Use dual-source blending for rendering the sky (#17672)

# Objective

Since previously we only had the alpha channel available, we stored the
mean of the transmittance in the aerial view lut, resulting in a grayer
fog than should be expected.

## Solution

- Calculate transmittance to scene in `render_sky` with two samples from
the transmittance lut
- use dual-source blending to effectively have per-component alpha
blending

crates/bevy_pbr/src/atmosphere/aerial_view_lut.wgsl

@@ -55,12 +55,9 @@ fn main(@builtin(global_invocation_id) idx: vec3<u32>) {
                 break;
             }
         }
-        // We only have one channel to store transmittance, so we store the mean
-        let mean_transmittance = (throughput.r + throughput.g + throughput.b) / 3.0;
 
         // Store in log space to allow linear interpolation of exponential values between slices
-        let log_transmittance = -log(max(mean_transmittance, 1e-6)); // Avoid log(0)
         let log_inscattering = log(max(total_inscattering, vec3(1e-6)));
-        textureStore(aerial_view_lut_out, vec3(vec2<u32>(idx.xy), slice_i), vec4(log_inscattering, log_transmittance));
+        textureStore(aerial_view_lut_out, vec3(vec2<u32>(idx.xy), slice_i), vec4(log_inscattering, 0.0));
     }
 }

crates/bevy_pbr/src/atmosphere/functions.wgsl

@@ -118,6 +118,27 @@ fn sample_transmittance_lut(r: f32, mu: f32) -> vec3<f32> {
     return textureSampleLevel(transmittance_lut, transmittance_lut_sampler, uv, 0.0).rgb;
 }
 
+// NOTICE: This function is copyrighted by Eric Bruneton and INRIA, and falls
+// under the license reproduced in bruneton_functions.wgsl (variant of MIT license)
+//
+// FIXME: this function should be in bruneton_functions.wgsl, but because naga_oil doesn't 
+// support cyclic imports it's stuck here
+fn sample_transmittance_lut_segment(r: f32, mu: f32, t: f32) -> vec3<f32> {
+    let r_t = get_local_r(r, mu, t);
+    let mu_t = clamp((r * mu + t) / r_t, -1.0, 1.0);
+
+    if ray_intersects_ground(r, mu) {
+        return min(
+            sample_transmittance_lut(r_t, -mu_t) / sample_transmittance_lut(r, -mu),
+            vec3(1.0)
+        );
+    } else {
+        return min(
+            sample_transmittance_lut(r, mu) / sample_transmittance_lut(r_t, mu_t), vec3(1.0)
+        );
+    }
+}
+
 fn sample_multiscattering_lut(r: f32, mu: f32) -> vec3<f32> {
     let uv = multiscattering_lut_r_mu_to_uv(r, mu);
     return textureSampleLevel(multiscattering_lut, multiscattering_lut_sampler, uv, 0.0).rgb;
@@ -130,23 +151,31 @@ fn sample_sky_view_lut(r: f32, ray_dir_as: vec3<f32>) -> vec3<f32> {
     return textureSampleLevel(sky_view_lut, sky_view_lut_sampler, uv, 0.0).rgb;
 }
 
+fn ndc_to_camera_dist(ndc: vec3<f32>) -> f32 {
+    let view_pos = view.view_from_clip * vec4(ndc, 1.0);
+    let t = length(view_pos.xyz / view_pos.w) * settings.scene_units_to_m;
+    return t;
+}
+
 // RGB channels: total inscattered light along the camera ray to the current sample.
 // A channel: average transmittance across all wavelengths to the current sample.
-fn sample_aerial_view_lut(uv: vec2<f32>, depth: f32) -> vec4<f32> {
-    let view_pos = view.view_from_clip * vec4(uv_to_ndc(uv), depth, 1.0);
-    let dist = length(view_pos.xyz / view_pos.w) * settings.scene_units_to_m;
+fn sample_aerial_view_lut(uv: vec2<f32>, t: f32) -> vec3<f32> {
     let t_max = settings.aerial_view_lut_max_distance;
     let num_slices = f32(settings.aerial_view_lut_size.z);
-    // Offset the W coordinate by -0.5 over the max distance in order to 
-    // align sampling position with slice boundaries, since each texel 
-    // stores the integral over its entire slice
-    let uvw = vec3(uv, saturate(dist / t_max - 0.5 / num_slices));
+    // Each texel stores the value of the scattering integral over the whole slice,
+    // which requires us to offset the w coordinate by half a slice. For
+    // example, if we wanted the value of the integral at the boundary between slices,
+    // we'd need to sample at the center of the previous slice, and vice-versa for
+    // sampling in the center of a slice.
+    let uvw = vec3(uv, saturate(t / t_max - 0.5 / num_slices));
     let sample = textureSampleLevel(aerial_view_lut, aerial_view_lut_sampler, uvw, 0.0);
-    // Treat the first slice specially since there is 0 scattering at the camera
-    let delta_slice = t_max / num_slices;
-    let fade = saturate(dist / delta_slice);
+    // Since sampling anywhere between w=0 and w=t_slice will clamp to the first slice,
+    // we need to do a linear step over the first slice towards zero at the camera's
+    // position to recover the correct integral value.
+    let t_slice = t_max / num_slices;
+    let fade = saturate(t / t_slice);
     // Recover the values from log space
-    return exp(sample) * fade;
+    return exp(sample.rgb) * fade;
 }
 
 // PHASE FUNCTIONS

crates/bevy_pbr/src/atmosphere/mod.rs

@@ -46,6 +46,8 @@ use bevy_reflect::Reflect;
 use bevy_render::{
     extract_component::UniformComponentPlugin,
     render_resource::{DownlevelFlags, ShaderType, SpecializedRenderPipelines},
+    renderer::RenderDevice,
+    settings::WgpuFeatures,
 };
 use bevy_render::{
     extract_component::{ExtractComponent, ExtractComponentPlugin},
@@ -157,6 +159,15 @@ impl Plugin for AtmospherePlugin {
         };
 
         let render_adapter = render_app.world().resource::<RenderAdapter>();
+        let render_device = render_app.world().resource::<RenderDevice>();
+
+        if !render_device
+            .features()
+            .contains(WgpuFeatures::DUAL_SOURCE_BLENDING)
+        {
+            warn!("AtmospherePlugin not loaded. GPU lacks support for dual-source blending.");
+            return;
+        }
 
         if !render_adapter
             .get_downlevel_capabilities()

crates/bevy_pbr/src/atmosphere/render_sky.wgsl

@@ -2,10 +2,10 @@
     types::{Atmosphere, AtmosphereSettings},
     bindings::{atmosphere, view, atmosphere_transforms},
     functions::{
-        sample_transmittance_lut, sample_sky_view_lut, 
-        direction_world_to_atmosphere, uv_to_ray_direction,
-        uv_to_ndc, sample_aerial_view_lut, view_radius,
-        sample_sun_illuminance,
+        sample_transmittance_lut, sample_transmittance_lut_segment,
+        sample_sky_view_lut, direction_world_to_atmosphere,
+        uv_to_ray_direction, uv_to_ndc, sample_aerial_view_lut,
+        view_radius, sample_sun_illuminance, ndc_to_camera_dist
     },
 };
 #import bevy_render::view::View;
@@ -18,22 +18,30 @@
 @group(0) @binding(13) var depth_texture: texture_depth_2d;
 #endif
 
-@fragment
-fn main(in: FullscreenVertexOutput) -> @location(0) vec4<f32> {
-    let depth = textureLoad(depth_texture, vec2<i32>(in.position.xy), 0);
-    if depth == 0.0 {
-        let ray_dir_ws = uv_to_ray_direction(in.uv);
-        let ray_dir_as = direction_world_to_atmosphere(ray_dir_ws.xyz);
+struct RenderSkyOutput {
+    @location(0) inscattering: vec4<f32>,
+    @location(0) @second_blend_source transmittance: vec4<f32>,
+}
 
+@fragment
+fn main(in: FullscreenVertexOutput) -> RenderSkyOutput {
+    let depth = textureLoad(depth_texture, vec2<i32>(in.position.xy), 0);
+
+    let ray_dir_ws = uv_to_ray_direction(in.uv);
     let r = view_radius();
     let mu = ray_dir_ws.y;
 
-        let transmittance = sample_transmittance_lut(r, mu);
-        let inscattering = sample_sky_view_lut(r, ray_dir_as);
-
-        let sun_illuminance = sample_sun_illuminance(ray_dir_ws.xyz, transmittance);
-        return vec4(inscattering + sun_illuminance, (transmittance.r + transmittance.g + transmittance.b) / 3.0);
+    var transmittance: vec3<f32>;
+    var inscattering: vec3<f32>;
+    if depth == 0.0 {
+        let ray_dir_as = direction_world_to_atmosphere(ray_dir_ws.xyz);
+        transmittance = sample_transmittance_lut(r, mu);
+        inscattering += sample_sky_view_lut(r, ray_dir_as);
+        inscattering += sample_sun_illuminance(ray_dir_ws.xyz, transmittance);
     } else {
-        return sample_aerial_view_lut(in.uv, depth);
+        let t = ndc_to_camera_dist(vec3(uv_to_ndc(in.uv), depth));
+        inscattering = sample_aerial_view_lut(in.uv, t);
+        transmittance = sample_transmittance_lut_segment(r, mu, t);
     }
+    return RenderSkyOutput(vec4(inscattering, 0.0), vec4(transmittance, 1.0));
 }

crates/bevy_pbr/src/atmosphere/resources.rs

@@ -367,7 +367,7 @@ impl SpecializedRenderPipeline for RenderSkyBindGroupLayouts {
                     blend: Some(BlendState {
                         color: BlendComponent {
                             src_factor: BlendFactor::One,
-                            dst_factor: BlendFactor::SrcAlpha,
+                            dst_factor: BlendFactor::Src1,
                             operation: BlendOperation::Add,
                         },
                         alpha: BlendComponent {