Add emissive_exposure_weight to the StandardMaterial (bevyengine#13350)

# Objective

- The emissive color gets multiplied by the camera exposure value. But
this cancels out almost any emissive effect.
- Fixes bevyengine#13133
- Closes PR bevyengine#13337 

## Solution
- Add emissive_exposure_weight to the StandardMaterial
- In the shader this value is stored in the alpha channel of the
emissive color.
- This value defines how much the exposure influences the emissive
color.
- It's equal to Google's Filament:
https://google.github.io/filament/Materials.html#emissive

https://github.com/google/filament/blob/4f021583f1c721486baaa9291be5943216c244ec/shaders/src/shading_lit.fs#L287

## Testing

- The result of
[EmissiveStrengthTest](https://github.com/KhronosGroup/glTF-Sample-Models/tree/main/2.0/EmissiveStrengthTest)
with the default value of 0.0:

without bloom:

![emissive_fix](https://github.com/bevyengine/bevy/assets/688816/8f8c131a-464a-4d7b-a9e4-4e28d679ee5d)

with bloom:

![emissive_fix_bloom](https://github.com/bevyengine/bevy/assets/688816/89f200ee-3bd5-4daa-bf64-8999b56df3fa)

crates/bevy_pbr/src/pbr_material.rs

@@ -90,6 +90,13 @@ pub struct StandardMaterial {
     /// it just adds a value to the color seen on screen.
     pub emissive: Color,
 
+    /// The weight in which the camera exposure influences the emissive color.
+    /// A value of `0.0` means the emissive color is not affected by the camera exposure.
+    /// In opposition, a value of `1.0` means the emissive color is multiplied by the camera exposure.
+    ///
+    /// Defaults to `0.0`
+    pub emissive_exposure_weight: f32,
+
     /// The UV channel to use for the [`StandardMaterial::emissive_texture`].
     ///
     /// Defaults to [`UvChannel::Uv0`].
@@ -683,6 +690,7 @@ impl Default for StandardMaterial {
             base_color_channel: UvChannel::Uv0,
             base_color_texture: None,
             emissive: Color::BLACK,
+            emissive_exposure_weight: 0.0,
             emissive_channel: UvChannel::Uv0,
             emissive_texture: None,
             // Matches Blender's default roughness.
@@ -964,9 +972,12 @@ impl AsBindGroupShaderType<StandardMaterialUniform> for StandardMaterial {
             flags |= StandardMaterialFlags::ATTENUATION_ENABLED;
         }
 
+        let mut emissive = LinearRgba::from(self.emissive).to_f32_array();
+        emissive[3] = self.emissive_exposure_weight;
+
         StandardMaterialUniform {
             base_color: LinearRgba::from(self.base_color).to_f32_array().into(),
-            emissive: LinearRgba::from(self.emissive).to_f32_array().into(),
+            emissive: emissive.into(),
             roughness: self.perceptual_roughness,
             metallic: self.metallic,
             reflectance: self.reflectance,

crates/bevy_pbr/src/render/pbr_fragment.wgsl

@@ -178,7 +178,6 @@ fn pbr_input_from_standard_material(
         pbr_input.material.alpha_cutoff = pbr_bindings::material.alpha_cutoff;
 
         // emissive
-        // TODO use .a for exposure compensation in HDR
         var emissive: vec4<f32> = pbr_bindings::material.emissive;
 #ifdef VERTEX_UVS
         if ((pbr_bindings::material.flags & pbr_types::STANDARD_MATERIAL_FLAGS_EMISSIVE_TEXTURE_BIT) != 0u) {
@@ -191,7 +190,7 @@ fn pbr_input_from_standard_material(
                 uv,
 #endif
                 bias,
-            ).rgb, 1.0);
+            ).rgb, emissive.a);
         }
 #endif
         pbr_input.material.emissive = emissive;

crates/bevy_pbr/src/render/pbr_functions.wgsl

@@ -224,7 +224,6 @@ fn apply_pbr_lighting(
 ) -> vec4<f32> {
     var output_color: vec4<f32> = in.material.base_color;
 
-    // TODO use .a for exposure compensation in HDR
     let emissive = in.material.emissive;
 
     // calculate non-linear roughness from linear perceptualRoughness
@@ -564,6 +563,8 @@ fn apply_pbr_lighting(
     emissive_light = emissive_light * (0.04 + (1.0 - 0.04) * pow(1.0 - clearcoat_NdotV, 5.0));
 #endif
 
+    emissive_light = emissive_light * mix(1.0, view_bindings::view.exposure, emissive.a);
+
 #ifdef STANDARD_MATERIAL_SPECULAR_TRANSMISSION
     transmitted_light += transmission::specular_transmissive_light(in.world_position, in.frag_coord.xyz, view_z, in.N, in.V, F0, ior, thickness, perceptual_roughness, specular_transmissive_color, specular_transmitted_environment_light).rgb;
 
@@ -585,7 +586,7 @@ fn apply_pbr_lighting(
 
     // Total light
     output_color = vec4<f32>(
-        view_bindings::view.exposure * (transmitted_light + direct_light + indirect_light + emissive_light),
+        (view_bindings::view.exposure * (transmitted_light + direct_light + indirect_light)) + emissive_light,
         output_color.a
     );
 

examples/3d/bloom_3d.rs

@@ -41,15 +41,15 @@ fn setup_scene(
     ));
 
     let material_emissive1 = materials.add(StandardMaterial {
-        emissive: Color::linear_rgb(23000.0, 9000.0, 3000.0), // 4. Put something bright in a dark environment to see the effect
+        emissive: Color::linear_rgb(13.99, 5.32, 2.0), // 4. Put something bright in a dark environment to see the effect
         ..default()
     });
     let material_emissive2 = materials.add(StandardMaterial {
-        emissive: Color::linear_rgb(3000.0, 23000.0, 9000.0),
+        emissive: Color::linear_rgb(2.0, 13.99, 5.32),
         ..default()
     });
     let material_emissive3 = materials.add(StandardMaterial {
-        emissive: Color::linear_rgb(9000.0, 3000.0, 23000.0),
+        emissive: Color::linear_rgb(5.32, 2.0, 13.99),
         ..default()
     });
     let material_non_emissive = materials.add(StandardMaterial {

examples/3d/lightmaps.rs

@@ -35,11 +35,6 @@ fn add_lightmaps_to_meshes(
 ) {
     let exposure = 250.0;
     for (entity, name, material) in meshes.iter() {
-        if &**name == "Light" {
-            materials.get_mut(material).unwrap().emissive = Color::Srgba(Srgba::WHITE * exposure);
-            continue;
-        }
-
         if &**name == "large_box" {
             materials.get_mut(material).unwrap().lightmap_exposure = exposure;
             commands.entity(entity).insert(Lightmap {

examples/3d/spotlight.rs

@@ -79,12 +79,12 @@ fn setup(
     let sphere_mesh_direction = meshes.add(Sphere::new(0.1).mesh().uv(32, 18));
     let red_emissive = materials.add(StandardMaterial {
         base_color: RED.into(),
-        emissive: Color::linear_rgba(100.0, 0.0, 0.0, 0.0),
+        emissive: Color::linear_rgba(1.0, 0.0, 0.0, 0.0),
         ..default()
     });
     let maroon_emissive = materials.add(StandardMaterial {
         base_color: MAROON.into(),
-        emissive: Color::linear_rgba(50.0, 0.0, 0.0, 0.0),
+        emissive: Color::linear_rgba(0.369, 0.0, 0.0, 0.0),
         ..default()
     });
 

examples/3d/transmission.rs

@@ -137,8 +137,8 @@ fn setup(
     ));
 
     // Candle Flame
-    let scaled_white = LinearRgba::from(ANTIQUE_WHITE) * 80.;
-    let scaled_orange = LinearRgba::from(ORANGE_RED) * 16.;
+    let scaled_white = LinearRgba::from(ANTIQUE_WHITE) * 20.;
+    let scaled_orange = LinearRgba::from(ORANGE_RED) * 4.;
     let emissive = LinearRgba {
         red: scaled_white.red + scaled_orange.red,
         green: scaled_white.green + scaled_orange.green,

examples/ecs/iter_combinations.rs

@@ -99,7 +99,7 @@ fn generate_bodies(
                     mesh: meshes.add(Sphere::new(1.0).mesh().ico(5).unwrap()),
                     material: materials.add(StandardMaterial {
                         base_color: ORANGE_RED.into(),
-                        emissive: (LinearRgba::from(ORANGE_RED) * 18.).into(),
+                        emissive: (LinearRgba::from(ORANGE_RED) * 2.).into(),
                         ..default()
                     }),
                     ..default()