Take DirectionalLight's GlobalTransform into account when calculating…

… shadow map volume (not just direction) (bevyengine#6384)

# Objective

This PR fixes bevyengine#5789, by enabling movable (and scalable) directional light shadow volumes.

## Solution

This PR changes `ExtractedDirectionalLight` to hold a copy of the `DirectionalLight` entity's `GlobalTransform`, instead of just a `direction` vector. This allows the shadow map volume (as defined by the light's `shadow_projection` field) to be transformed honoring translation _and_ scale transforms, and not just rotation.

It also augments the texel size calculation (used to determine the `shadow_normal_bias`) so that it now takes into account the upper bound of the x/y/z scale of the `GlobalTransform`.

This change makes the directional light extraction code more consistent with point and spot lights (that already use `transform`), and allows easily moving and scaling the shadow volume along with a player entity based on camera distance/angle, immediately enabling more real world use cases until we have a more sophisticated adaptive implementation, such as the one described in bevyengine#3629.

**Note:** While it was previously possible to update the projection achieving a similar effect, depending on the light direction and distance to the origin, the fact that the shadow map camera was always positioned at the origin with a hardcoded `Vec3::Y` up value meant you would get sub-optimal or inconsistent/incorrect results.

---

## Changelog

### Changed

- `DirectionalLight` shadow volumes now honor translation and scale transforms

## Migration Guide

- If your directional lights were positioned at the origin and not scaled (the default, most common scenario) no changes are needed on your part; it just works as before;
- If you previously had a system for dynamically updating directional light shadow projections, you might now be able to simplify your code by updating the directional light entity's transform instead;
- In the unlikely scenario that a scene with directional lights that previously rendered shadows correctly has missing shadows, make sure your directional lights are positioned at (0, 0, 0) and are not scaled to a size that's too large or too small.

crates/bevy_pbr/src/light.rs

@@ -167,13 +167,51 @@ impl Default for SpotLight {
 /// | 32,000–100,000    | Direct sunlight                                |
 ///
 /// Source: [Wikipedia](https://en.wikipedia.org/wiki/Lux)
+///
+/// ## Shadows
+///
+/// To enable shadows, set the `shadows_enabled` property to `true`.
+///
+/// While directional lights contribute to the illumination of meshes regardless
+/// of their (or the meshes') positions, currently only a limited region of the scene
+/// (the _shadow volume_) can cast and receive shadows for any given directional light.
+///
+/// The shadow volume is a _rectangular cuboid_, with left/right/bottom/top/near/far
+/// planes controllable via the `shadow_projection` field. It is affected by the
+/// directional light entity's [`GlobalTransform`], and as such can be freely repositioned in the
+/// scene, (or even scaled!) without affecting illumination in any other way, by simply
+/// moving (or scaling) the entity around. The shadow volume is always oriented towards the
+/// light entity's forward direction.
+///
+/// For smaller scenes, a static directional light with a preset volume is typically
+/// sufficient. For larger scenes with movable cameras, you might want to introduce
+/// a system that dynamically repositions and scales the light entity (and therefore
+/// its shadow volume) based on the scene subject's position (e.g. a player character)
+/// and its relative distance to the camera.
+///
+/// Shadows are produced via [shadow mapping](https://en.wikipedia.org/wiki/Shadow_mapping).
+/// To control the resolution of the shadow maps, use the [`DirectionalLightShadowMap`] resource:
+///
+/// ```
+/// # use bevy_app::prelude::*;
+/// # use bevy_pbr::DirectionalLightShadowMap;
+/// App::new()
+///     .insert_resource(DirectionalLightShadowMap { size: 2048 });
+/// ```
+///
+/// **Note:** Very large shadow map resolutions (> 4K) can have non-negligible performance and
+/// memory impact, and not work properly under mobile or lower-end hardware. To improve the visual
+/// fidelity of shadow maps, it's typically advisable to first reduce the `shadow_projection`
+/// left/right/top/bottom to a scene-appropriate size, before ramping up the shadow map
+/// resolution.
 #[derive(Component, Debug, Clone, Reflect)]
 #[reflect(Component, Default)]
 pub struct DirectionalLight {
     pub color: Color,
     /// Illuminance in lux
     pub illuminance: f32,
     pub shadows_enabled: bool,
+    /// A projection that controls the volume in which shadow maps are rendered
     pub shadow_projection: OrthographicProjection,
     pub shadow_depth_bias: f32,
     /// A bias applied along the direction of the fragment's surface normal. It is scaled to the
@@ -208,6 +246,7 @@ impl DirectionalLight {
     pub const DEFAULT_SHADOW_NORMAL_BIAS: f32 = 0.6;
 }
 
+/// Controls the resolution of [`DirectionalLight`] shadow maps.
 #[derive(Resource, Clone, Debug, Reflect)]
 #[reflect(Resource)]
 pub struct DirectionalLightShadowMap {

crates/bevy_pbr/src/render/light.rs

@@ -10,7 +10,7 @@ use bevy_ecs::{
     prelude::*,
     system::{lifetimeless::*, SystemParamItem},
 };
-use bevy_math::{Mat4, UVec3, UVec4, Vec2, Vec3, Vec3Swizzles, Vec4, Vec4Swizzles};
+use bevy_math::{Mat4, UVec3, UVec4, Vec2, Vec3, Vec3A, Vec3Swizzles, Vec4, Vec4Swizzles};
 use bevy_render::{
     camera::{Camera, CameraProjection},
     color::Color,
@@ -66,7 +66,7 @@ pub struct ExtractedPointLight {
 pub struct ExtractedDirectionalLight {
     color: Color,
     illuminance: f32,
-    direction: Vec3,
+    transform: GlobalTransform,
     projection: Mat4,
     shadows_enabled: bool,
     shadow_depth_bias: f32,
@@ -550,32 +550,27 @@ pub fn extract_lights(
             continue;
         }
 
-        // Calulate the directional light shadow map texel size using the largest x,y dimension of
+        // Calculate the directional light shadow map texel size using the scaled x,y length of
         // the orthographic projection divided by the shadow map resolution
         // NOTE: When using various PCF kernel sizes, this will need to be adjusted, according to:
         // https://catlikecoding.com/unity/tutorials/custom-srp/directional-shadows/
-        let largest_dimension = (directional_light.shadow_projection.right
-            - directional_light.shadow_projection.left)
-            .max(
-                directional_light.shadow_projection.top
-                    - directional_light.shadow_projection.bottom,
-            );
-        let directional_light_texel_size =
-            largest_dimension / directional_light_shadow_map.size as f32;
+        let directional_light_texel_size = transform.radius_vec3a(Vec3A::new(
+            directional_light.shadow_projection.right - directional_light.shadow_projection.left,
+            directional_light.shadow_projection.top - directional_light.shadow_projection.bottom,
+            0.,
+        )) / directional_light_shadow_map.size as f32;
         // TODO: As above
         let render_visible_entities = visible_entities.clone();
         commands.get_or_spawn(entity).insert((
             ExtractedDirectionalLight {
                 color: directional_light.color,
                 illuminance: directional_light.illuminance,
-                direction: transform.forward(),
+                transform: *transform,
                 projection: directional_light.shadow_projection.get_projection_matrix(),
                 shadows_enabled: directional_light.shadows_enabled,
                 shadow_depth_bias: directional_light.shadow_depth_bias,
-                // The factor of SQRT_2 is for the worst-case diagonal offset
                 shadow_normal_bias: directional_light.shadow_normal_bias
-                    * directional_light_texel_size
-                    * std::f32::consts::SQRT_2,
+                    * directional_light_texel_size,
             },
             render_visible_entities,
         ));
@@ -947,7 +942,7 @@ pub fn prepare_lights(
         }
 
         // direction is negated to be ready for N.L
-        let dir_to_light = -light.direction;
+        let dir_to_light = light.transform.back();
 
         // convert from illuminance (lux) to candelas
         //
@@ -961,9 +956,8 @@ pub fn prepare_lights(
         let exposure = 1.0 / (f32::powf(2.0, ev100) * 1.2);
         let intensity = light.illuminance * exposure;
 
-        // NOTE: A directional light seems to have to have an eye position on the line along the direction of the light
-        // through the world origin. I (Rob Swain) do not yet understand why it cannot be translated away from this.
-        let view = Mat4::look_at_rh(Vec3::ZERO, light.direction, Vec3::Y);
+        // NOTE: For the purpose of rendering shadow maps, we apply the directional light's transform to an orthographic camera
+        let view = light.transform.compute_matrix().inverse();
         // NOTE: This orthographic projection defines the volume within which shadows from a directional light can be cast
         let projection = light.projection;
 
@@ -1175,10 +1169,6 @@ pub fn prepare_lights(
             .enumerate()
             .take(directional_shadow_maps_count)
         {
-            // NOTE: A directional light seems to have to have an eye position on the line along the direction of the light
-            // through the world origin. I (Rob Swain) do not yet understand why it cannot be translated away from this.
-            let view = Mat4::look_at_rh(Vec3::ZERO, light.direction, Vec3::Y);
-
             let depth_texture_view =
                 directional_light_depth_texture
                     .texture
@@ -1206,7 +1196,7 @@ pub fn prepare_lights(
                             directional_light_shadow_map.size as u32,
                             directional_light_shadow_map.size as u32,
                         ),
-                        transform: GlobalTransform::from(view.inverse()),
+                        transform: light.transform,
                         projection: light.projection,
                         hdr: false,
                     },