelude::Entity, reflect::ReflectComponent};
 use bevy_reflect::Reflect;
 use bevy_render::{
     mesh::Mesh,
-    primitives::{CubemapFrusta, Frustum},
+    primitives::{CascadesFrusta, CubemapFrusta, Frustum},
     view::{ComputedVisibility, Visibility, VisibleEntities},
 };
 use bevy_transform::components::{GlobalTransform, Transform};
+use bevy_utils::HashMap;
 
 /// A component bundle for PBR entities with a [`Mesh`] and a [`StandardMaterial`].
 pub type PbrBundle = MaterialMeshBundle<StandardMaterial>;
@@ -63,6 +67,14 @@ impl CubemapVisibleEntities {
     }
 }
 
+#[derive(Component, Clone, Debug, Default, Reflect)]
+#[reflect(Component)]
+pub struct CascadesVisibleEntities {
+    /// Map of view entity to the visible entities for each cascade frustum.
+    #[reflect(ignore)]
+    pub entities: HashMap<Entity, Vec<VisibleEntities>>,
+}
+
 /// A component bundle for [`PointLight`] entities.
 #[derive(Debug, Bundle, Default)]
 pub struct PointLightBundle {
@@ -95,8 +107,10 @@ pub struct SpotLightBundle {
 #[derive(Debug, Bundle, Default)]
 pub struct DirectionalLightBundle {
     pub directional_light: DirectionalLight,
-    pub frustum: Frustum,
-    pub visible_entities: VisibleEntities,
+    pub frusta: CascadesFrusta,
+    pub cascades: Cascades,
+    pub cascade_shadow_config: CascadeShadowConfig,
+    pub visible_entities: CascadesVisibleEntities,
     pub transform: Transform,
     pub global_transform: GlobalTransform,
     /// Enables or disables the light

crates/bevy_pbr/src/lib.rs

@@ -145,17 +145,20 @@ impl Plugin for PbrPlugin {
             Shader::from_wgsl
         );
 
-        app.register_type::<CubemapVisibleEntities>()
-            .register_type::<DirectionalLight>()
-            .register_type::<PointLight>()
-            .register_type::<SpotLight>()
-            .register_asset_reflect::<StandardMaterial>()
+        app.register_asset_reflect::<StandardMaterial>()
             .register_type::<AmbientLight>()
-            .register_type::<DirectionalLightShadowMap>()
+            .register_type::<CascadeShadowConfig>()
+            .register_type::<Cascades>()
+            .register_type::<CascadesVisibleEntities>()
             .register_type::<ClusterConfig>()
-            .register_type::<ClusterZConfig>()
             .register_type::<ClusterFarZMode>()
+            .register_type::<ClusterZConfig>()
+            .register_type::<CubemapVisibleEntities>()
+            .register_type::<DirectionalLight>()
+            .register_type::<DirectionalLightShadowMap>()
+            .register_type::<PointLight>()
             .register_type::<PointLightShadowMap>()
+            .register_type::<SpotLight>()
             .add_plugin(MeshRenderPlugin)
             .add_plugin(MaterialPlugin::<StandardMaterial> {
                 prepass_enabled: self.prepass_enabled,
@@ -183,13 +186,20 @@ impl Plugin for PbrPlugin {
                     .after(CameraUpdateSystem)
                     .after(ModifiesWindows),
             )
+            .add_system_to_stage(
+                CoreStage::PostUpdate,
+                update_directional_light_cascades
+                    .label(SimulationLightSystems::UpdateDirectionalLightCascades)
+                    .after(TransformSystem::TransformPropagate),
+            )
             .add_system_to_stage(
                 CoreStage::PostUpdate,
                 update_directional_light_frusta
                     .label(SimulationLightSystems::UpdateLightFrusta)
                     // This must run after CheckVisibility because it relies on ComputedVisibility::is_visible()
                     .after(VisibilitySystems::CheckVisibility)
                     .after(TransformSystem::TransformPropagate)
+                    .after(SimulationLightSystems::UpdateDirectionalLightCascades)
                     // We assume that no entity will be both a directional light and a spot light,
                     // so these systems will run independently of one another.
                     // FIXME: Add an archetype invariant for this https://github.com/bevyengine/bevy/issues/1481.