1enable wgpu_ray_query;
2
3#import bevy_core_pipeline::tonemapping::tonemapping_luminance as luminance
4#import bevy_pbr::pbr_functions::calculate_F0
5#import bevy_pbr::utils::{rand_f, rand_vec2f}
6#import bevy_render::maths::{PI, orthonormalize}
7#import bevy_render::view::View
8#import bevy_solari::brdf::{evaluate_brdf, evaluate_and_sample_brdf, fresnel}
9#import bevy_solari::sampling::{sample_random_light, random_emissive_light_pdf, ggx_vndf_pdf, power_heuristic}
10#import bevy_solari::scene_bindings::{trace_ray, resolve_ray_hit_full, ResolvedRayHitFull, RAY_T_MIN, RAY_T_MAX, MIRROR_ROUGHNESS_THRESHOLD}
11
12@group(1) @binding(0) var accumulation_texture: texture_storage_2d<rgba32float, read_write>;
13@group(1) @binding(1) var view_output: texture_storage_2d<rgba16float, write>;
14@group(1) @binding(2) var<uniform> view: View;
15
16@compute @workgroup_size(8, 8, 1)
17fn pathtrace(@builtin(global_invocation_id) global_id: vec3<u32>) {
18    if any(global_id.xy >= vec2u(view.viewport.zw)) {
19        return;
20    }
21
22    let old_color = textureLoad(accumulation_texture, global_id.xy);
23
24    // Setup RNG
25    let pixel_index = global_id.x + global_id.y * u32(view.viewport.z);
26    let frame_index = u32(old_color.a) * 5782582u;
27    var rng = pixel_index + frame_index;
28
29    // Shoot the first ray from the camera
30    let pixel_center = vec2<f32>(global_id.xy) + 0.5;
31    let jitter = rand_vec2f(&rng) - 0.5;
32    let pixel_uv = (pixel_center + jitter) / view.viewport.zw;
33    let pixel_ndc = (pixel_uv * 2.0) - 1.0;
34    let primary_ray_target = view.world_from_clip * vec4(pixel_ndc.x, -pixel_ndc.y, 1.0, 1.0);
35    var ray_origin = view.world_position;
36    var ray_direction = normalize((primary_ray_target.xyz / primary_ray_target.w) - ray_origin);
37    var ray_t_min = 0.0;
38
39    // Path trace
40    var radiance = vec3(0.0);
41    var throughput = vec3(1.0);
42    var p_bounce = 0.0;
43    loop {
44        let ray = trace_ray(ray_origin, ray_direction, ray_t_min, RAY_T_MAX, RAY_FLAG_NONE);
45        if ray.kind != RAY_QUERY_INTERSECTION_NONE {
46            let ray_hit = resolve_ray_hit_full(ray);
47            let wo = -ray_direction;
48
49            // Emissive contribution
50            var mis_weight = 1.0;
51            if p_bounce != 0.0 { // Not first bounce
52                let p_light = random_emissive_light_pdf(ray_hit);
53                mis_weight = power_heuristic(p_bounce, p_light);
54            }
55            radiance += mis_weight * throughput * ray_hit.material.emissive;
56
57            // Sample direct lighting, but only if the surface is not mirror-like
58            // TODO: randomly choose to use NEE or not with probability proportional to roughness and metallicness
59            let is_perfectly_specular = ray_hit.material.roughness <= MIRROR_ROUGHNESS_THRESHOLD && ray_hit.material.metallic > 0.9999;
60            if !is_perfectly_specular {
61                let direct_lighting = sample_random_light(ray_hit.world_position, ray_hit.world_normal, &rng);
62
63                mis_weight = 1.0;
64                if direct_lighting.brdf_rays_can_hit {
65                    let pdf_of_bounce = brdf_pdf(wo, direct_lighting.wi, ray_hit);
66                    mis_weight = power_heuristic(1.0 / direct_lighting.inverse_pdf, pdf_of_bounce);
67                }
68
69                let direct_lighting_brdf = evaluate_brdf(wo, direct_lighting.wi, ray_hit.world_normal, ray_hit.material);
70                radiance += mis_weight * throughput * direct_lighting.radiance * direct_lighting.inverse_pdf * direct_lighting_brdf;
71            }
72
73            // Sample new ray direction from the material BRDF for next bounce and apply BRDF
74            let next_bounce = evaluate_and_sample_brdf(wo, ray_hit.world_normal, ray_hit.material, &rng);
75            if next_bounce.pdf == 0.0 { break; }
76            ray_direction = next_bounce.wi;
77            ray_origin = ray_hit.world_position + (ray_hit.geometric_world_normal * RAY_T_MIN);
78            ray_t_min = RAY_T_MIN;
79            p_bounce = next_bounce.pdf;
80            throughput *= next_bounce.throughput;
81
82            // Russian roulette for early termination
83            let p = luminance(throughput);
84            if rand_f(&rng) > p { break; }
85            throughput /= p;
86        } else { break; }
87    }
88
89    // Camera exposure
90    radiance *= view.exposure;
91
92    // Accumulation over time via running average
93    let new_color = mix(old_color.rgb, radiance, 1.0 / (old_color.a + 1.0));
94    textureStore(accumulation_texture, global_id.xy, vec4(new_color, old_color.a + 1.0));
95    textureStore(view_output, global_id.xy, vec4(new_color, 1.0));
96}
97
98fn brdf_pdf(wo: vec3<f32>, wi: vec3<f32>, ray_hit: ResolvedRayHitFull) -> f32 {
99    let NdotV = max(dot(ray_hit.world_normal, wo), 0.0001);
100    let F0 = calculate_F0(ray_hit.material.base_color, ray_hit.material.metallic, vec3(ray_hit.material.reflectance));
101    let df = 1.0 - luminance(fresnel(F0, NdotV));
102
103    let diffuse_weight = mix(df, 0.0, ray_hit.material.metallic);
104    let specular_weight = 1.0 - diffuse_weight;
105
106    let TBN = orthonormalize(ray_hit.world_normal);
107    let T = TBN[0];
108    let B = TBN[1];
109    let N = TBN[2];
110
111    let wo_tangent = vec3(dot(wo, T), dot(wo, B), dot(wo, N));
112    let wi_tangent = vec3(dot(wi, T), dot(wi, B), dot(wi, N));
113
114    let diffuse_pdf = wi_tangent.z / PI;
115    let specular_pdf = ggx_vndf_pdf(wo_tangent, wi_tangent, ray_hit.material.roughness);
116    let pdf = (diffuse_weight * diffuse_pdf) + (specular_weight * specular_pdf);
117    return pdf;
118}