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/* SPDX-License-Identifier: Apache-2.0
* Copyright 2011-2022 Blender Foundation */
/*
* ASHIKHMIN SHIRLEY BSDF
*
* Implementation of
* Michael Ashikhmin and Peter Shirley: "An Anisotropic Phong BRDF Model" (2000)
*
* The Fresnel factor is missing to get a separable bsdf (intensity*color), as is
* the case with all other microfacet-based BSDF implementations in Cycles.
*
* Other than that, the implementation directly follows the paper.
*/
#pragma once
CCL_NAMESPACE_BEGIN
ccl_device int bsdf_ashikhmin_shirley_setup(ccl_private MicrofacetBsdf *bsdf)
{
bsdf->alpha_x = clamp(bsdf->alpha_x, 1e-4f, 1.0f);
bsdf->alpha_y = clamp(bsdf->alpha_y, 1e-4f, 1.0f);
bsdf->type = CLOSURE_BSDF_ASHIKHMIN_SHIRLEY_ID;
return SD_BSDF | SD_BSDF_HAS_EVAL;
}
ccl_device void bsdf_ashikhmin_shirley_blur(ccl_private ShaderClosure *sc, float roughness)
{
ccl_private MicrofacetBsdf *bsdf = (ccl_private MicrofacetBsdf *)sc;
bsdf->alpha_x = fmaxf(roughness, bsdf->alpha_x);
bsdf->alpha_y = fmaxf(roughness, bsdf->alpha_y);
}
ccl_device_inline float bsdf_ashikhmin_shirley_roughness_to_exponent(float roughness)
{
return 2.0f / (roughness * roughness) - 2.0f;
}
ccl_device_forceinline Spectrum
bsdf_ashikhmin_shirley_eval_reflect(ccl_private const ShaderClosure *sc,
const float3 I,
const float3 omega_in,
ccl_private float *pdf)
{
ccl_private const MicrofacetBsdf *bsdf = (ccl_private const MicrofacetBsdf *)sc;
float3 N = bsdf->N;
float NdotI = dot(N, I); /* in Cycles/OSL convention I is omega_out */
float NdotO = dot(N, omega_in); /* and consequently we use for O omaga_in ;) */
float out = 0.0f;
if (fmaxf(bsdf->alpha_x, bsdf->alpha_y) <= 1e-4f) {
*pdf = 0.0f;
return zero_spectrum();
}
if (NdotI > 0.0f && NdotO > 0.0f) {
NdotI = fmaxf(NdotI, 1e-6f);
NdotO = fmaxf(NdotO, 1e-6f);
float3 H = normalize(omega_in + I);
float HdotI = fmaxf(fabsf(dot(H, I)), 1e-6f);
float HdotN = fmaxf(dot(H, N), 1e-6f);
/* pump from original paper
* (first derivative disc., but cancels the HdotI in the pdf nicely) */
float pump = 1.0f / fmaxf(1e-6f, (HdotI * fmaxf(NdotO, NdotI)));
/* pump from d-brdf paper */
/*float pump = 1.0f / fmaxf(1e-4f, ((NdotO + NdotI) * (NdotO*NdotI))); */
float n_x = bsdf_ashikhmin_shirley_roughness_to_exponent(bsdf->alpha_x);
float n_y = bsdf_ashikhmin_shirley_roughness_to_exponent(bsdf->alpha_y);
if (n_x == n_y) {
/* isotropic */
float e = n_x;
float lobe = powf(HdotN, e);
float norm = (n_x + 1.0f) / (8.0f * M_PI_F);
out = NdotO * norm * lobe * pump;
/* this is p_h / 4(H.I) (conversion from 'wh measure' to 'wi measure', eq. 8 in paper). */
*pdf = norm * lobe / HdotI;
}
else {
/* anisotropic */
float3 X, Y;
make_orthonormals_tangent(N, bsdf->T, &X, &Y);
float HdotX = dot(H, X);
float HdotY = dot(H, Y);
float lobe;
if (HdotN < 1.0f) {
float e = (n_x * HdotX * HdotX + n_y * HdotY * HdotY) / (1.0f - HdotN * HdotN);
lobe = powf(HdotN, e);
}
else {
lobe = 1.0f;
}
float norm = sqrtf((n_x + 1.0f) * (n_y + 1.0f)) / (8.0f * M_PI_F);
out = NdotO * norm * lobe * pump;
*pdf = norm * lobe / HdotI;
}
}
return make_spectrum(out);
}
ccl_device Spectrum bsdf_ashikhmin_shirley_eval_transmit(ccl_private const ShaderClosure *sc,
const float3 I,
const float3 omega_in,
ccl_private float *pdf)
{
*pdf = 0.0f;
return zero_spectrum();
}
ccl_device_inline void bsdf_ashikhmin_shirley_sample_first_quadrant(float n_x,
float n_y,
float randu,
float randv,
ccl_private float *phi,
ccl_private float *cos_theta)
{
*phi = atanf(sqrtf((n_x + 1.0f) / (n_y + 1.0f)) * tanf(M_PI_2_F * randu));
float cos_phi = cosf(*phi);
float sin_phi = sinf(*phi);
*cos_theta = powf(randv, 1.0f / (n_x * cos_phi * cos_phi + n_y * sin_phi * sin_phi + 1.0f));
}
ccl_device int bsdf_ashikhmin_shirley_sample(ccl_private const ShaderClosure *sc,
float3 Ng,
float3 I,
float3 dIdx,
float3 dIdy,
float randu,
float randv,
ccl_private Spectrum *eval,
ccl_private float3 *omega_in,
ccl_private float3 *domega_in_dx,
ccl_private float3 *domega_in_dy,
ccl_private float *pdf)
{
ccl_private const MicrofacetBsdf *bsdf = (ccl_private const MicrofacetBsdf *)sc;
float3 N = bsdf->N;
int label = LABEL_REFLECT | LABEL_GLOSSY;
float NdotI = dot(N, I);
if (NdotI > 0.0f) {
float n_x = bsdf_ashikhmin_shirley_roughness_to_exponent(bsdf->alpha_x);
float n_y = bsdf_ashikhmin_shirley_roughness_to_exponent(bsdf->alpha_y);
/* get x,y basis on the surface for anisotropy */
float3 X, Y;
if (n_x == n_y)
make_orthonormals(N, &X, &Y);
else
make_orthonormals_tangent(N, bsdf->T, &X, &Y);
/* sample spherical coords for h in tangent space */
float phi;
float cos_theta;
if (n_x == n_y) {
/* isotropic sampling */
phi = M_2PI_F * randu;
cos_theta = powf(randv, 1.0f / (n_x + 1.0f));
}
else {
/* anisotropic sampling */
if (randu < 0.25f) { /* first quadrant */
float remapped_randu = 4.0f * randu;
bsdf_ashikhmin_shirley_sample_first_quadrant(
n_x, n_y, remapped_randu, randv, &phi, &cos_theta);
}
else if (randu < 0.5f) { /* second quadrant */
float remapped_randu = 4.0f * (.5f - randu);
bsdf_ashikhmin_shirley_sample_first_quadrant(
n_x, n_y, remapped_randu, randv, &phi, &cos_theta);
phi = M_PI_F - phi;
}
else if (randu < 0.75f) { /* third quadrant */
float remapped_randu = 4.0f * (randu - 0.5f);
bsdf_ashikhmin_shirley_sample_first_quadrant(
n_x, n_y, remapped_randu, randv, &phi, &cos_theta);
phi = M_PI_F + phi;
}
else { /* fourth quadrant */
float remapped_randu = 4.0f * (1.0f - randu);
bsdf_ashikhmin_shirley_sample_first_quadrant(
n_x, n_y, remapped_randu, randv, &phi, &cos_theta);
phi = 2.0f * M_PI_F - phi;
}
}
/* get half vector in tangent space */
float sin_theta = sqrtf(fmaxf(0.0f, 1.0f - cos_theta * cos_theta));
float cos_phi = cosf(phi);
float sin_phi = sinf(phi); /* no sqrt(1-cos^2) here b/c it causes artifacts */
float3 h = make_float3(sin_theta * cos_phi, sin_theta * sin_phi, cos_theta);
/* half vector to world space */
float3 H = h.x * X + h.y * Y + h.z * N;
float HdotI = dot(H, I);
if (HdotI < 0.0f)
H = -H;
/* reflect I on H to get omega_in */
*omega_in = -I + (2.0f * HdotI) * H;
if (fmaxf(bsdf->alpha_x, bsdf->alpha_y) <= 1e-4f) {
/* Some high number for MIS. */
*pdf = 1e6f;
*eval = make_spectrum(1e6f);
label = LABEL_REFLECT | LABEL_SINGULAR;
}
else {
/* leave the rest to eval_reflect */
*eval = bsdf_ashikhmin_shirley_eval_reflect(sc, I, *omega_in, pdf);
}
#ifdef __RAY_DIFFERENTIALS__
/* just do the reflection thing for now */
*domega_in_dx = (2.0f * dot(N, dIdx)) * N - dIdx;
*domega_in_dy = (2.0f * dot(N, dIdy)) * N - dIdy;
#endif
}
return label;
}
CCL_NAMESPACE_END
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