Atmosphere LUT parameterization improvements

crates/bevy_pbr/src/atmosphere/aerial_view_lut.wgsl

@@ -32,10 +32,7 @@ fn main(@builtin(global_invocation_id) idx: vec3<u32>) {
 
     for (var slice_i: u32 = 0; slice_i < settings.aerial_view_lut_size.z; slice_i++) {
         for (var step_i: u32 = 0; step_i < settings.aerial_view_lut_samples; step_i++) {
-            // Offset by -0.5 to align sampling position with slice boundaries, 
-            // since each texel stores the integral over its entire slice
-            var t_i = t_max * (f32(slice_i) - 0.5 + ((f32(step_i) + MIDPOINT_RATIO) / f32(settings.aerial_view_lut_samples))) / f32(settings.aerial_view_lut_size.z);
-            t_i = max(t_i, 0.0);
+            let t_i = t_max * (f32(slice_i) + ((f32(step_i) + MIDPOINT_RATIO) / f32(settings.aerial_view_lut_samples))) / f32(settings.aerial_view_lut_size.z);
             let dt = (t_i - prev_t);
             prev_t = t_i;
 
@@ -61,9 +58,9 @@ fn main(@builtin(global_invocation_id) idx: vec3<u32>) {
         // 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 the optical depth so that it can be linearly sampled between slices
-        let optical_depth = -log(max(mean_transmittance, 1e-6)); // Avoid log(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, optical_depth));
+        textureStore(aerial_view_lut_out, vec3(vec2<u32>(idx.xy), slice_i), vec4(log_inscattering, log_transmittance));
     }
 }

crates/bevy_pbr/src/atmosphere/functions.wgsl

@@ -135,10 +135,18 @@ fn sample_sky_view_lut(r: f32, ray_dir_as: vec3<f32>) -> vec3<f32> {
 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;
-    let uvw = vec3(uv, dist / settings.aerial_view_lut_max_distance);
+    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));
     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);
     // Recover the values from log space
-    return vec4(exp(sample.rgb), exp(-sample.a));
+    return vec4(exp(sample.rgb) * fade, exp(sample.a) * fade);
 }
 
 // PHASE FUNCTIONS