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Cycles: Switch microfacet code to non-separable shadowing-masking term 

This gives closer results to what I've seen in papers and other renderers when
using the code to precompute albedo later (to replace MultiGGX).

It's usually a tiny difference, the only case where I've seen it matter is
in the `shader/node_group_float.blend` test - but that's a (single-scatter) GGX
closure with 0.9 roughness, so it's not too surprising. In any case, the new
result looks closer to Eevee, so that's good I guess.

Differential Revision: https://developer.blender.org/D17099
This commit is contained in:
Lukas Stockner 2023-01-24 00:03:50 +01:00
parent ce25e3e581
commit fdcb55b285
1 changed files with 50 additions and 40 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

90
intern/cycles/kernel/closure/bsdf_microfacet.h
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@ -46,7 +46,7 @@ ccl_device_inline void microfacet_beckmann_sample_slopes(KernelGlobals kg,
float randv,
ccl_private float *slope_x,
ccl_private float *slope_y,
ccl_private float *G1i)
ccl_private float *lambda_i)
{
/* Special case (normal incidence). */
if (cos_theta_i >= 0.99999f) {
@ -54,7 +54,7 @@ ccl_device_inline void microfacet_beckmann_sample_slopes(KernelGlobals kg,
const float phi = M_2PI_F * randv;
*slope_x = r * cosf(phi);
*slope_y = r * sinf(phi);
*G1i = 1.0f;
*lambda_i = 0.0f;
return;
}
@ -67,9 +67,8 @@ ccl_device_inline void microfacet_beckmann_sample_slopes(KernelGlobals kg,
const float exp_a2 = expf(-cot_theta_i * cot_theta_i);
const float SQRT_PI_INV = 0.56418958354f;
const float Lambda = 0.5f * (erf_a - 1.0f) + (0.5f * SQRT_PI_INV) * (exp_a2 * inv_a);
const float G1 = 1.0f / (1.0f + Lambda); /* masking */
*G1i = G1;
*lambda_i = Lambda;
/* Based on paper from Wenzel Jakob
* An Improved Visible Normal Sampling Routine for the Beckmann Distribution
@ -132,7 +131,7 @@ ccl_device_inline void microfacet_ggx_sample_slopes(const float cos_theta_i,
float randv,
ccl_private float *slope_x,
ccl_private float *slope_y,
ccl_private float *G1i)
ccl_private float *lambda_i)
{
/* Special case (normal incidence). */
if (cos_theta_i >= 0.99999f) {
@ -140,7 +139,7 @@ ccl_device_inline void microfacet_ggx_sample_slopes(const float cos_theta_i,
const float phi = M_2PI_F * randv;
*slope_x = r * cosf(phi);
*slope_y = r * sinf(phi);
*G1i = 1.0f;
*lambda_i = 0.0f;
return;
}
@ -149,7 +148,7 @@ ccl_device_inline void microfacet_ggx_sample_slopes(const float cos_theta_i,
const float tan_theta_i = sin_theta_i / cos_theta_i;
const float G1_inv = 0.5f * (1.0f + safe_sqrtf(1.0f + tan_theta_i * tan_theta_i));
*G1i = 1.0f / G1_inv;
*lambda_i = G1_inv - 1.0f;
/* Sample slope_x. */
const float A = 2.0f * randu * G1_inv - 1.0f;
@ -186,7 +185,7 @@ ccl_device_forceinline float3 microfacet_sample_stretched(KernelGlobals kg,
const float alpha_y,
const float randu,
const float randv,
ccl_private float *G1i)
ccl_private float *lambda_i)
{
/* 1. stretch wi */
float3 wi_ = make_float3(alpha_x * wi.x, alpha_y * wi.y, wi.z);
@ -212,10 +211,10 @@ ccl_device_forceinline float3 microfacet_sample_stretched(KernelGlobals kg,
if (m_type == MicrofacetType::BECKMANN) {
microfacet_beckmann_sample_slopes(
kg, costheta_, sintheta_, randu, randv, &slope_x, &slope_y, G1i);
kg, costheta_, sintheta_, randu, randv, &slope_x, &slope_y, lambda_i);
}
else {
microfacet_ggx_sample_slopes(costheta_, sintheta_, randu, randv, &slope_x, &slope_y, G1i);
microfacet_ggx_sample_slopes(costheta_, sintheta_, randu, randv, &slope_x, &slope_y, lambda_i);
}
/* 3. rotate */
@ -271,37 +270,48 @@ ccl_device_forceinline float bsdf_clearcoat_D(float alpha2, float cos_NH)
return (alpha2 - 1.0f) / (M_PI_F * logf(alpha2) * t);
}
/* Monodirectional shadowing-masking term. */
/* Smith shadowing-masking term, here in the non-separable form.
* For details, see:
* Understanding the Masking-Shadowing Function in Microfacet-Based BRDFs.
* Eric Heitz, JCGT Vol. 3, No. 2, 2014.
* https://jcgt.org/published/0003/02/03/ */
template<MicrofacetType m_type>
ccl_device_inline float bsdf_G1_from_sqr_alpha_tan_n(float sqr_alpha_tan_n)
ccl_device_inline float bsdf_lambda_from_sqr_alpha_tan_n(float sqr_alpha_tan_n)
{
if (m_type == MicrofacetType::GGX) {
return 2.0f / (1.0f + sqrtf(1.0f + sqr_alpha_tan_n));
/* Equation 72. */
return 0.5f * (sqrtf(1.0f + sqr_alpha_tan_n) - 1.0f);
}
else {
/* m_type == MicrofacetType::BECKMANN */
/* m_type == MicrofacetType::BECKMANN
* Approximation from below Equation 69. */
if (sqr_alpha_tan_n < 0.39f) {
/* Equivalent to a >= 1.6f, but also handles sqr_alpha_tan_n == 0.0f cleanly. */
return 0.0f;
}
const float a = inversesqrtf(sqr_alpha_tan_n);
return (a > 1.6f) ? 1.0f : ((2.181f * a + 3.535f) * a) / ((2.577f * a + 2.276f) * a + 1.0f);
return ((0.396f * a - 1.259f) * a + 1.0f) / ((2.181f * a + 3.535f) * a);
}
}
template<MicrofacetType m_type> ccl_device_inline float bsdf_G1(float alpha2, float cos_N)
template<MicrofacetType m_type> ccl_device_inline float bsdf_lambda(float alpha2, float cos_N)
{
return bsdf_G1_from_sqr_alpha_tan_n<m_type>(alpha2 * fmaxf(1.0f / (cos_N * cos_N) - 1.0f, 0.0f));
return bsdf_lambda_from_sqr_alpha_tan_n<m_type>(alpha2 * fmaxf(1.0f / sqr(cos_N) - 1.0f, 0.0f));
}
template<MicrofacetType m_type>
ccl_device_inline float bsdf_aniso_G1(float alpha_x, float alpha_y, float3 V)
ccl_device_inline float bsdf_aniso_lambda(float alpha_x, float alpha_y, float3 V)
{
return bsdf_G1_from_sqr_alpha_tan_n<m_type>((sqr(alpha_x * V.x) + sqr(alpha_y * V.y)) /
sqr(V.z));
const float sqr_alpha_tan_n = (sqr(alpha_x * V.x) + sqr(alpha_y * V.y)) / sqr(V.z);
return bsdf_lambda_from_sqr_alpha_tan_n<m_type>(sqr_alpha_tan_n);
}
/* Smith's separable shadowing-masking term. */
/* Combined shadowing-masking term. */
template<MicrofacetType m_type>
ccl_device_inline float bsdf_G(float alpha2, float cos_NI, float cos_NO)
{
return bsdf_G1<m_type>(alpha2, cos_NI) * bsdf_G1<m_type>(alpha2, cos_NO);
return 1.0f / (1.0f + bsdf_lambda<m_type>(alpha2, cos_NI) + bsdf_lambda<m_type>(alpha2, cos_NO));
}
/* Normal distribution function. */
@ -382,7 +392,7 @@ ccl_device Spectrum bsdf_microfacet_eval(ccl_private const ShaderClosure *sc,
H *= inv_len_H;
const float cos_NH = dot(N, H);
float D, G1i, G1o;
float D, lambdaI, lambdaO;
/* TODO: add support for anisotropic transmission. */
if (alpha_x == alpha_y || m_refractive) { /* Isotropic. */
@ -399,8 +409,8 @@ ccl_device Spectrum bsdf_microfacet_eval(ccl_private const ShaderClosure *sc,
D = bsdf_D<m_type>(alpha2, cos_NH);
}
G1i = bsdf_G1<m_type>(alpha2, cos_NI);
G1o = bsdf_G1<m_type>(alpha2, cos_NO);
lambdaI = bsdf_lambda<m_type>(alpha2, cos_NI);
lambdaO = bsdf_lambda<m_type>(alpha2, cos_NO);
}
else { /* Anisotropic. */
float3 X, Y;
@ -412,20 +422,20 @@ ccl_device Spectrum bsdf_microfacet_eval(ccl_private const ShaderClosure *sc,
D = bsdf_aniso_D<m_type>(alpha_x, alpha_y, local_H);
G1i = bsdf_aniso_G1<m_type>(alpha_x, alpha_y, local_I);
G1o = bsdf_aniso_G1<m_type>(alpha_x, alpha_y, local_O);
lambdaI = bsdf_aniso_lambda<m_type>(alpha_x, alpha_y, local_I);
lambdaO = bsdf_aniso_lambda<m_type>(alpha_x, alpha_y, local_O);
}
const float common = G1i * D / cos_NI *
const float common = D / cos_NI *
(m_refractive ?
sqr(bsdf->ior * inv_len_H) * fabsf(dot(H, wi) * dot(H, wo)) :
0.25f);
*pdf = common;
*pdf = common / (1.0f + lambdaI);
const Spectrum F = m_refractive ? one_spectrum() : reflection_color(bsdf, wo, H);
return F * G1o * common;
return F * common / (1.0f + lambdaO + lambdaI);
}
template<MicrofacetType m_type>
@ -466,10 +476,10 @@ ccl_device int bsdf_microfacet_sample(KernelGlobals kg,
/* Importance sampling with distribution of visible normals. Vectors are transformed to local
* space before and after sampling. */
float G1i;
float lambdaI;
const float3 local_I = make_float3(dot(X, wi), dot(Y, wi), cos_NI);
const float3 local_H = microfacet_sample_stretched<m_type>(
kg, local_I, alpha_x, alpha_y, randu, randv, &G1i);
kg, local_I, alpha_x, alpha_y, randu, randv, &lambdaI);
const float3 H = X * local_H.x + Y * local_H.y + N * local_H.z;
const float cos_NH = local_H.z;
@ -514,7 +524,7 @@ ccl_device int bsdf_microfacet_sample(KernelGlobals kg,
else {
label |= LABEL_GLOSSY;
float cos_NO = dot(N, *wo);
float D, G1o;
float D, lambdaO;
/* TODO: add support for anisotropic transmission. */
if (alpha_x == alpha_y || m_refractive) { /* Isotropic. */
@ -527,33 +537,33 @@ ccl_device int bsdf_microfacet_sample(KernelGlobals kg,
* => alpha2 = 0.25 * 0.25 */
alpha2 = 0.0625f;
/* Recalculate G1i. */
G1i = bsdf_G1<m_type>(alpha2, cos_NI);
/* Recalculate lambdaI. */
lambdaI = bsdf_lambda<m_type>(alpha2, cos_NI);
}
else {
D = bsdf_D<m_type>(alpha2, cos_NH);
}
G1o = bsdf_G1<m_type>(alpha2, cos_NO);
lambdaO = bsdf_lambda<m_type>(alpha2, cos_NO);
}
else { /* Anisotropic. */
const float3 local_O = make_float3(dot(X, *wo), dot(Y, *wo), cos_NO);
D = bsdf_aniso_D<m_type>(alpha_x, alpha_y, local_H);
G1o = bsdf_aniso_G1<m_type>(alpha_x, alpha_y, local_O);
lambdaO = bsdf_aniso_lambda<m_type>(alpha_x, alpha_y, local_O);
}
const float cos_HO = dot(H, *wo);
const float common = G1i * D / cos_NI *
const float common = D / cos_NI *
(m_refractive ? fabsf(cos_HI * cos_HO) / sqr(cos_HO + cos_HI / m_eta) :
0.25f);
*pdf = common;
*pdf = common / (1.0f + lambdaI);
Spectrum F = m_refractive ? one_spectrum() : reflection_color(bsdf, *wo, H);
*eval = G1o * common * F;
*eval = F * common / (1.0f + lambdaI + lambdaO);
}
*sampled_roughness = make_float2(alpha_x, alpha_y);