Approximate indirect specular occlusion (bevyengine#11152)

# Objective

- The current PBR renderer over-brightens indirect specular reflections,
which tends to cause objects to appear to glow, because specular
occlusion is not accounted for.

## Solution

- Attenuate indirect specular term with an approximation for specular
occlusion, using [[Lagarde et al., 2014] (pg.
76)](https://seblagarde.files.wordpress.com/2015/07/course_notes_moving_frostbite_to_pbr_v32.pdf).

| Before | After | Animation |
| --- | --- | --- |
| <img width="1840" alt="before bike"
src="https://github.com/bevyengine/bevy/assets/2632925/b6e10d15-a998-4a94-875a-1c2b1e98348a">
| <img width="1840" alt="after bike"
src="https://github.com/bevyengine/bevy/assets/2632925/53b1479c-b1e4-427f-b140-53df26ca7193">
|
![ezgif-1-fbcbaf272b](https://github.com/bevyengine/bevy/assets/2632925/c2dece1c-eb3d-4e05-92a2-46cf83052c7c)
|
| <img width="1840" alt="classroom before"
src="https://github.com/bevyengine/bevy/assets/2632925/b16c0e74-741e-4f40-a7df-8863eaa62596">
| <img width="1840" alt="classroom after"
src="https://github.com/bevyengine/bevy/assets/2632925/26f9e971-0c63-4ee9-9544-964e5703d65e">
|
![ezgif-1-0f390edd06](https://github.com/bevyengine/bevy/assets/2632925/d8894e52-380f-4528-aa0d-1ca249108178)
|

---

## Changelog

- Ambient occlusion now applies to indirect specular reflections to
approximate how objects occlude specular light.

## Migration Guide

- Renamed `PbrInput::occlusion` to `diffuse_occlusion`, and added
`specular_occlusion`.

crates/bevy_pbr/src/deferred/deferred_lighting.wgsl

@@ -4,6 +4,7 @@
     pbr_functions,
     pbr_deferred_functions::pbr_input_from_deferred_gbuffer,
     pbr_deferred_types::unpack_unorm3x4_plus_unorm_20_,
+    lighting,
     mesh_view_bindings::deferred_prepass_texture,
 }
 
@@ -64,7 +65,15 @@ fn fragment(in: FullscreenVertexOutput) -> @location(0) vec4<f32> {
 #ifdef SCREEN_SPACE_AMBIENT_OCCLUSION
         let ssao = textureLoad(screen_space_ambient_occlusion_texture, vec2<i32>(in.position.xy), 0i).r;
         let ssao_multibounce = gtao_multibounce(ssao, pbr_input.material.base_color.rgb);
-        pbr_input.occlusion = min(pbr_input.occlusion, ssao_multibounce);
+        pbr_input.diffuse_occlusion = min(pbr_input.diffuse_occlusion, ssao_multibounce);
+
+        // Neubelt and Pettineo 2013, "Crafting a Next-gen Material Pipeline for The Order: 1886"
+        let NdotV = max(dot(pbr_input.N, pbr_input.V), 0.0001); 
+        var perceptual_roughness: f32 = pbr_input.material.perceptual_roughness;
+        let roughness = lighting::perceptualRoughnessToRoughness(perceptual_roughness);
+        // Use SSAO to estimate the specular occlusion.
+        // Lagarde and Rousiers 2014, "Moving Frostbite to Physically Based Rendering"
+        pbr_input.specular_occlusion =  saturate(pow(NdotV + ssao, exp2(-16.0 * roughness - 1.0)) - 1.0 + ssao);
 #endif // SCREEN_SPACE_AMBIENT_OCCLUSION
 
         output_color = pbr_functions::apply_pbr_lighting(pbr_input);

crates/bevy_pbr/src/deferred/pbr_deferred_functions.wgsl

@@ -22,18 +22,18 @@ fn deferred_gbuffer_from_pbr_input(in: PbrInput) -> vec4<u32> {
      // Real time occlusion is applied in the deferred lighting pass.
      // Deriving luminance via Rec. 709. coefficients
      // https://en.wikipedia.org/wiki/Rec._709
-    let occlusion = dot(in.occlusion, vec3<f32>(0.2126, 0.7152, 0.0722));
+    let diffuse_occlusion = dot(in.diffuse_occlusion, vec3<f32>(0.2126, 0.7152, 0.0722));
 #ifdef WEBGL2 // More crunched for webgl so we can also fit depth.
     var props = deferred_types::pack_unorm3x4_plus_unorm_20_(vec4(
         in.material.reflectance,
         in.material.metallic,
-        occlusion, 
+        diffuse_occlusion, 
         in.frag_coord.z));
 #else
     var props = deferred_types::pack_unorm4x8_(vec4(
         in.material.reflectance, // could be fewer bits
         in.material.metallic, // could be fewer bits
-        occlusion, // is this worth including?
+        diffuse_occlusion, // is this worth including?
         0.0)); // spare
 #endif // WEBGL2
     let flags = deferred_types::deferred_flags_from_mesh_material_flags(in.flags, in.material.flags);
@@ -85,7 +85,7 @@ fn pbr_input_from_deferred_gbuffer(frag_coord: vec4<f32>, gbuffer: vec4<u32>) ->
     pbr.material.reflectance = props.r;
 #endif // WEBGL2
     pbr.material.metallic = props.g;
-    pbr.occlusion = vec3(props.b);
+    pbr.diffuse_occlusion = vec3(props.b);
     let octahedral_normal = deferred_types::unpack_24bit_normal(gbuffer.a);
     let N = octahedral_decode(octahedral_normal);
 

crates/bevy_pbr/src/render/pbr_fragment.wgsl

@@ -5,6 +5,7 @@
     pbr_bindings,
     pbr_types,
     prepass_utils,
+    lighting,
     mesh_bindings::mesh,
     mesh_view_bindings::view,
     parallax_mapping::parallaxed_uv,
@@ -68,6 +69,9 @@ fn pbr_input_from_standard_material(
     pbr_input.material.base_color *= pbr_bindings::material.base_color;
     pbr_input.material.deferred_lighting_pass_id = pbr_bindings::material.deferred_lighting_pass_id;
 
+    // Neubelt and Pettineo 2013, "Crafting a Next-gen Material Pipeline for The Order: 1886"
+    let NdotV = max(dot(pbr_input.N, pbr_input.V), 0.0001);
+
 #ifdef VERTEX_UVS
     var uv = in.uv;
 
@@ -120,6 +124,7 @@ fn pbr_input_from_standard_material(
         // metallic and perceptual roughness
         var metallic: f32 = pbr_bindings::material.metallic;
         var perceptual_roughness: f32 = pbr_bindings::material.perceptual_roughness;
+        let roughness = lighting::perceptualRoughnessToRoughness(perceptual_roughness);
 #ifdef VERTEX_UVS
         if ((pbr_bindings::material.flags & pbr_types::STANDARD_MATERIAL_FLAGS_METALLIC_ROUGHNESS_TEXTURE_BIT) != 0u) {
             let metallic_roughness = textureSampleBias(pbr_bindings::metallic_roughness_texture, pbr_bindings::metallic_roughness_sampler, uv, view.mip_bias);
@@ -159,20 +164,23 @@ fn pbr_input_from_standard_material(
 #endif
         pbr_input.material.diffuse_transmission = diffuse_transmission;
 
-        // occlusion
-        // TODO: Split into diffuse/specular occlusion?
-        var occlusion: vec3<f32> = vec3(1.0);
+        var diffuse_occlusion: vec3<f32> = vec3(1.0);
+        var specular_occlusion: f32 = 1.0;
 #ifdef VERTEX_UVS
         if ((pbr_bindings::material.flags & pbr_types::STANDARD_MATERIAL_FLAGS_OCCLUSION_TEXTURE_BIT) != 0u) {
-            occlusion = vec3(textureSampleBias(pbr_bindings::occlusion_texture, pbr_bindings::occlusion_sampler, uv, view.mip_bias).r);
+            diffuse_occlusion = vec3(textureSampleBias(pbr_bindings::occlusion_texture, pbr_bindings::occlusion_sampler, uv, view.mip_bias).r);
         }
 #endif
 #ifdef SCREEN_SPACE_AMBIENT_OCCLUSION
         let ssao = textureLoad(screen_space_ambient_occlusion_texture, vec2<i32>(in.position.xy), 0i).r;
         let ssao_multibounce = gtao_multibounce(ssao, pbr_input.material.base_color.rgb);
-        occlusion = min(occlusion, ssao_multibounce);
+        diffuse_occlusion = min(diffuse_occlusion, ssao_multibounce);
+        // Use SSAO to estimate the specular occlusion.
+        // Lagarde and Rousiers 2014, "Moving Frostbite to Physically Based Rendering"
+        specular_occlusion =  saturate(pow(NdotV + ssao, exp2(-16.0 * roughness - 1.0)) - 1.0 + ssao);
 #endif
-        pbr_input.occlusion = occlusion;
+        pbr_input.diffuse_occlusion = diffuse_occlusion;
+        pbr_input.specular_occlusion = specular_occlusion;
 
         // N (normal vector)
 #ifndef LOAD_PREPASS_NORMALS

crates/bevy_pbr/src/render/pbr_functions.wgsl

@@ -164,7 +164,8 @@ fn apply_pbr_lighting(
 
     let specular_transmissive_color = specular_transmission * in.material.base_color.rgb;
 
-    let occlusion = in.occlusion;
+    let diffuse_occlusion = in.diffuse_occlusion;
+    let specular_occlusion = in.specular_occlusion;
 
     // Neubelt and Pettineo 2013, "Crafting a Next-gen Material Pipeline for The Order: 1886"
     let NdotV = max(dot(in.N, in.V), 0.0001);
@@ -306,7 +307,7 @@ fn apply_pbr_lighting(
     }
 
     // Ambient light (indirect)
-    var indirect_light = ambient::ambient_light(in.world_position, in.N, in.V, NdotV, diffuse_color, F0, perceptual_roughness, occlusion);
+    var indirect_light = ambient::ambient_light(in.world_position, in.N, in.V, NdotV, diffuse_color, F0, perceptual_roughness, diffuse_occlusion);
 
     if diffuse_transmission > 0.0 {
         // NOTE: We use the diffuse transmissive color, the second Lambertian lobe's calculated
@@ -316,14 +317,14 @@ fn apply_pbr_lighting(
         // perceptual_roughness = 1.0;
         // NdotV = 1.0;
         // F0 = vec3<f32>(0.0)
-        // occlusion = vec3<f32>(1.0)
+        // diffuse_occlusion = vec3<f32>(1.0)
         transmitted_light += ambient::ambient_light(diffuse_transmissive_lobe_world_position, -in.N, -in.V, 1.0, diffuse_transmissive_color, vec3<f32>(0.0), 1.0, vec3<f32>(1.0));
     }
 
     // Environment map light (indirect)
 #ifdef ENVIRONMENT_MAP
     let environment_light = environment_map::environment_map_light(perceptual_roughness, roughness, diffuse_color, NdotV, f_ab, in.N, R, F0);
-    indirect_light += (environment_light.diffuse * occlusion) + environment_light.specular;
+    indirect_light += (environment_light.diffuse * diffuse_occlusion) + (environment_light.specular * specular_occlusion);
 
     // we'll use the specular component of the transmitted environment
     // light in the call to `specular_transmissive_light()` below
@@ -338,7 +339,7 @@ fn apply_pbr_lighting(
         // NdotV = 1.0;
         // R = T // see definition below
         // F0 = vec3<f32>(1.0)
-        // occlusion = 1.0
+        // diffuse_occlusion = 1.0
         //
         // (This one is slightly different from the other light types above, because the environment
         // map light returns both diffuse and specular components separately, and we want to use both)

crates/bevy_pbr/src/render/pbr_types.wgsl

@@ -80,7 +80,8 @@ fn standard_material_new() -> StandardMaterial {
 
 struct PbrInput {
     material: StandardMaterial,
-    occlusion: vec3<f32>,
+    diffuse_occlusion: vec3<f32>,
+    specular_occlusion: f32,
     frag_coord: vec4<f32>,
     world_position: vec4<f32>,
     // Normalized world normal used for shadow mapping as normal-mapping is not used for shadow
@@ -101,7 +102,8 @@ fn pbr_input_new() -> PbrInput {
     var pbr_input: PbrInput;
 
     pbr_input.material = standard_material_new();
-    pbr_input.occlusion = vec3<f32>(1.0);
+    pbr_input.diffuse_occlusion = vec3<f32>(1.0);
+    pbr_input.specular_occlusion = 1.0;
 
     pbr_input.frag_coord = vec4<f32>(0.0, 0.0, 0.0, 1.0);
     pbr_input.world_position = vec4<f32>(0.0, 0.0, 0.0, 1.0);