blender/intern/cycles/kernel/kernel_subsurface.h

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/*
* Copyright 2011-2013 Blender Foundation
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License
*/
CCL_NAMESPACE_BEGIN
/* NEW BSSRDF: See "BSSRDF Importance Sampling", SIGGRAPH 2013 */
/* TODO:
* - test using power heuristic for combing bssrdfs
* - try to reduce one sample model variance
*/
#define BSSRDF_MULTI_EVAL
ccl_device ShaderClosure *subsurface_scatter_pick_closure(KernelGlobals *kg, ShaderData *sd, float *probability)
{
/* sum sample weights of bssrdf and bsdf */
float bsdf_sum = 0.0f;
float bssrdf_sum = 0.0f;
for(int i = 0; i < sd->num_closure; i++) {
ShaderClosure *sc = &sd->closure[i];
if(CLOSURE_IS_BSDF(sc->type))
bsdf_sum += sc->sample_weight;
else if(CLOSURE_IS_BSSRDF(sc->type))
bssrdf_sum += sc->sample_weight;
}
/* use bsdf or bssrdf? */
float r = sd->randb_closure*(bsdf_sum + bssrdf_sum);
if(r < bsdf_sum) {
/* use bsdf, and adjust randb so we can reuse it for picking a bsdf */
sd->randb_closure = r/bsdf_sum;
*probability = (bsdf_sum > 0.0f)? (bsdf_sum + bssrdf_sum)/bsdf_sum: 1.0f;
return NULL;
}
/* use bssrdf */
r -= bsdf_sum;
float sum = 0.0f;
for(int i = 0; i < sd->num_closure; i++) {
ShaderClosure *sc = &sd->closure[i];
if(CLOSURE_IS_BSSRDF(sc->type)) {
sum += sc->sample_weight;
if(r <= sum) {
sd->randb_closure = (r - (sum - sc->sample_weight))/sc->sample_weight;
#ifdef BSSRDF_MULTI_EVAL
*probability = (bssrdf_sum > 0.0f)? (bsdf_sum + bssrdf_sum)/bssrdf_sum: 1.0f;
#else
*probability = (bssrdf_sum > 0.0f)? (bsdf_sum + bssrdf_sum)/sc->sample_weight: 1.0f;
#endif
return sc;
}
}
}
/* should never happen */
sd->randb_closure = 0.0f;
*probability = 1.0f;
return NULL;
}
ccl_device float3 subsurface_scatter_eval(ShaderData *sd, ShaderClosure *sc, float disk_r, float r, bool all)
{
#ifdef BSSRDF_MULTI_EVAL
/* this is the veach one-sample model with balance heuristic, some pdf
* factors drop out when using balance heuristic weighting */
float3 eval_sum = make_float3(0.0f, 0.0f, 0.0f);
float pdf_sum = 0.0f;
float sample_weight_sum = 0.0f;
int num_bssrdf = 0;
for(int i = 0; i < sd->num_closure; i++) {
sc = &sd->closure[i];
if(CLOSURE_IS_BSSRDF(sc->type)) {
float sample_weight = (all)? 1.0f: sc->sample_weight;
sample_weight_sum += sample_weight;
}
}
float sample_weight_inv = 1.0f/sample_weight_sum;
//printf("num closures %d\n", sd->num_closure);
for(int i = 0; i < sd->num_closure; i++) {
sc = &sd->closure[i];
if(CLOSURE_IS_BSSRDF(sc->type)) {
/* in case of branched path integrate we sample all bssrdf's once,
* for path trace we pick one, so adjust pdf for that */
float sample_weight = (all)? 1.0f: sc->sample_weight * sample_weight_inv;
/* compute pdf */
float pdf = bssrdf_pdf(sc, r);
float disk_pdf = bssrdf_pdf(sc, disk_r);
/* TODO power heuristic is not working correct here */
eval_sum += sc->weight*pdf; //*sample_weight*disk_pdf;
pdf_sum += sample_weight*disk_pdf; //*sample_weight*disk_pdf;
num_bssrdf++;
}
}
return (pdf_sum > 0.0f)? eval_sum / pdf_sum : make_float3(0.0f, 0.0f, 0.0f);
#else
float pdf = bssrdf_pdf(pick_sc, r);
float disk_pdf = bssrdf_pdf(pick_sc, disk_r);
return pick_sc->weight * pdf / disk_pdf;
#endif
}
/* replace closures with a single diffuse bsdf closure after scatter step */
ccl_device void subsurface_scatter_setup_diffuse_bsdf(ShaderData *sd, float3 weight, bool hit, float3 N)
{
sd->flag &= ~SD_CLOSURE_FLAGS;
sd->randb_closure = 0.0f;
if(hit) {
ShaderClosure *sc = &sd->closure[0];
sd->num_closure = 1;
sc->weight = weight;
sc->sample_weight = 1.0f;
sc->data0 = 0.0f;
sc->data1 = 0.0f;
sc->N = N;
sd->flag |= bsdf_diffuse_setup(sc);
/* replace CLOSURE_BSDF_DIFFUSE_ID with this special ID so render passes
* can recognize it as not being a regular diffuse closure */
sc->type = CLOSURE_BSDF_BSSRDF_ID;
}
else
sd->num_closure = 0;
}
/* optionally do blurring of color and/or bump mapping, at the cost of a shader evaluation */
ccl_device float3 subsurface_color_pow(float3 color, float exponent)
{
color = max(color, make_float3(0.0f, 0.0f, 0.0f));
if(exponent == 1.0f) {
/* nothing to do */
}
else if(exponent == 0.5f) {
color.x = sqrtf(color.x);
color.y = sqrtf(color.y);
color.z = sqrtf(color.z);
}
else {
color.x = powf(color.x, exponent);
color.y = powf(color.y, exponent);
color.z = powf(color.z, exponent);
}
return color;
}
ccl_device void subsurface_color_bump_blur(KernelGlobals *kg, ShaderData *out_sd, ShaderData *in_sd, int state_flag, float3 *eval, float3 *N)
{
/* average color and texture blur at outgoing point */
float texture_blur;
float3 out_color = shader_bssrdf_sum(out_sd, NULL, &texture_blur);
/* do we have bump mapping? */
bool bump = (out_sd->flag & SD_HAS_BSSRDF_BUMP) != 0;
if(bump || texture_blur > 0.0f) {
/* average color and normal at incoming point */
shader_eval_surface(kg, in_sd, 0.0f, state_flag, SHADER_CONTEXT_SSS);
float3 in_color = shader_bssrdf_sum(in_sd, (bump)? N: NULL, NULL);
/* we simply divide out the average color and multiply with the average
* of the other one. we could try to do this per closure but it's quite
* tricky to match closures between shader evaluations, their number and
* order may change, this is simpler */
if(texture_blur > 0.0f) {
out_color = subsurface_color_pow(out_color, texture_blur);
in_color = subsurface_color_pow(in_color, texture_blur);
*eval *= safe_divide_color(in_color, out_color);
}
}
}
/* subsurface scattering step, from a point on the surface to other nearby points on the same object */
ccl_device int subsurface_scatter_multi_step(KernelGlobals *kg, ShaderData *sd, ShaderData bssrdf_sd[BSSRDF_MAX_HITS],
int state_flag, ShaderClosure *sc, uint *lcg_state, float disk_u, float disk_v, bool all)
{
/* pick random axis in local frame and point on disk */
float3 disk_N, disk_T, disk_B;
float pick_pdf_N, pick_pdf_T, pick_pdf_B;
disk_N = sd->Ng;
make_orthonormals(disk_N, &disk_T, &disk_B);
/* reusing variable for picking the closure gives a bit nicer stratification
* for path tracer, for branched we do all closures so it doesn't help */
float axisu = (all)? disk_u: sd->randb_closure;
if(axisu < 0.5f) {
pick_pdf_N = 0.5f;
pick_pdf_T = 0.25f;
pick_pdf_B = 0.25f;
if(all)
disk_u *= 2.0f;
}
else if(axisu < 0.75f) {
float3 tmp = disk_N;
disk_N = disk_T;
disk_T = tmp;
pick_pdf_N = 0.25f;
pick_pdf_T = 0.5f;
pick_pdf_B = 0.25f;
if(all)
disk_u = (disk_u - 0.5f)*4.0f;
}
else {
float3 tmp = disk_N;
disk_N = disk_B;
disk_B = tmp;
pick_pdf_N = 0.25f;
pick_pdf_T = 0.25f;
pick_pdf_B = 0.5f;
if(all)
disk_u = (disk_u - 0.75f)*4.0f;
}
/* sample point on disk */
float phi = M_2PI_F * disk_u;
float disk_r = disk_v;
float disk_height;
bssrdf_sample(sc, disk_r, &disk_r, &disk_height);
float3 disk_P = (disk_r*cosf(phi)) * disk_T + (disk_r*sinf(phi)) * disk_B;
/* create ray */
Ray ray;
ray.P = sd->P + disk_N*disk_height + disk_P;
ray.D = -disk_N;
ray.t = 2.0f*disk_height;
ray.dP = sd->dP;
ray.dD = differential3_zero();
ray.time = sd->time;
/* intersect with the same object. if multiple intersections are found it
* will use at most BSSRDF_MAX_HITS hits, a random subset of all hits */
Intersection isect[BSSRDF_MAX_HITS];
uint num_hits = scene_intersect_subsurface(kg, &ray, isect, sd->object, lcg_state, BSSRDF_MAX_HITS);
/* evaluate bssrdf */
float3 eval = make_float3(0.0f, 0.0f, 0.0f);
int num_eval_hits = min(num_hits, BSSRDF_MAX_HITS);
for(int hit = 0; hit < num_eval_hits; hit++) {
ShaderData *bsd = &bssrdf_sd[hit];
/* setup new shading point */
*bsd = *sd;
shader_setup_from_subsurface(kg, bsd, &isect[hit], &ray);
/* probability densities for local frame axes */
float pdf_N = pick_pdf_N * fabsf(dot(disk_N, bsd->Ng));
float pdf_T = pick_pdf_T * fabsf(dot(disk_T, bsd->Ng));
float pdf_B = pick_pdf_B * fabsf(dot(disk_B, bsd->Ng));
/* multiple importance sample between 3 axes, power heuristic
* found to be slightly better than balance heuristic */
float mis_weight = power_heuristic_3(pdf_N, pdf_T, pdf_B);
/* real distance to sampled point */
float r = len(bsd->P - sd->P);
/* evaluate */
float w = mis_weight / pdf_N;
if(num_hits > BSSRDF_MAX_HITS)
w *= num_hits/(float)BSSRDF_MAX_HITS;
eval = subsurface_scatter_eval(bsd, sc, disk_r, r, all) * w;
/* optionally blur colors and bump mapping */
float3 N = bsd->N;
subsurface_color_bump_blur(kg, sd, bsd, state_flag, &eval, &N);
/* setup diffuse bsdf */
subsurface_scatter_setup_diffuse_bsdf(bsd, eval, true, N);
}
return num_eval_hits;
}
/* subsurface scattering step, from a point on the surface to another nearby point on the same object */
ccl_device void subsurface_scatter_step(KernelGlobals *kg, ShaderData *sd,
int state_flag, ShaderClosure *sc, uint *lcg_state, float disk_u, float disk_v, bool all)
{
float3 eval = make_float3(0.0f, 0.0f, 0.0f);
uint num_hits = 0;
/* pick random axis in local frame and point on disk */
float3 disk_N, disk_T, disk_B;
float pick_pdf_N, pick_pdf_T, pick_pdf_B;
disk_N = sd->Ng;
make_orthonormals(disk_N, &disk_T, &disk_B);
if(sd->randb_closure < 0.5f) {
pick_pdf_N = 0.5f;
pick_pdf_T = 0.25f;
pick_pdf_B = 0.25f;
}
else if(sd->randb_closure < 0.75f) {
float3 tmp = disk_N;
disk_N = disk_T;
disk_T = tmp;
pick_pdf_N = 0.25f;
pick_pdf_T = 0.5f;
pick_pdf_B = 0.25f;
}
else {
float3 tmp = disk_N;
disk_N = disk_B;
disk_B = tmp;
pick_pdf_N = 0.25f;
pick_pdf_T = 0.25f;
pick_pdf_B = 0.5f;
}
/* sample point on disk */
float phi = M_2PI_F * disk_u;
float disk_r = disk_v;
float disk_height;
bssrdf_sample(sc, disk_r, &disk_r, &disk_height);
float3 disk_P = (disk_r*cosf(phi)) * disk_T + (disk_r*sinf(phi)) * disk_B;
/* create ray */
Ray ray;
ray.P = sd->P + disk_N*disk_height + disk_P;
ray.D = -disk_N;
ray.t = 2.0f*disk_height;
ray.dP = sd->dP;
ray.dD = differential3_zero();
ray.time = sd->time;
/* intersect with the same object. if multiple intersections are
* found it will randomly pick one of them */
Intersection isect;
num_hits = scene_intersect_subsurface(kg, &ray, &isect, sd->object, lcg_state, 1);
/* evaluate bssrdf */
if(num_hits > 0) {
float3 origP = sd->P;
/* setup new shading point */
shader_setup_from_subsurface(kg, sd, &isect, &ray);
/* probability densities for local frame axes */
float pdf_N = pick_pdf_N * fabsf(dot(disk_N, sd->Ng));
float pdf_T = pick_pdf_T * fabsf(dot(disk_T, sd->Ng));
float pdf_B = pick_pdf_B * fabsf(dot(disk_B, sd->Ng));
/* multiple importance sample between 3 axes, power heuristic
* found to be slightly better than balance heuristic */
float mis_weight = power_heuristic_3(pdf_N, pdf_T, pdf_B);
/* real distance to sampled point */
float r = len(sd->P - origP);
/* evaluate */
float w = (mis_weight * num_hits) / pdf_N;
eval = subsurface_scatter_eval(sd, sc, disk_r, r, all) * w;
}
/* optionally blur colors and bump mapping */
float3 N = sd->N;
subsurface_color_bump_blur(kg, sd, sd, state_flag, &eval, &N);
/* setup diffuse bsdf */
subsurface_scatter_setup_diffuse_bsdf(sd, eval, (num_hits > 0), N);
}
CCL_NAMESPACE_END