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/* SPDX-License-Identifier: Apache-2.0
* Copyright 2011-2022 Blender Foundation */
#pragma once
CCL_NAMESPACE_BEGIN
/* VOLUME EXTINCTION */
ccl_device void volume_extinction_setup(ccl_private ShaderData *sd, float3 weight)
{
if (sd->flag & SD_EXTINCTION) {
sd->closure_transparent_extinction += weight;
}
else {
sd->flag |= SD_EXTINCTION;
sd->closure_transparent_extinction = weight;
}
}
/* HENYEY-GREENSTEIN CLOSURE */
typedef struct HenyeyGreensteinVolume {
SHADER_CLOSURE_BASE;
float g;
} HenyeyGreensteinVolume;
static_assert(sizeof(ShaderClosure) >= sizeof(HenyeyGreensteinVolume),
"HenyeyGreensteinVolume is too large!");
/* Given cosine between rays, return probability density that a photon bounces
* to that direction. The g parameter controls how different it is from the
* uniform sphere. g=0 uniform diffuse-like, g=1 close to sharp single ray. */
ccl_device float single_peaked_henyey_greenstein(float cos_theta, float g)
{
return ((1.0f - g * g) / safe_powf(1.0f + g * g - 2.0f * g * cos_theta, 1.5f)) *
(M_1_PI_F * 0.25f);
};
ccl_device int volume_henyey_greenstein_setup(ccl_private HenyeyGreensteinVolume *volume)
{
volume->type = CLOSURE_VOLUME_HENYEY_GREENSTEIN_ID;
/* clamp anisotropy to avoid delta function */
volume->g = signf(volume->g) * min(fabsf(volume->g), 1.0f - 1e-3f);
return SD_SCATTER;
}
ccl_device float3 volume_henyey_greenstein_eval_phase(ccl_private const ShaderVolumeClosure *svc,
const float3 I,
float3 omega_in,
ccl_private float *pdf)
{
float g = svc->g;
/* note that I points towards the viewer */
if (fabsf(g) < 1e-3f) {
*pdf = M_1_PI_F * 0.25f;
}
else {
float cos_theta = dot(-I, omega_in);
*pdf = single_peaked_henyey_greenstein(cos_theta, g);
}
return make_float3(*pdf, *pdf, *pdf);
}
ccl_device float3
henyey_greenstrein_sample(float3 D, float g, float randu, float randv, ccl_private float *pdf)
{
/* match pdf for small g */
float cos_theta;
bool isotropic = fabsf(g) < 1e-3f;
if (isotropic) {
cos_theta = (1.0f - 2.0f * randu);
if (pdf) {
*pdf = M_1_PI_F * 0.25f;
}
}
else {
float k = (1.0f - g * g) / (1.0f - g + 2.0f * g * randu);
cos_theta = (1.0f + g * g - k * k) / (2.0f * g);
if (pdf) {
*pdf = single_peaked_henyey_greenstein(cos_theta, g);
}
}
float sin_theta = safe_sqrtf(1.0f - cos_theta * cos_theta);
float phi = M_2PI_F * randv;
float3 dir = make_float3(sin_theta * cosf(phi), sin_theta * sinf(phi), cos_theta);
float3 T, B;
make_orthonormals(D, &T, &B);
dir = dir.x * T + dir.y * B + dir.z * D;
return dir;
}
ccl_device int volume_henyey_greenstein_sample(ccl_private const ShaderVolumeClosure *svc,
float3 I,
float3 dIdx,
float3 dIdy,
float randu,
float randv,
ccl_private float3 *eval,
ccl_private float3 *omega_in,
ccl_private float3 *domega_in_dx,
ccl_private float3 *domega_in_dy,
ccl_private float *pdf)
{
float g = svc->g;
/* note that I points towards the viewer and so is used negated */
*omega_in = henyey_greenstrein_sample(-I, g, randu, randv, pdf);
*eval = make_float3(*pdf, *pdf, *pdf); /* perfect importance sampling */
#ifdef __RAY_DIFFERENTIALS__
/* todo: implement ray differential estimation */
*domega_in_dx = make_float3(0.0f, 0.0f, 0.0f);
*domega_in_dy = make_float3(0.0f, 0.0f, 0.0f);
#endif
return LABEL_VOLUME_SCATTER;
}
/* VOLUME CLOSURE */
ccl_device float3 volume_phase_eval(ccl_private const ShaderData *sd,
ccl_private const ShaderVolumeClosure *svc,
float3 omega_in,
ccl_private float *pdf)
{
return volume_henyey_greenstein_eval_phase(svc, sd->I, omega_in, pdf);
}
ccl_device int volume_phase_sample(ccl_private const ShaderData *sd,
ccl_private const ShaderVolumeClosure *svc,
float randu,
float randv,
ccl_private float3 *eval,
ccl_private float3 *omega_in,
ccl_private differential3 *domega_in,
ccl_private float *pdf)
{
return volume_henyey_greenstein_sample(svc,
sd->I,
sd->dI.dx,
sd->dI.dy,
randu,
randv,
eval,
omega_in,
&domega_in->dx,
&domega_in->dy,
pdf);
}
/* Volume sampling utilities. */
/* todo: this value could be tweaked or turned into a probability to avoid
* unnecessary work in volumes and subsurface scattering. */
#define VOLUME_THROUGHPUT_EPSILON 1e-6f
ccl_device float3 volume_color_transmittance(float3 sigma, float t)
{
return exp3(-sigma * t);
}
ccl_device float volume_channel_get(float3 value, int channel)
{
return (channel == 0) ? value.x : ((channel == 1) ? value.y : value.z);
}
ccl_device int volume_sample_channel(float3 albedo,
float3 throughput,
float rand,
ccl_private float3 *pdf)
{
/* Sample color channel proportional to throughput and single scattering
* albedo, to significantly reduce noise with many bounce, following:
*
* "Practical and Controllable Subsurface Scattering for Production Path
* Tracing". Matt Jen-Yuan Chiang, Peter Kutz, Brent Burley. SIGGRAPH 2016. */
float3 weights = fabs(throughput * albedo);
float sum_weights = weights.x + weights.y + weights.z;
if (sum_weights > 0.0f) {
*pdf = weights / sum_weights;
}
else {
*pdf = make_float3(1.0f / 3.0f, 1.0f / 3.0f, 1.0f / 3.0f);
}
if (rand < pdf->x) {
return 0;
}
else if (rand < pdf->x + pdf->y) {
return 1;
}
else {
return 2;
}
}
CCL_NAMESPACE_END
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