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Fix #158426: Improve energy conservation for multi-scatter glass BSDF

Browse source 
This patch improves the multi-scatter approximations for isotropic GGX
models to pass the furnace tests. The resolution of the look up tables
is increased as well as the mu (cosine(theta)) parametrization is
optimized to increase resolution at grazing angles.

Pull Request: https://projects.blender.org/blender/blender/pulls/159635
This commit is contained in:
Sebastian 2026-08-27 09:42:40 +02:00 • committed by Sebastian Herholz
parent c94b716c80
commit 88b7cc8a52
23 changed files with 4483 additions and 1145 deletions
Download patch file Download diff file Expand all files Collapse all files

81
intern/cycles/app/cycles_precompute.cpp
View file

@ -138,6 +138,13 @@ inline float ior_parametrization(const float z)
return ior_from_F0(sqr(sqr(z)));
}
inline float mu_parametrization(const float y)
{
/* This parametrization increases the resolution at grazing angles, where the Fresnel
* can have a significant influence on the directional albedo. */
return y * y;
}
struct PrecomputeTerm {
int samples;
int nx, ny, nz;
@ -148,30 +155,42 @@ static bool cycles_precompute(std::string name)
{
std::map<string, PrecomputeTerm> precompute_terms;
/* Overall albedo of the GGX microfacet BRDF, depending on cosI and roughness. */
precompute_terms["ggx_E"] = {
1 << 23, 32, 32, 1, [](const float rough, const float mu, float, const float3 rand) {
return precompute_ggx_E(rough, mu, rand);
}};
precompute_terms["ggx_E"] = {1 << 23,
GGX_E_RES_ROUGH,
GGX_E_RES_MU,
1,
[](const float rough, const float y, float, const float3 rand) {
const float mu = mu_parametrization(y);
return precompute_ggx_E(rough, mu, rand);
}};
/* Overall albedo of the GGX microfacet BRDF, averaged over cosI */
precompute_terms["ggx_Eavg"] = {
1 << 26, 32, 1, 1, [](const float rough, const float mu, float, const float3 rand) {
return 2.0f * mu * precompute_ggx_E(rough, mu, rand);
}};
precompute_terms["ggx_Eavg"] = {1 << 26,
GGX_E_RES_ROUGH,
1,
1,
[](const float rough, const float mu, float, const float3 rand) {
return 2.0f * mu * precompute_ggx_E(rough, mu, rand);
}};
/* Overall albedo of the GGX microfacet BSDF with dielectric Fresnel,
* depending on cosI and roughness, for IOR>1. */
precompute_terms["ggx_glass_E"] = {
1 << 23,
16,
16,
16,
[](const float rough, const float mu, const float z, const float3 rand) {
GGX_GLASS_E_RES_ROUGH,
GGX_GLASS_E_RES_MU,
GGX_GLASS_E_RES_IOR,
[](const float rough, const float y, const float z, const float3 rand) {
const float ior = ior_parametrization(z);
const float mu = mu_parametrization(y);
return precompute_ggx_glass_E(rough, mu, ior, rand);
}};
/* Overall albedo of the GGX microfacet BSDF with dielectric Fresnel,
* averaged over cosI, for IOR>1. */
precompute_terms["ggx_glass_Eavg"] = {
1 << 26, 16, 1, 16, [](const float rough, const float mu, const float z, const float3 rand) {
1 << 26,
GGX_GLASS_E_RES_ROUGH,
1,
GGX_GLASS_E_RES_IOR,
[](const float rough, const float mu, const float z, const float3 rand) {
const float ior = ior_parametrization(z);
return 2.0f * mu * precompute_ggx_glass_E(rough, mu, ior, rand);
}};
@ -179,17 +198,22 @@ static bool cycles_precompute(std::string name)
* depending on cosI and roughness, for IOR<1. */
precompute_terms["ggx_glass_inv_E"] = {
1 << 23,
16,
16,
16,
[](const float rough, const float mu, const float z, const float3 rand) {
GGX_GLASS_E_RES_ROUGH,
GGX_GLASS_E_RES_MU,
GGX_GLASS_E_RES_IOR,
[](const float rough, const float y, const float z, const float3 rand) {
const float ior = ior_parametrization(z);
const float mu = mu_parametrization(y);
return precompute_ggx_glass_E(rough, mu, 1.0f / ior, rand);
}};
/* Overall albedo of the GGX microfacet BSDF with dielectric Fresnel,
* averaged over cosI, for IOR<1. */
precompute_terms["ggx_glass_inv_Eavg"] = {
1 << 26, 16, 1, 16, [](const float rough, const float mu, const float z, const float3 rand) {
1 << 26,
GGX_GLASS_E_RES_ROUGH,
1,
GGX_GLASS_E_RES_IOR,
[](const float rough, const float mu, const float z, const float3 rand) {
const float ior = ior_parametrization(z);
return 2.0f * mu * precompute_ggx_glass_E(rough, mu, 1.0f / ior, rand);
}};
@ -198,11 +222,12 @@ static bool cycles_precompute(std::string name)
* depending on cosI and roughness, for IOR>1, using dielectric Fresnel mode. */
precompute_terms["ggx_gen_schlick_ior_s"] = {
1 << 20,
16,
16,
16,
[](const float rough, const float mu, const float z, const float3 rand) {
GGX_GEN_SCHLICK_IOR_S_RES_ROUGH,
GGX_GEN_SCHLICK_IOR_S_RES_MU,
GGX_GEN_SCHLICK_IOR_S_RES_IOR,
[](const float rough, const float y, const float z, const float3 rand) {
const float ior = ior_parametrization(z);
const float mu = mu_parametrization(y);
return precompute_ggx_gen_schlick_s(rough, mu, ior, -1.0f, rand);
}};
@ -210,12 +235,13 @@ static bool cycles_precompute(std::string name)
* depending on cosI and roughness, for IOR>1. */
precompute_terms["ggx_gen_schlick_s"] = {
1 << 20,
16,
16,
16,
[](const float rough, const float mu, const float z, const float3 rand) {
GGX_GEN_SCHLICK_S_RES_ROUGH,
GGX_GEN_SCHLICK_S_RES_MU,
GGX_GEN_SCHLICK_S_RES_IOR,
[](const float rough, const float y, const float z, const float3 rand) {
/* Remap 0..1 to 0..inf, with 0.5 mapping to 5 (the default value). */
const float exponent = 5.0f * ((1.0f - z) / z);
const float mu = mu_parametrization(y);
return precompute_ggx_gen_schlick_s(rough, mu, 1.0f, exponent, rand);
}};
@ -242,7 +268,8 @@ static bool cycles_precompute(std::string name)
const float4 rand = sobol_burley_sample_4D(sample, 0, seed, 0xffffffff);
const float rough = (nx == 1) ? 0.0f : clamp(float(x) / float(nx - 1), 1e-4f, 1.0f);
const float mu = (ny == 1) ? rand.w : clamp(float(y) / float(ny - 1), 1e-4f, 1.0f);
/* We use 1e-2f as min for mu since to the y*y mu parametrization results in 1e-4f. */
const float mu = (ny == 1) ? rand.w : clamp(float(y) / float(ny - 1), 1e-2f, 1.0f);
const float ior = (nz == 1) ? 0.0f : clamp(float(z) / float(nz - 1), 1e-4f, 0.99f);
float value = term.evaluation(rough, mu, ior, make_float3(rand));

1
intern/cycles/kernel/CMakeLists.txt
View file

@ -249,6 +249,7 @@ set(SRC_KERNEL_UTIL_HEADERS
)
set(SRC_KERNEL_TYPES_HEADERS
constants.h
data_arrays.h
data_template.h
features.h

96
intern/cycles/kernel/closure/bsdf_microfacet.h
View file

@ -9,6 +9,7 @@
#include "kernel/closure/bsdf_transparent.h"
#include "kernel/closure/bsdf_util.h"
#include "kernel/constants.h"
#include "kernel/sample/mapping.h"
#include "kernel/util/lookup_table.h"
@ -131,6 +132,30 @@ struct Coat {
Spectrum weight;
};
/* -------------------------------------------------------------------- */
/** \name GGX LUT Index Mapping Functions
*
* This section contains functions to map from GGX alphas, IOR, and mu to the indicies of
* 2D and 3D LUTs (e.g., for multis-scatter GGX energy compensation or scattering albedos).
* \{ */
ccl_device_forceinline float alpha_to_roughness_index(const float alpha_x, const float alpha_y)
{
return sqrtf(sqrtf(alpha_x * alpha_y));
}
ccl_device_forceinline float ior_to_z_index(const float ior)
{
return sqrtf(fabsf((ior - 1.0f) / (ior + 1.0f)));
}
ccl_device_forceinline float mu_to_y_index(const float mu)
{
return safe_sqrtf(mu);
}
/** \} */
ccl_device_inline void adjust_thin_film_ior_at_backface(ccl_private float &film_ior,
const float inv_bulk_ior)
{
@ -512,15 +537,17 @@ ccl_device_inline void microfacet_ggx_preserve_energy(KernelGlobals kg,
const Spectrum Fss)
{
const float mu = dot(wi, bsdf->N);
const float rough = sqrtf(sqrtf(bsdf->alpha_x * bsdf->alpha_y));
const float y = mu_to_y_index(mu);
const float rough = alpha_to_roughness_index(bsdf->alpha_x, bsdf->alpha_y);
float E;
float E_avg;
if (bsdf->type == CLOSURE_BSDF_MICROFACET_GGX_ID ||
bsdf->type == CLOSURE_BSDF_THIN_GLASS_TRANSMISSION_ID)
{
E = lookup_table_read_2D(kg, rough, mu, kernel_data.tables.ggx_E, 32, 32);
E_avg = lookup_table_read(kg, rough, kernel_data.tables.ggx_Eavg, 32);
E = lookup_table_read_2D(
kg, rough, y, kernel_data.tables.ggx_E, GGX_E_RES_ROUGH, GGX_E_RES_MU);
E_avg = lookup_table_read(kg, rough, kernel_data.tables.ggx_Eavg, GGX_E_RES_ROUGH);
}
else if (bsdf->type == CLOSURE_BSDF_MICROFACET_GGX_GLASS_ID) {
int ofs = kernel_data.tables.ggx_glass_E;
@ -531,10 +558,11 @@ ccl_device_inline void microfacet_ggx_preserve_energy(KernelGlobals kg,
ofs = kernel_data.tables.ggx_glass_inv_E;
avg_ofs = kernel_data.tables.ggx_glass_inv_Eavg;
}
/* TODO: Bias mu towards more precision for low values. */
const float z = sqrtf(fabsf((ior - 1.0f) / (ior + 1.0f)));
E = lookup_table_read_3D(kg, rough, mu, z, ofs, 16, 16, 16);
E_avg = lookup_table_read_2D(kg, rough, z, avg_ofs, 16, 16);
const float z = ior_to_z_index(ior);
E = lookup_table_read_3D(
kg, rough, y, z, ofs, GGX_GLASS_E_RES_ROUGH, GGX_GLASS_E_RES_MU, GGX_GLASS_E_RES_IOR);
E_avg = lookup_table_read_2D(
kg, rough, z, avg_ofs, GGX_GLASS_E_RES_ROUGH, GGX_GLASS_E_RES_IOR);
}
else {
kernel_assert(false);
@ -593,17 +621,31 @@ ccl_device Spectrum bsdf_microfacet_estimate_albedo(KernelGlobals kg,
* reflection approximation from the microfacet_fresnel call above in that case. */
}
else {
const float rough = sqrtf(sqrtf(bsdf->alpha_x * bsdf->alpha_y));
const float rough = alpha_to_roughness_index(bsdf->alpha_x, bsdf->alpha_y);
float s;
if (fresnel->exponent < 0.0f) {
const float z = sqrtf(fabsf((bsdf->ior - 1.0f) / (bsdf->ior + 1.0f)));
s = lookup_table_read_3D(
kg, rough, cos_NI, z, kernel_data.tables.ggx_gen_schlick_ior_s, 16, 16, 16);
const float z = ior_to_z_index(bsdf->ior);
const float y = mu_to_y_index(cos_NI);
s = lookup_table_read_3D(kg,
rough,
y,
z,
kernel_data.tables.ggx_gen_schlick_ior_s,
GGX_GEN_SCHLICK_IOR_S_RES_ROUGH,
GGX_GEN_SCHLICK_IOR_S_RES_MU,
GGX_GEN_SCHLICK_IOR_S_RES_IOR);
}
else {
const float z = 1.0f / (0.2f * fresnel->exponent + 1.0f);
s = lookup_table_read_3D(
kg, rough, cos_NI, z, kernel_data.tables.ggx_gen_schlick_s, 16, 16, 16);
const float y = mu_to_y_index(cos_NI);
s = lookup_table_read_3D(kg,
rough,
y,
z,
kernel_data.tables.ggx_gen_schlick_s,
GGX_GEN_SCHLICK_S_RES_ROUGH,
GGX_GEN_SCHLICK_S_RES_MU,
GGX_GEN_SCHLICK_S_RES_IOR);
}
coeff *= fresnel->tint * mix(fresnel->f0, fresnel->f90, s);
return coeff.sum();
@ -617,9 +659,16 @@ ccl_device Spectrum bsdf_microfacet_estimate_albedo(KernelGlobals kg,
* reflection approximation from the microfacet_fresnel call above in that case. */
}
else {
const float rough = sqrtf(sqrtf(bsdf->alpha_x * bsdf->alpha_y));
const float s = lookup_table_read_3D(
kg, rough, cos_NI, 0.5f, kernel_data.tables.ggx_gen_schlick_s, 16, 16, 16);
const float rough = alpha_to_roughness_index(bsdf->alpha_x, bsdf->alpha_y);
const float y = mu_to_y_index(cos_NI);
const float s = lookup_table_read_3D(kg,
rough,
y,
0.5f,
kernel_data.tables.ggx_gen_schlick_s,
GGX_GEN_SCHLICK_S_RES_ROUGH,
GGX_GEN_SCHLICK_S_RES_MU,
GGX_GEN_SCHLICK_S_RES_IOR);
/* TODO: Precompute B factor term and account for it here. */
const Spectrum reflectance = mix(fresnel->f0, one_spectrum(), s) * float(eval_reflection);
return reflectance;
@ -631,10 +680,17 @@ ccl_device Spectrum bsdf_microfacet_estimate_albedo(KernelGlobals kg,
{
/* We can re-use the ggx_gen_schlick_ior_s table here, since it's already precomputed for our
* exponent<0 corner case where we use the real dielectric Fresnel. */
const float rough = sqrtf(sqrtf(bsdf->alpha_x * bsdf->alpha_y));
const float z = sqrtf(fabsf((bsdf->ior - 1.0f) / (bsdf->ior + 1.0f)));
const float s = lookup_table_read_3D(
kg, rough, cos_NI, z, kernel_data.tables.ggx_gen_schlick_ior_s, 16, 16, 16);
const float rough = alpha_to_roughness_index(bsdf->alpha_x, bsdf->alpha_y);
const float z = ior_to_z_index(bsdf->ior);
const float y = mu_to_y_index(cos_NI);
const float s = lookup_table_read_3D(kg,
rough,
y,
z,
kernel_data.tables.ggx_gen_schlick_ior_s,
GGX_GEN_SCHLICK_IOR_S_RES_ROUGH,
GGX_GEN_SCHLICK_IOR_S_RES_MU,
GGX_GEN_SCHLICK_IOR_S_RES_IOR);
#if defined(__KERNEL_HIP__)
/* Temporary workaround for a HIP compiler bug under Windows in combination with AMD 6800 XT

6
intern/cycles/kernel/closure/bsdf_principled_hair_huang.h
View file

@ -8,6 +8,7 @@
#pragma once
#include "kernel/constants.h"
#include "kernel/types.h"
#include "kernel/closure/bsdf_microfacet.h"
@ -319,7 +320,10 @@ ccl_device_forceinline float bsdf_hair_huang_energy_scale(KernelGlobals kg,
const bool inv_table = (ior < 1.0f);
const int ofs = inv_table ? kernel_data.tables.ggx_glass_inv_E : kernel_data.tables.ggx_glass_E;
const float z = sqrtf(fabsf((ior - 1.0f) / (ior + 1.0f)));
return 1.0f / lookup_table_read_3D(kg, rough, mu, z, ofs, 16, 16, 16);
const float y = mu_to_y_index(mu);
return 1.0f /
lookup_table_read_3D(
kg, rough, y, z, ofs, GGX_GLASS_E_RES_ROUGH, GGX_GLASS_E_RES_MU, GGX_GLASS_E_RES_IOR);
}
/* Sample microfacets from a tilted mesonormal. */

19
intern/cycles/kernel/closure/bsdf_sheen.h
View file

@ -12,6 +12,7 @@
#include "kernel/closure/alloc.h"
#include "kernel/sample/mapping.h"
#include "kernel/constants.h"
#include "kernel/util/lookup_table.h"
CCL_NAMESPACE_BEGIN
@ -46,10 +47,20 @@ ccl_device Spectrum bsdf_sheen_setup(KernelGlobals kg,
const float cosNI = dot(bsdf->N, sd->wi);
const int offset = kernel_data.tables.sheen_ltc;
bsdf->transformA = lookup_table_read_2D(kg, cosNI, bsdf->roughness, offset, 32, 32);
bsdf->transformB = lookup_table_read_2D(kg, cosNI, bsdf->roughness, offset + 32 * 32, 32, 32);
const float albedo = lookup_table_read_2D(
kg, cosNI, bsdf->roughness, offset + 2 * 32 * 32, 32, 32);
bsdf->transformA = lookup_table_read_2D(
kg, cosNI, bsdf->roughness, offset, SHEEN_RES_MU, SHEEN_RES_ROUGH);
bsdf->transformB = lookup_table_read_2D(kg,
cosNI,
bsdf->roughness,
offset + SHEEN_RES_MU * SHEEN_RES_ROUGH,
SHEEN_RES_MU,
SHEEN_RES_ROUGH);
const float albedo = lookup_table_read_2D(kg,
cosNI,
bsdf->roughness,
offset + 2 * SHEEN_RES_MU * SHEEN_RES_ROUGH,
SHEEN_RES_MU,
SHEEN_RES_ROUGH);
/* If the given roughness and angle result in an invalid LTC, skip the closure. */
if (fabsf(bsdf->transformA) < 1e-5f || albedo < 1e-5f) {

25
intern/cycles/kernel/constants.h Normal file
View file

@ -0,0 +1,25 @@
/* SPDX-FileCopyrightText: 2026 Blender Foundation
*
* SPDX-License-Identifier: Apache-2.0 */
#pragma once
/* Constant defines for the resolutions of the albedo look up tables. */
#define GGX_E_RES_ROUGH 32
#define GGX_E_RES_MU 32
#define GGX_GLASS_E_RES_ROUGH 32
#define GGX_GLASS_E_RES_MU 32
#define GGX_GLASS_E_RES_IOR 32
#define GGX_GEN_SCHLICK_S_RES_ROUGH 32
#define GGX_GEN_SCHLICK_S_RES_MU 32
#define GGX_GEN_SCHLICK_S_RES_IOR 32
#define GGX_GEN_SCHLICK_IOR_S_RES_ROUGH 32
#define GGX_GEN_SCHLICK_IOR_S_RES_MU 32
#define GGX_GEN_SCHLICK_IOR_S_RES_IOR 32
#define SHEEN_RES_ROUGH 32
#define SHEEN_RES_MU 32

1
intern/cycles/scene/CMakeLists.txt
View file

@ -47,6 +47,7 @@ set(SRC
curves.cpp
scene.cpp
shader.cpp
shader.tables
shader_graph.cpp
shader_nodes.cpp
stats.cpp

5354
intern/cycles/scene/shader.tables
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3
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