blender/intern/cycles/kernel/kernel_volume.h
Brecht Van Lommel 5fa68133c9 Cycles: volume sampling method can now be set per material/world.
This gives you "Multiple Importance", "Distance" and "Equiangular" choices.

What multiple importance sampling does is make things more robust to certain
types of noise at the cost of a bit more noise in cases where the individual
strategies are always better.

So if you've got a pretty dense volume that's lit from far away then distance
sampling is usually more efficient. If you've got a light inside or near the
volume then equiangular sampling is better. If you have a combination of both,
then the multiple importance sampling will be better.
2014-06-14 13:49:56 +02:00

996 lines
31 KiB
C

/*
* 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
/* Events for probalistic scattering */
typedef enum VolumeIntegrateResult {
VOLUME_PATH_SCATTERED = 0,
VOLUME_PATH_ATTENUATED = 1,
VOLUME_PATH_MISSED = 2
} VolumeIntegrateResult;
/* Volume shader properties
*
* extinction coefficient = absorption coefficient + scattering coefficient
* sigma_t = sigma_a + sigma_s */
typedef struct VolumeShaderCoefficients {
float3 sigma_a;
float3 sigma_s;
float3 emission;
} VolumeShaderCoefficients;
/* evaluate shader to get extinction coefficient at P */
ccl_device bool volume_shader_extinction_sample(KernelGlobals *kg, ShaderData *sd, PathState *state, float3 P, float3 *extinction)
{
sd->P = P;
shader_eval_volume(kg, sd, state->volume_stack, PATH_RAY_SHADOW, SHADER_CONTEXT_SHADOW);
if(!(sd->flag & (SD_ABSORPTION|SD_SCATTER)))
return false;
float3 sigma_t = make_float3(0.0f, 0.0f, 0.0f);
for(int i = 0; i < sd->num_closure; i++) {
const ShaderClosure *sc = &sd->closure[i];
if(CLOSURE_IS_VOLUME(sc->type))
sigma_t += sc->weight;
}
*extinction = sigma_t;
return true;
}
/* evaluate shader to get absorption, scattering and emission at P */
ccl_device bool volume_shader_sample(KernelGlobals *kg, ShaderData *sd, PathState *state, float3 P, VolumeShaderCoefficients *coeff)
{
sd->P = P;
shader_eval_volume(kg, sd, state->volume_stack, state->flag, SHADER_CONTEXT_VOLUME);
if(!(sd->flag & (SD_ABSORPTION|SD_SCATTER|SD_EMISSION)))
return false;
coeff->sigma_a = make_float3(0.0f, 0.0f, 0.0f);
coeff->sigma_s = make_float3(0.0f, 0.0f, 0.0f);
coeff->emission = make_float3(0.0f, 0.0f, 0.0f);
for(int i = 0; i < sd->num_closure; i++) {
const ShaderClosure *sc = &sd->closure[i];
if(sc->type == CLOSURE_VOLUME_ABSORPTION_ID)
coeff->sigma_a += sc->weight;
else if(sc->type == CLOSURE_EMISSION_ID)
coeff->emission += sc->weight;
else if(CLOSURE_IS_VOLUME(sc->type))
coeff->sigma_s += sc->weight;
}
/* when at the max number of bounces, treat scattering as absorption */
if(sd->flag & SD_SCATTER) {
if(state->volume_bounce >= kernel_data.integrator.max_volume_bounce) {
coeff->sigma_a += coeff->sigma_s;
coeff->sigma_s = make_float3(0.0f, 0.0f, 0.0f);
sd->flag &= ~SD_SCATTER;
sd->flag |= SD_ABSORPTION;
}
}
return true;
}
ccl_device float3 volume_color_transmittance(float3 sigma, float t)
{
return make_float3(expf(-sigma.x * t), expf(-sigma.y * t), expf(-sigma.z * t));
}
ccl_device float kernel_volume_channel_get(float3 value, int channel)
{
return (channel == 0)? value.x: ((channel == 1)? value.y: value.z);
}
ccl_device bool volume_stack_is_heterogeneous(KernelGlobals *kg, VolumeStack *stack)
{
for(int i = 0; stack[i].shader != SHADER_NONE; i++) {
int shader_flag = kernel_tex_fetch(__shader_flag, (stack[i].shader & SHADER_MASK)*2);
if(shader_flag & SD_HETEROGENEOUS_VOLUME)
return true;
}
return false;
}
ccl_device int volume_stack_sampling_method(KernelGlobals *kg, VolumeStack *stack)
{
if(kernel_data.integrator.num_all_lights == 0)
return 0;
int method = -1;
for(int i = 0; stack[i].shader != SHADER_NONE; i++) {
int shader_flag = kernel_tex_fetch(__shader_flag, (stack[i].shader & SHADER_MASK)*2);
if(shader_flag & SD_VOLUME_MIS) {
return SD_VOLUME_MIS;
}
else if(shader_flag & SD_VOLUME_EQUIANGULAR) {
if(method == 0)
return SD_VOLUME_MIS;
method = SD_VOLUME_EQUIANGULAR;
}
else {
if(method == SD_VOLUME_EQUIANGULAR)
return SD_VOLUME_MIS;
method = 0;
}
}
return method;
}
/* Volume Shadows
*
* These functions are used to attenuate shadow rays to lights. Both absorption
* and scattering will block light, represented by the extinction coefficient. */
/* homogeneous volume: assume shader evaluation at the starts gives
* the extinction coefficient for the entire line segment */
ccl_device void kernel_volume_shadow_homogeneous(KernelGlobals *kg, PathState *state, Ray *ray, ShaderData *sd, float3 *throughput)
{
float3 sigma_t;
if(volume_shader_extinction_sample(kg, sd, state, ray->P, &sigma_t))
*throughput *= volume_color_transmittance(sigma_t, ray->t);
}
/* heterogeneous volume: integrate stepping through the volume until we
* reach the end, get absorbed entirely, or run out of iterations */
ccl_device void kernel_volume_shadow_heterogeneous(KernelGlobals *kg, PathState *state, Ray *ray, ShaderData *sd, float3 *throughput)
{
float3 tp = *throughput;
const float tp_eps = 1e-6f; /* todo: this is likely not the right value */
/* prepare for stepping */
int max_steps = kernel_data.integrator.volume_max_steps;
float step = kernel_data.integrator.volume_step_size;
float random_jitter_offset = lcg_step_float(&state->rng_congruential) * step;
/* compute extinction at the start */
float t = 0.0f;
for(int i = 0; i < max_steps; i++) {
/* advance to new position */
float new_t = min(ray->t, (i+1) * step);
float dt = new_t - t;
/* use random position inside this segment to sample shader */
if(new_t == ray->t)
random_jitter_offset = lcg_step_float(&state->rng_congruential) * dt;
float3 new_P = ray->P + ray->D * (t + random_jitter_offset);
float3 sigma_t;
/* compute attenuation over segment */
if(volume_shader_extinction_sample(kg, sd, state, new_P, &sigma_t)) {
/* todo: we could avoid computing expf() for each step by summing,
* because exp(a)*exp(b) = exp(a+b), but we still want a quick
* tp_eps check too */
tp *= volume_color_transmittance(sigma_t, new_t - t);
/* stop if nearly all light blocked */
if(tp.x < tp_eps && tp.y < tp_eps && tp.z < tp_eps)
break;
}
/* stop if at the end of the volume */
t = new_t;
if(t == ray->t)
break;
}
*throughput = tp;
}
/* get the volume attenuation over line segment defined by ray, with the
* assumption that there are no surfaces blocking light between the endpoints */
ccl_device_noinline void kernel_volume_shadow(KernelGlobals *kg, PathState *state, Ray *ray, float3 *throughput)
{
ShaderData sd;
shader_setup_from_volume(kg, &sd, ray, state->bounce, state->transparent_bounce);
if(volume_stack_is_heterogeneous(kg, state->volume_stack))
kernel_volume_shadow_heterogeneous(kg, state, ray, &sd, throughput);
else
kernel_volume_shadow_homogeneous(kg, state, ray, &sd, throughput);
}
/* Equi-angular sampling as in:
* "Importance Sampling Techniques for Path Tracing in Participating Media" */
ccl_device float kernel_volume_equiangular_sample(Ray *ray, float3 light_P, float xi, float *pdf)
{
float t = ray->t;
float delta = dot((light_P - ray->P) , ray->D);
float D = sqrtf(len_squared(light_P - ray->P) - delta * delta);
float theta_a = -atan2f(delta, D);
float theta_b = atan2f(t - delta, D);
float t_ = D * tanf((xi * theta_b) + (1 - xi) * theta_a);
*pdf = D / ((theta_b - theta_a) * (D * D + t_ * t_));
return min(t, delta + t_); /* min is only for float precision errors */
}
ccl_device float kernel_volume_equiangular_pdf(Ray *ray, float3 light_P, float sample_t)
{
float delta = dot((light_P - ray->P) , ray->D);
float D = sqrtf(len_squared(light_P - ray->P) - delta * delta);
float t = ray->t;
float t_ = sample_t - delta;
float theta_a = -atan2f(delta, D);
float theta_b = atan2f(t - delta, D);
float pdf = D / ((theta_b - theta_a) * (D * D + t_ * t_));
return pdf;
}
/* Distance sampling */
ccl_device float kernel_volume_distance_sample(float max_t, float3 sigma_t, int channel, float xi, float3 *transmittance, float3 *pdf)
{
/* xi is [0, 1[ so log(0) should never happen, division by zero is
* avoided because sample_sigma_t > 0 when SD_SCATTER is set */
float sample_sigma_t = kernel_volume_channel_get(sigma_t, channel);
float3 full_transmittance = volume_color_transmittance(sigma_t, max_t);
float sample_transmittance = kernel_volume_channel_get(full_transmittance, channel);
float sample_t = min(max_t, -logf(1.0f - xi*(1.0f - sample_transmittance))/sample_sigma_t);
*transmittance = volume_color_transmittance(sigma_t, sample_t);
*pdf = (sigma_t * *transmittance)/(make_float3(1.0f, 1.0f, 1.0f) - full_transmittance);
/* todo: optimization: when taken together with hit/miss decision,
* the full_transmittance cancels out drops out and xi does not
* need to be remapped */
return sample_t;
}
ccl_device float3 kernel_volume_distance_pdf(float max_t, float3 sigma_t, float sample_t)
{
float3 full_transmittance = volume_color_transmittance(sigma_t, max_t);
float3 transmittance = volume_color_transmittance(sigma_t, sample_t);
return (sigma_t * transmittance)/(make_float3(1.0f, 1.0f, 1.0f) - full_transmittance);
}
/* Emission */
ccl_device float3 kernel_volume_emission_integrate(VolumeShaderCoefficients *coeff, int closure_flag, float3 transmittance, float t)
{
/* integral E * exp(-sigma_t * t) from 0 to t = E * (1 - exp(-sigma_t * t))/sigma_t
* this goes to E * t as sigma_t goes to zero
*
* todo: we should use an epsilon to avoid precision issues near zero sigma_t */
float3 emission = coeff->emission;
if(closure_flag & SD_ABSORPTION) {
float3 sigma_t = coeff->sigma_a + coeff->sigma_s;
emission.x *= (sigma_t.x > 0.0f)? (1.0f - transmittance.x)/sigma_t.x: t;
emission.y *= (sigma_t.y > 0.0f)? (1.0f - transmittance.y)/sigma_t.y: t;
emission.z *= (sigma_t.z > 0.0f)? (1.0f - transmittance.z)/sigma_t.z: t;
}
else
emission *= t;
return emission;
}
/* Volume Path */
/* homogeneous volume: assume shader evaluation at the start gives
* the volume shading coefficient for the entire line segment */
ccl_device VolumeIntegrateResult kernel_volume_integrate_homogeneous(KernelGlobals *kg,
PathState *state, Ray *ray, ShaderData *sd, PathRadiance *L, float3 *throughput,
RNG *rng, bool probalistic_scatter)
{
VolumeShaderCoefficients coeff;
if(!volume_shader_sample(kg, sd, state, ray->P, &coeff))
return VOLUME_PATH_MISSED;
int closure_flag = sd->flag;
float t = ray->t;
float3 new_tp;
/* randomly scatter, and if we do t is shortened */
if(closure_flag & SD_SCATTER) {
/* extinction coefficient */
float3 sigma_t = coeff.sigma_a + coeff.sigma_s;
/* pick random color channel, we use the Veach one-sample
* model with balance heuristic for the channels */
float rphase = path_state_rng_1D_for_decision(kg, rng, state, PRNG_PHASE);
int channel = (int)(rphase*3.0f);
sd->randb_closure = rphase*3.0f - channel;
/* decide if we will hit or miss */
bool scatter = true;
float xi = path_state_rng_1D_for_decision(kg, rng, state, PRNG_SCATTER_DISTANCE);
if(probalistic_scatter) {
float sample_sigma_t = kernel_volume_channel_get(sigma_t, channel);
float sample_transmittance = expf(-sample_sigma_t * t);
if(1.0f - xi >= sample_transmittance) {
scatter = true;
/* rescale random number so we can reuse it */
xi = 1.0f - (1.0f - xi - sample_transmittance)/(1.0f - sample_transmittance);
}
else
scatter = false;
}
if(scatter) {
/* scattering */
float3 pdf;
float3 transmittance;
float sample_t;
/* distance sampling */
sample_t = kernel_volume_distance_sample(ray->t, sigma_t, channel, xi, &transmittance, &pdf);
/* modifiy pdf for hit/miss decision */
if(probalistic_scatter)
pdf *= make_float3(1.0f, 1.0f, 1.0f) - volume_color_transmittance(sigma_t, t);
new_tp = *throughput * coeff.sigma_s * transmittance / average(pdf);
t = sample_t;
}
else {
/* no scattering */
float3 transmittance = volume_color_transmittance(sigma_t, t);
float pdf = average(transmittance);
new_tp = *throughput * transmittance / pdf;
}
}
else if(closure_flag & SD_ABSORPTION) {
/* absorption only, no sampling needed */
float3 transmittance = volume_color_transmittance(coeff.sigma_a, t);
new_tp = *throughput * transmittance;
}
/* integrate emission attenuated by extinction */
if(L && (closure_flag & SD_EMISSION)) {
float3 sigma_t = coeff.sigma_a + coeff.sigma_s;
float3 transmittance = volume_color_transmittance(sigma_t, ray->t);
float3 emission = kernel_volume_emission_integrate(&coeff, closure_flag, transmittance, ray->t);
path_radiance_accum_emission(L, *throughput, emission, state->bounce);
}
/* modify throughput */
if(closure_flag & (SD_ABSORPTION|SD_SCATTER)) {
*throughput = new_tp;
/* prepare to scatter to new direction */
if(t < ray->t) {
/* adjust throughput and move to new location */
sd->P = ray->P + t*ray->D;
return VOLUME_PATH_SCATTERED;
}
}
return VOLUME_PATH_ATTENUATED;
}
/* heterogeneous volume distance sampling: integrate stepping through the
* volume until we reach the end, get absorbed entirely, or run out of
* iterations. this does probalistically scatter or get transmitted through
* for path tracing where we don't want to branch. */
ccl_device VolumeIntegrateResult kernel_volume_integrate_heterogeneous_distance(KernelGlobals *kg,
PathState *state, Ray *ray, ShaderData *sd, PathRadiance *L, float3 *throughput, RNG *rng)
{
float3 tp = *throughput;
const float tp_eps = 1e-6f; /* todo: this is likely not the right value */
/* prepare for stepping */
int max_steps = kernel_data.integrator.volume_max_steps;
float step_size = kernel_data.integrator.volume_step_size;
float random_jitter_offset = lcg_step_float(&state->rng_congruential) * step_size;
/* compute coefficients at the start */
float t = 0.0f;
float3 accum_transmittance = make_float3(1.0f, 1.0f, 1.0f);
/* pick random color channel, we use the Veach one-sample
* model with balance heuristic for the channels */
float xi = path_state_rng_1D_for_decision(kg, rng, state, PRNG_SCATTER_DISTANCE);
float rphase = path_state_rng_1D_for_decision(kg, rng, state, PRNG_PHASE);
int channel = (int)(rphase*3.0f);
sd->randb_closure = rphase*3.0f - channel;
bool has_scatter = false;
for(int i = 0; i < max_steps; i++) {
/* advance to new position */
float new_t = min(ray->t, (i+1) * step_size);
float dt = new_t - t;
/* use random position inside this segment to sample shader */
if(new_t == ray->t)
random_jitter_offset = lcg_step_float(&state->rng_congruential) * dt;
float3 new_P = ray->P + ray->D * (t + random_jitter_offset);
VolumeShaderCoefficients coeff;
/* compute segment */
if(volume_shader_sample(kg, sd, state, new_P, &coeff)) {
int closure_flag = sd->flag;
float3 new_tp;
float3 transmittance;
bool scatter = false;
/* distance sampling */
if((closure_flag & SD_SCATTER) || (has_scatter && (closure_flag & SD_ABSORPTION))) {
has_scatter = true;
float3 sigma_t = coeff.sigma_a + coeff.sigma_s;
float3 sigma_s = coeff.sigma_s;
/* compute transmittance over full step */
transmittance = volume_color_transmittance(sigma_t, dt);
/* decide if we will scatter or continue */
float sample_transmittance = kernel_volume_channel_get(transmittance, channel);
if(1.0f - xi >= sample_transmittance) {
/* compute sampling distance */
float sample_sigma_t = kernel_volume_channel_get(sigma_t, channel);
float new_dt = -logf(1.0f - xi)/sample_sigma_t;
new_t = t + new_dt;
/* transmittance and pdf */
float3 new_transmittance = volume_color_transmittance(sigma_t, new_dt);
float3 pdf = sigma_t * new_transmittance;
/* throughput */
new_tp = tp * sigma_s * new_transmittance / average(pdf);
scatter = true;
}
else {
/* throughput */
float pdf = average(transmittance);
new_tp = tp * transmittance / pdf;
/* remap xi so we can reuse it and keep thing stratified */
xi = 1.0f - (1.0f - xi)/sample_transmittance;
}
}
else if(closure_flag & SD_ABSORPTION) {
/* absorption only, no sampling needed */
float3 sigma_a = coeff.sigma_a;
transmittance = volume_color_transmittance(sigma_a, dt);
new_tp = tp * transmittance;
}
/* integrate emission attenuated by absorption */
if(L && (closure_flag & SD_EMISSION)) {
float3 emission = kernel_volume_emission_integrate(&coeff, closure_flag, transmittance, dt);
path_radiance_accum_emission(L, tp, emission, state->bounce);
}
/* modify throughput */
if(closure_flag & (SD_ABSORPTION|SD_SCATTER)) {
tp = new_tp;
/* stop if nearly all light blocked */
if(tp.x < tp_eps && tp.y < tp_eps && tp.z < tp_eps) {
tp = make_float3(0.0f, 0.0f, 0.0f);
break;
}
}
/* prepare to scatter to new direction */
if(scatter) {
/* adjust throughput and move to new location */
sd->P = ray->P + new_t*ray->D;
*throughput = tp;
return VOLUME_PATH_SCATTERED;
}
else {
/* accumulate transmittance */
accum_transmittance *= transmittance;
}
}
/* stop if at the end of the volume */
t = new_t;
if(t == ray->t)
break;
}
*throughput = tp;
return VOLUME_PATH_ATTENUATED;
}
/* get the volume attenuation and emission over line segment defined by
* ray, with the assumption that there are no surfaces blocking light
* between the endpoints. distance sampling is used to decide if we will
* scatter or not. */
ccl_device_noinline VolumeIntegrateResult kernel_volume_integrate(KernelGlobals *kg,
PathState *state, ShaderData *sd, Ray *ray, PathRadiance *L, float3 *throughput, RNG *rng)
{
/* workaround to fix correlation bug in T38710, can find better solution
* in random number generator later, for now this is done here to not impact
* performance of rendering without volumes */
RNG tmp_rng = cmj_hash(*rng, state->rng_offset);
bool heterogeneous = volume_stack_is_heterogeneous(kg, state->volume_stack);
shader_setup_from_volume(kg, sd, ray, state->bounce, state->transparent_bounce);
if(heterogeneous)
return kernel_volume_integrate_heterogeneous_distance(kg, state, ray, sd, L, throughput, &tmp_rng);
else
return kernel_volume_integrate_homogeneous(kg, state, ray, sd, L, throughput, &tmp_rng, true);
}
/* Decoupled Volume Sampling
*
* VolumeSegment is list of coefficients and transmittance stored at all steps
* through a volume. This can then latter be used for decoupled sampling as in:
* "Importance Sampling Techniques for Path Tracing in Participating Media"
*
* On the GPU this is only supported for homogeneous volumes (1 step), due to
* no support for malloc/free and too much stack usage with a fix size array. */
typedef struct VolumeStep {
float3 sigma_s; /* scatter coefficient */
float3 sigma_t; /* extinction coefficient */
float3 accum_transmittance; /* accumulated transmittance including this step */
float3 cdf_distance; /* cumulative density function for distance sampling */
float t; /* distance at end of this step */
float shade_t; /* jittered distance where shading was done in step */
int closure_flag; /* shader evaluation closure flags */
} VolumeStep;
typedef struct VolumeSegment {
#ifdef __KERNEL_CPU__
VolumeStep *steps; /* recorded steps */
#else
VolumeStep steps[1]; /* recorded steps */
#endif
int numsteps; /* number of steps */
int closure_flag; /* accumulated closure flags from all steps */
float3 accum_emission; /* accumulated emission at end of segment */
float3 accum_transmittance; /* accumulated transmittance at end of segment */
int sampling_method; /* volume sampling method */
} VolumeSegment;
/* record volume steps to the end of the volume.
*
* it would be nice if we could only record up to the point that we need to scatter,
* but the entire segment is needed to do always scattering, rather than probalistically
* hitting or missing the volume. if we don't know the transmittance at the end of the
* volume we can't generate stratitied distance samples up to that transmittance */
ccl_device void kernel_volume_decoupled_record(KernelGlobals *kg, PathState *state,
Ray *ray, ShaderData *sd, VolumeSegment *segment, bool heterogeneous)
{
const float tp_eps = 1e-6f; /* todo: this is likely not the right value */
/* prepare for volume stepping */
int max_steps;
float step_size, random_jitter_offset;
if(heterogeneous) {
max_steps = kernel_data.integrator.volume_max_steps;
step_size = kernel_data.integrator.volume_step_size;
random_jitter_offset = lcg_step_float(&state->rng_congruential) * step_size;
/* compute exact steps in advance for malloc */
max_steps = max((int)ceilf(ray->t/step_size), 1);
}
else {
max_steps = 1;
step_size = ray->t;
random_jitter_offset = 0.0f;
}
/* init accumulation variables */
float3 accum_emission = make_float3(0.0f, 0.0f, 0.0f);
float3 accum_transmittance = make_float3(1.0f, 1.0f, 1.0f);
float3 cdf_distance = make_float3(0.0f, 0.0f, 0.0f);
float t = 0.0f;
segment->closure_flag = 0;
segment->numsteps = 0;
#ifdef __KERNEL_CPU__
segment->steps = (VolumeStep*)malloc(sizeof(VolumeStep)*max_steps);
#else
kernel_assert(max_steps == 1);
#endif
VolumeStep *step = segment->steps;
for(int i = 0; i < max_steps; i++, step++) {
/* advance to new position */
float new_t = min(ray->t, (i+1) * step_size);
float dt = new_t - t;
/* use random position inside this segment to sample shader */
if(heterogeneous && new_t == ray->t)
random_jitter_offset = lcg_step_float(&state->rng_congruential) * dt;
float3 new_P = ray->P + ray->D * (t + random_jitter_offset);
VolumeShaderCoefficients coeff;
/* compute segment */
if(volume_shader_sample(kg, sd, state, new_P, &coeff)) {
int closure_flag = sd->flag;
float3 sigma_t = coeff.sigma_a + coeff.sigma_s;
/* compute accumulated transmittance */
float3 transmittance = volume_color_transmittance(sigma_t, dt);
/* compute emission attenuated by absorption */
if(closure_flag & SD_EMISSION) {
float3 emission = kernel_volume_emission_integrate(&coeff, closure_flag, transmittance, dt);
accum_emission += accum_transmittance * emission;
}
accum_transmittance *= transmittance;
/* compute pdf for distance sampling */
float3 pdf_distance = dt * accum_transmittance * coeff.sigma_s;
cdf_distance = cdf_distance + pdf_distance;
/* write step data */
step->sigma_t = sigma_t;
step->sigma_s = coeff.sigma_s;
step->closure_flag = closure_flag;
segment->closure_flag |= closure_flag;
}
else {
/* store empty step (todo: skip consecutive empty steps) */
step->sigma_t = make_float3(0.0f, 0.0f, 0.0f);
step->sigma_s = make_float3(0.0f, 0.0f, 0.0f);
step->closure_flag = 0;
}
step->accum_transmittance = accum_transmittance;
step->cdf_distance = cdf_distance;
step->t = new_t;
step->shade_t = t + random_jitter_offset;
segment->numsteps++;
/* stop if at the end of the volume */
t = new_t;
if(t == ray->t)
break;
/* stop if nearly all light blocked */
if(accum_transmittance.x < tp_eps && accum_transmittance.y < tp_eps && accum_transmittance.z < tp_eps)
break;
}
/* store total emission and transmittance */
segment->accum_emission = accum_emission;
segment->accum_transmittance = accum_transmittance;
/* normalize cumulative density function for distance sampling */
VolumeStep *last_step = segment->steps + segment->numsteps - 1;
if(!is_zero(last_step->cdf_distance)) {
VolumeStep *step = &segment->steps[0];
int numsteps = segment->numsteps;
float3 inv_cdf_distance_sum = safe_invert_color(last_step->cdf_distance);
for(int i = 0; i < numsteps; i++, step++)
step->cdf_distance *= inv_cdf_distance_sum;
}
}
ccl_device void kernel_volume_decoupled_free(KernelGlobals *kg, VolumeSegment *segment)
{
#ifdef __KERNEL_CPU__
free(segment->steps);
#endif
}
/* scattering for homogeneous and heterogeneous volumes, using decoupled ray
* marching. unlike the non-decoupled functions, these do not do probalistic
* scattering, they always scatter if there is any non-zero scattering
* coefficient.
*
* these also do not do emission or modify throughput. */
ccl_device VolumeIntegrateResult kernel_volume_decoupled_scatter(
KernelGlobals *kg, PathState *state, Ray *ray, ShaderData *sd,
float3 *throughput, float rphase, float rscatter,
const VolumeSegment *segment, const float3 *light_P, bool probalistic_scatter)
{
int closure_flag = segment->closure_flag;
/* XXX add probalistic scattering! */
if(!(closure_flag & SD_SCATTER))
return VOLUME_PATH_MISSED;
/* pick random color channel, we use the Veach one-sample
* model with balance heuristic for the channels */
int channel = (int)(rphase*3.0f);
sd->randb_closure = rphase*3.0f - channel;
float xi = rscatter;
/* probalistic scattering decision based on transmittance */
if(probalistic_scatter) {
float sample_transmittance = kernel_volume_channel_get(segment->accum_transmittance, channel);
if(1.0f - xi >= sample_transmittance) {
/* rescale random number so we can reuse it */
xi = 1.0f - (1.0f - xi - sample_transmittance)/(1.0f - sample_transmittance);
}
else
return VOLUME_PATH_MISSED;
}
VolumeStep *step;
float3 transmittance;
float pdf, sample_t;
float mis_weight = 1.0f;
bool distance_sample = true;
bool use_mis = false;
if(segment->sampling_method && light_P) {
if(segment->sampling_method == SD_VOLUME_MIS) {
/* multiple importance sample: randomly pick between
* equiangular and distance sampling strategy */
if(xi < 0.5f) {
xi *= 2.0f;
}
else {
xi = (xi - 0.5f)*2.0f;
distance_sample = false;
}
use_mis = true;
}
else {
/* only equiangular sampling */
distance_sample = false;
}
}
/* distance sampling */
if(distance_sample) {
/* find step in cdf */
step = segment->steps;
float prev_t = 0.0f;
float3 step_pdf_distance = make_float3(1.0f, 1.0f, 1.0f);
if(segment->numsteps > 1) {
float prev_cdf = 0.0f;
float step_cdf = 1.0f;
float3 prev_cdf_distance = make_float3(0.0f, 0.0f, 0.0f);
for(int i = 0; ; i++, step++) {
/* todo: optimize using binary search */
step_cdf = kernel_volume_channel_get(step->cdf_distance, channel);
if(xi < step_cdf || i == segment->numsteps-1)
break;
prev_cdf = step_cdf;
prev_t = step->t;
prev_cdf_distance = step->cdf_distance;
}
/* remap xi so we can reuse it */
xi = (xi - prev_cdf)/(step_cdf - prev_cdf);
/* pdf for picking step */
step_pdf_distance = step->cdf_distance - prev_cdf_distance;
}
/* determine range in which we will sample */
float step_t = step->t - prev_t;
/* sample distance and compute transmittance */
float3 distance_pdf;
sample_t = prev_t + kernel_volume_distance_sample(step_t, step->sigma_t, channel, xi, &transmittance, &distance_pdf);
/* modifiy pdf for hit/miss decision */
if(probalistic_scatter)
distance_pdf *= make_float3(1.0f, 1.0f, 1.0f) - segment->accum_transmittance;
pdf = average(distance_pdf * step_pdf_distance);
/* multiple importance sampling */
if(use_mis) {
float equi_pdf = kernel_volume_equiangular_pdf(ray, *light_P, sample_t);
mis_weight = 2.0f*power_heuristic(pdf, equi_pdf);
}
}
/* equi-angular sampling */
else {
/* sample distance */
sample_t = kernel_volume_equiangular_sample(ray, *light_P, xi, &pdf);
/* find step in which sampled distance is located */
step = segment->steps;
float prev_t = 0.0f;
float3 step_pdf_distance = make_float3(1.0f, 1.0f, 1.0f);
if(segment->numsteps > 1) {
/* todo: optimize using binary search */
float3 prev_cdf_distance = make_float3(0.0f, 0.0f, 0.0f);
for(int i = 0; i < segment->numsteps-1; i++, step++) {
if(sample_t < step->t)
break;
prev_t = step->t;
prev_cdf_distance = step->cdf_distance;
}
/* pdf for picking step with distance sampling */
step_pdf_distance = step->cdf_distance - prev_cdf_distance;
}
/* determine range in which we will sample */
float step_t = step->t - prev_t;
float step_sample_t = sample_t - prev_t;
/* compute transmittance */
transmittance = volume_color_transmittance(step->sigma_t, step_sample_t);
/* multiple importance sampling */
if(use_mis) {
float3 distance_pdf3 = kernel_volume_distance_pdf(step_t, step->sigma_t, step_sample_t);
float distance_pdf = average(distance_pdf3 * step_pdf_distance);
mis_weight = 2.0f*power_heuristic(pdf, distance_pdf);
}
}
/* compute transmittance up to this step */
if(step != segment->steps)
transmittance *= (step-1)->accum_transmittance;
/* modify throughput */
*throughput *= step->sigma_s * transmittance * (mis_weight / pdf);
/* evaluate shader to create closures at shading point */
if(segment->numsteps > 1) {
sd->P = ray->P + step->shade_t*ray->D;
VolumeShaderCoefficients coeff;
volume_shader_sample(kg, sd, state, sd->P, &coeff);
}
/* move to new position */
sd->P = ray->P + sample_t*ray->D;
return VOLUME_PATH_SCATTERED;
}
/* decide if we need to use decoupled or not */
ccl_device bool kernel_volume_use_decoupled(KernelGlobals *kg, bool heterogeneous, bool direct, int sampling_method)
{
/* decoupled ray marching for heterogenous volumes not supported on the GPU,
* which also means equiangular and multiple importance sampling is not
* support for that case */
#ifdef __KERNEL_GPU__
if(heterogeneous)
return false;
#endif
/* equiangular and multiple importance sampling only implemented for decoupled */
if(sampling_method != 0)
return true;
/* for all light sampling use decoupled, reusing shader evaluations is
* typically faster in that case */
if(direct)
return kernel_data.integrator.sample_all_lights_direct;
else
return kernel_data.integrator.sample_all_lights_indirect;
}
/* Volume Stack
*
* This is an array of object/shared ID's that the current segment of the path
* is inside of. */
ccl_device void kernel_volume_stack_init(KernelGlobals *kg, VolumeStack *stack)
{
/* todo: this assumes camera is always in air, need to detect when it isn't */
if(kernel_data.background.volume_shader == SHADER_NONE) {
stack[0].shader = SHADER_NONE;
}
else {
stack[0].shader = kernel_data.background.volume_shader;
stack[0].object = PRIM_NONE;
stack[1].shader = SHADER_NONE;
}
}
ccl_device void kernel_volume_stack_enter_exit(KernelGlobals *kg, ShaderData *sd, VolumeStack *stack)
{
/* todo: we should have some way for objects to indicate if they want the
* world shader to work inside them. excluding it by default is problematic
* because non-volume objects can't be assumed to be closed manifolds */
if(!(sd->flag & SD_HAS_VOLUME))
return;
if(sd->flag & SD_BACKFACING) {
/* exit volume object: remove from stack */
for(int i = 0; stack[i].shader != SHADER_NONE; i++) {
if(stack[i].object == sd->object) {
/* shift back next stack entries */
do {
stack[i] = stack[i+1];
i++;
}
while(stack[i].shader != SHADER_NONE);
return;
}
}
}
else {
/* enter volume object: add to stack */
int i;
for(i = 0; stack[i].shader != SHADER_NONE; i++) {
/* already in the stack? then we have nothing to do */
if(stack[i].object == sd->object)
return;
}
/* if we exceed the stack limit, ignore */
if(i >= VOLUME_STACK_SIZE-1)
return;
/* add to the end of the stack */
stack[i].shader = sd->shader;
stack[i].object = sd->object;
stack[i+1].shader = SHADER_NONE;
}
}
CCL_NAMESPACE_END