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Improve layering in the solari BRDF #24243

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Merged
alice-i-cecile merged 7 commits into from
May 15, 2026
Merged

Improve layering in the solari BRDF #24243

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1cb0fe4
fix: use energy-conserving layering
dylansechet May 5, 2026
53c1416
refactor: address PR comments
dylansechet May 13, 2026
6afa995
fix: give mirrors an infinite pdf
dylansechet May 13, 2026
33c85db
fix: use evaluate_specular_brdf in mirror case
dylansechet May 13, 2026
f877012
chore: remove extra space
dylansechet May 13, 2026
4c523b3
fix: remove unused import
dylansechet May 15, 2026
5f63ce7
perf: avoid excess F_ab sampling
dylansechet May 15, 2026
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25 changes: 2 additions & 23 deletions crates/bevy_solari/src/pathtracer/pathtracer.wgsl
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Original file line number Diff line number Diff line change
Expand Up @@ -5,7 +5,7 @@ enable wgpu_ray_query;
#import bevy_pbr::utils::{rand_f, rand_vec2f}
#import bevy_render::maths::{PI, orthonormalize}
#import bevy_render::view::View
#import bevy_solari::brdf::{evaluate_brdf, evaluate_and_sample_brdf, fresnel}
#import bevy_solari::brdf::{evaluate_brdf, evaluate_and_sample_brdf, brdf_pdf, lobe_reflectances}
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#import bevy_solari::sampling::{sample_random_light, random_emissive_light_pdf, ggx_vndf_pdf, power_heuristic}
#import bevy_solari::scene_bindings::{trace_ray, resolve_ray_hit_full, ResolvedRayHitFull, RAY_T_MIN, RAY_T_MAX, MIRROR_ROUGHNESS_THRESHOLD}

Expand Down Expand Up @@ -62,7 +62,7 @@ fn pathtrace(@builtin(global_invocation_id) global_id: vec3<u32>) {

mis_weight = 1.0;
if direct_lighting.brdf_rays_can_hit {
let pdf_of_bounce = brdf_pdf(wo, direct_lighting.wi, ray_hit);
let pdf_of_bounce = brdf_pdf(wo, direct_lighting.wi, ray_hit.world_normal, ray_hit.material);
mis_weight = power_heuristic(1.0 / direct_lighting.inverse_pdf, pdf_of_bounce);
}

Expand Down Expand Up @@ -95,24 +95,3 @@ fn pathtrace(@builtin(global_invocation_id) global_id: vec3<u32>) {
textureStore(view_output, global_id.xy, vec4(new_color, 1.0));
}

fn brdf_pdf(wo: vec3<f32>, wi: vec3<f32>, ray_hit: ResolvedRayHitFull) -> f32 {
let NdotV = max(dot(ray_hit.world_normal, wo), 0.0001);
let F0 = calculate_F0(ray_hit.material.base_color, ray_hit.material.metallic, vec3(ray_hit.material.reflectance));
let df = 1.0 - luminance(fresnel(F0, NdotV));

let diffuse_weight = mix(df, 0.0, ray_hit.material.metallic);
let specular_weight = 1.0 - diffuse_weight;

let TBN = orthonormalize(ray_hit.world_normal);
let T = TBN[0];
let B = TBN[1];
let N = TBN[2];

let wo_tangent = vec3(dot(wo, T), dot(wo, B), dot(wo, N));
let wi_tangent = vec3(dot(wi, T), dot(wi, B), dot(wi, N));

let diffuse_pdf = wi_tangent.z / PI;
let specular_pdf = ggx_vndf_pdf(wo_tangent, wi_tangent, ray_hit.material.roughness);
let pdf = (diffuse_weight * diffuse_pdf) + (specular_weight * specular_pdf);
return pdf;
}
75 changes: 55 additions & 20 deletions crates/bevy_solari/src/scene/brdf.wgsl
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Expand Up @@ -4,7 +4,7 @@ enable wgpu_ray_query;

#import bevy_core_pipeline::tonemapping::tonemapping_luminance as luminance
#import bevy_pbr::lighting::{D_GGX, V_SmithGGXCorrelated, specular_multiscatter}
#import bevy_pbr::pbr_functions::{calculate_diffuse_color, calculate_F0}
#import bevy_pbr::pbr_functions::calculate_F0
#import bevy_pbr::utils::{rand_f, sample_cosine_hemisphere}
#import bevy_render::maths::{PI, orthonormalize}
#import bevy_solari::sampling::{sample_ggx_vndf, ggx_vndf_pdf, ggx_vndf_sample_invalid}
Expand All @@ -16,6 +16,22 @@ struct EvaluateAndSampleBrdfResult {
pdf: f32,
}

struct LobeReflectances {
specular: vec3<f32>,
diffuse: vec3<f32>,
}
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// Hemispherical reflectance of each lobe
fn lobe_reflectances(F0: vec3<f32>, material: ResolvedMaterial, NdotV: f32) -> LobeReflectances {
let F_ab = F_AB(material.perceptual_roughness, NdotV);
let multiscattering_factor = 1.0 / (F_ab.x + F_ab.y) - 1.0;
let rho_spec = (F0 * F_ab.x + F_ab.y) * (1.0 + F0 * multiscattering_factor);
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return LobeReflectances(
rho_spec,
(1.0 - material.metallic) * (1.0 - rho_spec) * material.base_color,
);
}

fn evaluate_and_sample_brdf(
wo: vec3<f32>,
world_normal: vec3<f32>,
Expand All @@ -25,10 +41,9 @@ fn evaluate_and_sample_brdf(
let NdotV = dot(world_normal, wo);
if NdotV < 0.0001 { return EvaluateAndSampleBrdfResult(vec3(0.0), vec3(0.0), 0.0); }
let F0 = calculate_F0(material.base_color, material.metallic, vec3(material.reflectance));
let df = 1.0 - luminance(fresnel(F0, NdotV));

let diffuse_weight = mix(df, 0.0, material.metallic);
let specular_weight = 1.0 - diffuse_weight;
let rho = lobe_reflectances(F0, material, NdotV);
let specular_weight = luminance(rho.specular) / luminance(rho.specular + rho.diffuse);
let diffuse_weight = 1.0 - specular_weight;

let TBN = orthonormalize(world_normal);
let T = TBN[0];
Expand All @@ -49,19 +64,21 @@ fn evaluate_and_sample_brdf(
return EvaluateAndSampleBrdfResult(vec3(0.0), vec3(0.0), 0.0);
}
wi = wi_tangent.x * T + wi_tangent.y * B + wi_tangent.z * N;

// Mirror specular is a delta function
if material.roughness <= MIRROR_ROUGHNESS_THRESHOLD {
return EvaluateAndSampleBrdfResult(
wi,
rho.specular / specular_weight,

@JMS55 JMS55 May 13, 2026

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I don't think rho.specular / specular_weight is right here. You need to evaluate the BRDF.

@dylansechet dylansechet May 13, 2026

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It should be the same thing, as the total reflected energy for a mirror is the same as the brdf evaluated for the reflected ray.
Doing a brdf evaluation does feel more readable though, and the lut at 0 roughness probably isn't that great.

bitcast<f32>(0x7F800000u) // INF
);
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}
}

let diffuse_pdf = wi_tangent.z / PI;
let specular_pdf = ggx_vndf_pdf(wo_tangent, wi_tangent, material.roughness);
let pdf = (diffuse_weight * diffuse_pdf) + (specular_weight * specular_pdf);

var throughput = evaluate_brdf(wo, wi, world_normal, material);
if diffuse_selected || material.roughness > MIRROR_ROUGHNESS_THRESHOLD {
throughput /= pdf;
} else {
throughput /= specular_weight;
}

let pdf = specular_weight * specular_pdf + diffuse_weight * diffuse_pdf;
let throughput = evaluate_brdf(wo, wi, world_normal, material) / pdf;
return EvaluateAndSampleBrdfResult(wi, throughput, pdf);
}

Expand All @@ -75,15 +92,12 @@ fn evaluate_brdf(
}

fn evaluate_diffuse_brdf(wo: vec3<f32>, wi: vec3<f32>, world_normal: vec3<f32>, material: ResolvedMaterial) -> vec3<f32> {
let diffuse_color = calculate_diffuse_color(material.base_color, material.metallic, 0.0, 0.0) / PI;

let NdotL = dot(world_normal, wi);
let NdotV = dot(world_normal, wo);
if NdotL < 0.0001 || NdotV < 0.0001 { return vec3(0.0); }
let F0 = calculate_F0(material.base_color, material.metallic, vec3(material.reflectance));
let layering = (1.0 - fresnel(F0, NdotL)) * (1.0 - fresnel(F0, NdotV));

return diffuse_color * layering * NdotL;
let rho = lobe_reflectances(F0, material, NdotV);
return rho.diffuse / PI * NdotL;
}

fn evaluate_specular_brdf(wo: vec3<f32>, wi: vec3<f32>, world_normal: vec3<f32>, material: ResolvedMaterial) -> vec3<f32> {
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Expand All @@ -93,7 +107,7 @@ fn evaluate_specular_brdf(wo: vec3<f32>, wi: vec3<f32>, world_normal: vec3<f32>,
let LdotH = dot(wi, H);
let NdotV = dot(world_normal, wo);
if NdotL < 0.0001 || NdotH < 0.0001 || LdotH < 0.0001 || NdotV < 0.0001 { return vec3(0.0); }

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let F0 = calculate_F0(material.base_color, material.metallic, vec3(material.reflectance));
let F = fresnel(F0, LdotH);

Expand All @@ -111,6 +125,27 @@ fn evaluate_specular_brdf(wo: vec3<f32>, wi: vec3<f32>, world_normal: vec3<f32>,
return specular_multiscatter(D, Vs, F, F0, F_ab, 1.0) * NdotL;
}

fn brdf_pdf(wo: vec3<f32>, wi: vec3<f32>, world_normal: vec3<f32>, material: ResolvedMaterial) -> f32 {
let NdotV = max(dot(world_normal, wo), 0.0001);
let F0 = calculate_F0(material.base_color, material.metallic, vec3(material.reflectance));
let rho = lobe_reflectances(F0, material, NdotV);
let specular_weight = luminance(rho.specular) / luminance(rho.specular + rho.diffuse);
let diffuse_weight = 1.0 - specular_weight;

let TBN = orthonormalize(world_normal);
let T = TBN[0];
let B = TBN[1];
let N = TBN[2];

let wo_tangent = vec3(dot(wo, T), dot(wo, B), dot(wo, N));
let wi_tangent = vec3(dot(wi, T), dot(wi, B), dot(wi, N));

let diffuse_pdf = wi_tangent.z / PI;
let specular_pdf = ggx_vndf_pdf(wo_tangent, wi_tangent, material.roughness);
return specular_weight * specular_pdf + diffuse_weight * diffuse_pdf;
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}


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fn fresnel(f0: vec3<f32>, LdotH: f32) -> vec3<f32> {
return f0 + (1.0 - f0) * pow(1.0 - LdotH, 5.0);
}
Expand Down
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