Use dedicated volume attenuation function

Author: Dahmen issam

crates/bevy_pbr/src/render/pbr_functions.wgsl

@@ -302,6 +302,28 @@ fn calculate_F0(base_color: vec3<f32>, metallic: f32, reflectance: vec3<f32>) ->
     return mix(calculate_F0_dielectric(reflectance), base_color, metallic);
 }
 
+#ifdef STANDARD_MATERIAL_SPECULAR_TRANSMISSION
+fn volume_attenuation(
+    radiance: vec3<f32>,
+    transmission_distance: f32,
+    attenuation_color: vec3<f32>,
+    attenuation_distance: f32,
+) -> vec3<f32> {
+    if attenuation_distance == 0.0 {
+        // Attenuation distance is +∞, indicated by zero, so the transmitted
+        // color is not attenuated.
+        return radiance;
+    } else {
+        // Compute light attenuation using Beer's law.
+        let transmittance = pow(
+            attenuation_color,
+            vec3<f32>(transmission_distance / attenuation_distance),
+        );
+        return transmittance * radiance;
+    }
+}
+#endif  // STANDARD_MATERIAL_SPECULAR_TRANSMISSION
+
 #ifdef CONTACT_SHADOWS
 #ifdef DEPTH_PREPASS
 fn calculate_contact_shadow(
@@ -852,19 +874,12 @@ fn apply_pbr_lighting(
     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;
 
     if (in.material.flags & pbr_types::STANDARD_MATERIAL_FLAGS_ATTENUATION_ENABLED_BIT) != 0u {
-        // We reuse the `atmospheric_fog()` function here, as it's fundamentally
-        // equivalent to the attenuation that takes place inside the material volume,
-        // and will allow us to eventually hook up subsurface scattering more easily
-        var attenuation_fog: mesh_view_types::Fog;
-        attenuation_fog.base_color.a = 1.0;
-        let attenuation_color = max(in.material.attenuation_color.rgb, vec3<f32>(1e-6));
-        attenuation_fog.be = -log(attenuation_color) / in.material.attenuation_distance;
-        // TODO: Add the subsurface scattering factor below
-        // attenuation_fog.bi = /* ... */
-        transmitted_light = bevy_pbr::fog::atmospheric_fog(
-            attenuation_fog, vec4<f32>(transmitted_light, 1.0), thickness,
-            vec3<f32>(0.0) // TODO: Pass in (pre-attenuated) scattered light contribution here
-        ).rgb;
+        transmitted_light = volume_attenuation(
+            transmitted_light,
+            thickness,
+            in.material.attenuation_color.rgb,
+            in.material.attenuation_distance,
+        );
     }
 #endif