1#import bevy_pbr::{
2    atmosphere::{
3        bindings::settings,
4        functions::{
5            sample_density_lut, sample_local_inscattering, uv_to_ray_direction, get_view_position,
6            MIDPOINT_RATIO, MIN_EXTINCTION, ABSORPTION_DENSITY, SCATTERING_DENSITY,
7        },
8    }
9}
10
11
12@group(0) @binding(13) var aerial_view_lut_out: texture_storage_3d<rgba16float, write>;
13
14@compute
15@workgroup_size(16, 16, 1)
16fn main(@builtin(global_invocation_id) idx: vec3<u32>) {
17    if any(idx.xy > settings.aerial_view_lut_size.xy) { return; }
18
19    let uv = (vec2<f32>(idx.xy) + 0.5) / vec2<f32>(settings.aerial_view_lut_size.xy);
20    let ray_dir = uv_to_ray_direction(uv);
21    let world_pos = get_view_position();
22
23    let r = length(world_pos);
24    let t_max = settings.aerial_view_lut_max_distance;
25
26    var prev_t = 0.0;
27    var total_inscattering = vec3(0.0);
28    var throughput = vec3(1.0);
29
30    for (var slice_i: u32 = 0; slice_i < settings.aerial_view_lut_size.z; slice_i++) {
31        for (var step_i: u32 = 0; step_i < settings.aerial_view_lut_samples; step_i++) {
32            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);
33            let dt = (t_i - prev_t);
34            prev_t = t_i;
35
36            let sample_pos = world_pos + ray_dir * t_i;
37            let local_r = length(sample_pos);
38            let local_up = normalize(sample_pos);
39
40            let absorption = sample_density_lut(local_r, ABSORPTION_DENSITY);
41            let scattering = sample_density_lut(local_r, SCATTERING_DENSITY);
42            let extinction = absorption + scattering;
43
44            let sample_optical_depth = extinction * dt;
45            let sample_transmittance = exp(-sample_optical_depth);
46
47            // evaluate one segment of the integral
48            var inscattering = sample_local_inscattering(scattering, ray_dir, sample_pos);
49
50            // Analytical integration of the single scattering term in the radiance transfer equation
51            let s_int = (inscattering - inscattering * sample_transmittance) / max(extinction, MIN_EXTINCTION);
52            total_inscattering += throughput * s_int;
53
54            throughput *= sample_transmittance;
55            if all(throughput < vec3(0.001)) {
56                break;
57            }
58        }
59
60        // Store in log space to allow linear interpolation of exponential values between slices
61        let log_inscattering = log(max(total_inscattering, vec3(1e-6)));
62        textureStore(aerial_view_lut_out, vec3(vec2<u32>(idx.xy), slice_i), vec4(log_inscattering, 0.0));
63    }
64}