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Fix #114780: Cycles: Principled Chiang Hair importance sampling corre…
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…ction

Principled Chiang hair longitudinal importance sampling correction,
according to the new pbrt fix here :
mmp/pbrt-v3#256

Pull Request: https://projects.blender.org/blender/blender/pulls/115241
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@HamilcarR @brechtvl
HamilcarR authored and brechtvl committed Jun 26, 2024
1 parent 74d9161 commit 52039dd
Showing 1 changed file with 30 additions and 28 deletions.
58 changes: 30 additions & 28 deletions src/kernel/closure/bsdf_principled_hair_chiang.h
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Original file line number Diff line number Diff line change
Expand Up @@ -142,11 +142,13 @@ ccl_device_inline float longitudinal_scattering(
if (v <= 0.1f) {
float i0 = log_bessel_I0(cos_arg);
float val = expf(i0 - sin_arg - inv_v + 0.6931f + logf(0.5f * inv_v));
kernel_assert(isfinite_safe(val));
return val;
}
else {
float i0 = bessel_I0(cos_arg);
float val = (expf(-sin_arg) * i0) / (sinhf(inv_v) * 2.0f * v);
kernel_assert(isfinite_safe(val));
return val;
}
}
Expand Down Expand Up @@ -185,7 +187,7 @@ ccl_device int bsdf_hair_chiang_setup(ccl_private ShaderData *sd, ccl_private Ch
kernel_assert(isfinite_safe(bsdf->h));

bsdf->N = Y;

bsdf->alpha = -bsdf->alpha;
return SD_BSDF | SD_BSDF_HAS_EVAL | SD_BSDF_HAS_TRANSMISSION;
}

Expand Down Expand Up @@ -224,9 +226,9 @@ ccl_device_inline void hair_attenuation(
Ap_energy[3] *= fac;
}

/* Given the tilt angle, generate the rotated theta_i for the different bounces. */
ccl_device_inline void hair_alpha_angles(float sin_theta_i,
float cos_theta_i,
/* Update sin_theta_o and cos_theta_o to account for scale tilt for each bounce. */
ccl_device_inline void hair_alpha_angles(float sin_theta_o,
float cos_theta_o,
float alpha,
ccl_private float *angles)
{
Expand All @@ -237,12 +239,12 @@ ccl_device_inline void hair_alpha_angles(float sin_theta_i,
float sin_4alpha = 2.0f * sin_2alpha * cos_2alpha;
float cos_4alpha = sqr(cos_2alpha) - sqr(sin_2alpha);

angles[0] = sin_theta_i * cos_2alpha + cos_theta_i * sin_2alpha;
angles[1] = fabsf(cos_theta_i * cos_2alpha - sin_theta_i * sin_2alpha);
angles[2] = sin_theta_i * cos_1alpha - cos_theta_i * sin_1alpha;
angles[3] = fabsf(cos_theta_i * cos_1alpha + sin_theta_i * sin_1alpha);
angles[4] = sin_theta_i * cos_4alpha - cos_theta_i * sin_4alpha;
angles[5] = fabsf(cos_theta_i * cos_4alpha + sin_theta_i * sin_4alpha);
angles[0] = sin_theta_o * cos_2alpha - cos_theta_o * sin_2alpha;
angles[1] = fabsf(cos_theta_o * cos_2alpha + sin_theta_o * sin_2alpha);
angles[2] = sin_theta_o * cos_1alpha + cos_theta_o * sin_1alpha;
angles[3] = fabsf(cos_theta_o * cos_1alpha - sin_theta_o * sin_1alpha);
angles[4] = sin_theta_o * cos_4alpha + cos_theta_o * sin_4alpha;
angles[5] = fabsf(cos_theta_o * cos_4alpha - sin_theta_o * sin_4alpha);
}

/* Evaluation function for our shader. */
Expand Down Expand Up @@ -293,17 +295,17 @@ ccl_device Spectrum bsdf_hair_chiang_eval(KernelGlobals kg,
const float phi = phi_i - phi_o;

float angles[6];
hair_alpha_angles(sin_theta_i, cos_theta_i, bsdf->alpha, angles);
hair_alpha_angles(sin_theta_o, cos_theta_o, bsdf->alpha, angles);

Spectrum F = zero_spectrum();
float F_energy = 0.0f;

/* Primary specular (R), Transmission (TT) and Secondary Specular (TRT). */
for (int i = 0; i < 3; i++) {
const float Mp = longitudinal_scattering(angles[2 * i],
const float Mp = longitudinal_scattering(sin_theta_i,
cos_theta_i,
angles[2 * i],
angles[2 * i + 1],
sin_theta_o,
cos_theta_o,
(i == 0) ? bsdf->m0_roughness :
(i == 1) ? 0.25f * bsdf->v :
4.0f * bsdf->v);
Expand Down Expand Up @@ -347,6 +349,7 @@ ccl_device int bsdf_hair_chiang_sample(KernelGlobals kg,
kernel_assert(fabsf(dot(X, Y)) < 1e-3f);
const float3 Z = safe_normalize(cross(X, Y));

/* wo in pbrt. */
const float3 local_O = make_float3(dot(sd->wi, X), dot(sd->wi, Y), dot(sd->wi, Z));

const float sin_theta_o = local_O.x;
Expand Down Expand Up @@ -387,18 +390,19 @@ ccl_device int bsdf_hair_chiang_sample(KernelGlobals kg,
v *= 4.0f;
}

rand.z = max(rand.z, 1e-5f);
const float fac = 1.0f + v * logf(rand.z + (1.0f - rand.z) * expf(-2.0f / v));
float sin_theta_i = -fac * sin_theta_o +
cos_from_sin(fac) * cosf(M_2PI_F * rand.y) * cos_theta_o;
float cos_theta_i = cos_from_sin(sin_theta_i);

float angles[6];
hair_alpha_angles(sin_theta_o, cos_theta_o, bsdf->alpha, angles);
float sin_theta_o_tilted = sin_theta_o;
float cos_theta_o_tilted = cos_theta_o;
if (p < 3) {
hair_alpha_angles(sin_theta_i, cos_theta_i, -bsdf->alpha, angles);
sin_theta_i = angles[2 * p];
cos_theta_i = angles[2 * p + 1];
sin_theta_o_tilted = angles[2 * p];
cos_theta_o_tilted = angles[2 * p + 1];
}
rand.z = max(rand.z, 1e-5f);
const float fac = 1.0f + v * logf(rand.z + (1.0f - rand.z) * expf(-2.0f / v));
float sin_theta_i = -fac * sin_theta_o_tilted +
sin_from_cos(fac) * cosf(M_2PI_F * rand.y) * cos_theta_o_tilted;
float cos_theta_i = cos_from_sin(sin_theta_i);

float phi;
if (p < 3) {
Expand All @@ -409,17 +413,15 @@ ccl_device int bsdf_hair_chiang_sample(KernelGlobals kg,
}
const float phi_i = phi_o + phi;

hair_alpha_angles(sin_theta_i, cos_theta_i, bsdf->alpha, angles);

Spectrum F = zero_spectrum();
float F_energy = 0.0f;

/* Primary specular (R), Transmission (TT) and Secondary Specular (TRT). */
for (int i = 0; i < 3; i++) {
const float Mp = longitudinal_scattering(angles[2 * i],
const float Mp = longitudinal_scattering(sin_theta_i,
cos_theta_i,
angles[2 * i],
angles[2 * i + 1],
sin_theta_o,
cos_theta_o,
(i == 0) ? bsdf->m0_roughness :
(i == 1) ? 0.25f * bsdf->v :
4.0f * bsdf->v);
Expand Down

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