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blender/intern/cycles/kernel/kernel_volume.h
Brecht Van Lommel 095a01a73a Cycles: slightly improve BSDF sample stratification for path tracing. 
Similar to what we did for area lights previously, this should help
preserve stratification when using multiple BSDFs in theory. Improvements
are not easily noticeable in practice though, because the number of BSDFs
is usually low. Still nice to eliminate one sampling dimension.
2017-09-20 19:38:08 +02:00

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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
/* 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_inline bool volume_shader_extinction_sample(KernelGlobals *kg,
ShaderData *sd,
ccl_addr_space PathState *state,
float3 P,
float3 *extinction)
{
sd->P = P;
shader_eval_volume(kg, sd, state, state->volume_stack, PATH_RAY_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_inline bool volume_shader_sample(KernelGlobals *kg,
ShaderData *sd,
ccl_addr_space PathState *state,
float3 P,
VolumeShaderCoefficients *coeff)
{
sd->P = P;
shader_eval_volume(kg, sd, state, state->volume_stack, state->flag);
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, ccl_addr_space 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)*SHADER_SIZE);
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)*SHADER_SIZE);
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,
ccl_addr_space 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,
ccl_addr_space 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_addrspace(&state->rng_congruential) * step;
/* compute extinction at the start */
float t = 0.0f;
float3 sum = make_float3(0.0f, 0.0f, 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_addrspace(&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)) {
/* Compute expf() only for every Nth step, to save some calculations
* because exp(a)*exp(b) = exp(a+b), also do a quick tp_eps check then. */
sum += (-sigma_t * (new_t - t));
if((i & 0x07) == 0) { /* ToDo: Other interval? */
tp = *throughput * make_float3(expf(sum.x), expf(sum.y), expf(sum.z));
/* stop if nearly all light is 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) {
/* Update throughput in case we haven't done it above */
tp = *throughput * make_float3(expf(sum.x), expf(sum.y), expf(sum.z));
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,
ShaderData *shadow_sd,
ccl_addr_space PathState *state,
Ray *ray,
float3 *throughput)
{
shader_setup_from_volume(kg, shadow_sd, ray);
if(volume_stack_is_heterogeneous(kg, state->volume_stack))
kernel_volume_shadow_heterogeneous(kg, state, ray, shadow_sd, throughput);
else
kernel_volume_shadow_homogeneous(kg, state, ray, shadow_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 = safe_sqrtf(len_squared(light_P - ray->P) - delta * delta);
if(UNLIKELY(D == 0.0f)) {
*pdf = 0.0f;
return 0.0f;
}
float theta_a = -atan2f(delta, D);
float theta_b = atan2f(t - delta, D);
float t_ = D * tanf((xi * theta_b) + (1 - xi) * theta_a);
if(UNLIKELY(theta_b == theta_a)) {
*pdf = 0.0f;
return 0.0f;
}
*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 = safe_sqrtf(len_squared(light_P - ray->P) - delta * delta);
if(UNLIKELY(D == 0.0f)) {
return 0.0f;
}
float t = ray->t;
float t_ = sample_t - delta;
float theta_a = -atan2f(delta, D);
float theta_b = atan2f(t - delta, D);
if(UNLIKELY(theta_b == theta_a)) {
return 0.0f;
}
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 = safe_divide_color(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 safe_divide_color(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,
ccl_addr_space PathState *state,
Ray *ray,
ShaderData *sd,
PathRadiance *L,
ccl_addr_space float3 *throughput,
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;
#ifdef __VOLUME_SCATTER__
/* 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(kg, state, PRNG_PHASE_CHANNEL);
int channel = (int)(rphase*3.0f);
/* decide if we will hit or miss */
bool scatter = true;
float xi = path_state_rng_1D(kg, 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);
/* modify 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
#endif
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, state, *throughput, emission);
}
/* 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 probabilistically 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,
ccl_addr_space PathState *state,
Ray *ray,
ShaderData *sd,
PathRadiance *L,
ccl_addr_space 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_size = kernel_data.integrator.volume_step_size;
float random_jitter_offset = lcg_step_float_addrspace(&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(kg, state, PRNG_SCATTER_DISTANCE);
float rphase = path_state_rng_1D(kg, state, PRNG_PHASE_CHANNEL);
int channel = (int)(rphase*3.0f);
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_addrspace(&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 */
#ifdef __VOLUME_SCATTER__
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
#endif
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, state, tp, emission);
}
/* 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,
ccl_addr_space PathState *state,
ShaderData *sd,
Ray *ray,
PathRadiance *L,
ccl_addr_space float3 *throughput,
bool heterogeneous)
{
shader_setup_from_volume(kg, sd, ray);
if(heterogeneous)
return kernel_volume_integrate_heterogeneous_distance(kg, state, ray, sd, L, throughput);
else
return kernel_volume_integrate_homogeneous(kg, state, ray, sd, L, throughput, true);
}
#ifndef __SPLIT_KERNEL__
/* Decoupled Volume Sampling
*
* VolumeSegment is list of coefficients and transmittance stored at all steps
* through a volume. This can then later be used for decoupled sampling as in:
* "Importance Sampling Techniques for Path Tracing in Participating Media"
*
* On the GPU this is only supported (but currently not enabled)
* 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 {
VolumeStep stack_step; /* stack storage for homogeneous step, to avoid malloc */
VolumeStep *steps; /* recorded steps */
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 probabilistically
* hitting or missing the volume. if we don't know the transmittance at the end of the
* volume we can't generate stratified distance samples up to that transmittance */
#ifdef __VOLUME_DECOUPLED__
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) {
const int global_max_steps = kernel_data.integrator.volume_max_steps;
step_size = kernel_data.integrator.volume_step_size;
/* compute exact steps in advance for malloc */
if(ray->t > global_max_steps*step_size) {
max_steps = global_max_steps;
step_size = ray->t / (float)max_steps;
}
else {
max_steps = max((int)ceilf(ray->t/step_size), 1);
}
#ifdef __KERNEL_CPU__
/* NOTE: For the branched path tracing it's possible to have direct
* and indirect light integration both having volume segments allocated.
* We detect this using index in the pre-allocated memory. Currently we
* only support two segments allocated at a time, if more needed some
* modifications to the KernelGlobals will be needed.
*
* This gives us restrictions that decoupled record should only happen
* in the stack manner, meaning if there's subsequent call of decoupled
* record it'll need to free memory before it's caller frees memory.
*/
const int index = kg->decoupled_volume_steps_index;
assert(index < sizeof(kg->decoupled_volume_steps) /
sizeof(*kg->decoupled_volume_steps));
if(kg->decoupled_volume_steps[index] == NULL) {
kg->decoupled_volume_steps[index] =
(VolumeStep*)malloc(sizeof(VolumeStep)*global_max_steps);
}
segment->steps = kg->decoupled_volume_steps[index];
++kg->decoupled_volume_steps_index;
#else
segment->steps = (VolumeStep*)malloc(sizeof(VolumeStep)*max_steps);
#endif
random_jitter_offset = lcg_step_float(&state->rng_congruential) * step_size;
}
else {
max_steps = 1;
step_size = ray->t;
random_jitter_offset = 0.0f;
segment->steps = &segment->stack_step;
}
/* 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->numsteps = 0;
segment->closure_flag = 0;
bool is_last_step_empty = false;
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;
is_last_step_empty = false;
segment->numsteps++;
}
else {
if(is_last_step_empty) {
/* consecutive empty step, merge */
step--;
}
else {
/* store empty step */
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;
segment->numsteps++;
is_last_step_empty = true;
}
}
step->accum_transmittance = accum_transmittance;
step->cdf_distance = cdf_distance;
step->t = new_t;
step->shade_t = t + random_jitter_offset;
/* 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)
{
if(segment->steps != &segment->stack_step) {
#ifdef __KERNEL_CPU__
/* NOTE: We only allow free last allocated segment.
* No random order of alloc/free is supported.
*/
assert(kg->decoupled_volume_steps_index > 0);
assert(segment->steps == kg->decoupled_volume_steps[kg->decoupled_volume_steps_index - 1]);
--kg->decoupled_volume_steps_index;
#else
free(segment->steps);
#endif
}
}
#endif /* __VOLUME_DECOUPLED__ */
/* scattering for homogeneous and heterogeneous volumes, using decoupled ray
* marching.
*
* function is expected to return VOLUME_PATH_SCATTERED when probalistic_scatter is false */
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)
{
kernel_assert(segment->closure_flag & SD_SCATTER);
/* pick random color channel, we use the Veach one-sample
* model with balance heuristic for the channels */
int channel = (int)(rphase*3.0f);
float xi = rscatter;
/* probabilistic 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 {
*throughput /= sample_transmittance;
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);
/* modify 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) {
float3 prev_cdf_distance = make_float3(0.0f, 0.0f, 0.0f);
int numsteps = segment->numsteps;
int high = numsteps - 1;
int low = 0;
int mid;
while(low < high) {
mid = (low + high) >> 1;
if(sample_t < step[mid].t)
high = mid;
else if(sample_t >= step[mid + 1].t)
low = mid + 1;
else {
/* found our interval in step[mid] .. step[mid+1] */
prev_t = step[mid].t;
prev_cdf_distance = step[mid].cdf_distance;
step += mid+1;
break;
}
}
if(low >= numsteps - 1) {
prev_t = step[numsteps - 1].t;
prev_cdf_distance = step[numsteps-1].cdf_distance;
step += numsteps - 1;
}
/* 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);
}
}
if(sample_t < 0.0f || pdf == 0.0f) {
return VOLUME_PATH_MISSED;
}
/* 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;
}
#endif /* __SPLIT_KERNEL */
/* 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 heterogeneous 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,
ShaderData *stack_sd,
ccl_addr_space const PathState *state,
ccl_addr_space const Ray *ray,
ccl_addr_space VolumeStack *stack)
{
/* NULL ray happens in the baker, does it need proper initialization of
* camera in volume?
*/
if(!kernel_data.cam.is_inside_volume || ray == NULL) {
/* Camera is guaranteed to be in the air, only take background volume
* into account in this case.
*/
if(kernel_data.background.volume_shader != SHADER_NONE) {
stack[0].shader = kernel_data.background.volume_shader;
stack[0].object = PRIM_NONE;
stack[1].shader = SHADER_NONE;
}
else {
stack[0].shader = SHADER_NONE;
}
return;
}
kernel_assert(state->flag & PATH_RAY_CAMERA);
Ray volume_ray = *ray;
volume_ray.t = FLT_MAX;
const uint visibility = (state->flag & PATH_RAY_ALL_VISIBILITY);
int stack_index = 0, enclosed_index = 0;
#ifdef __VOLUME_RECORD_ALL__
Intersection hits[2*VOLUME_STACK_SIZE + 1];
uint num_hits = scene_intersect_volume_all(kg,
&volume_ray,
hits,
2*VOLUME_STACK_SIZE,
visibility);
if(num_hits > 0) {
int enclosed_volumes[VOLUME_STACK_SIZE];
Intersection *isect = hits;
qsort(hits, num_hits, sizeof(Intersection), intersections_compare);
for(uint hit = 0; hit < num_hits; ++hit, ++isect) {
shader_setup_from_ray(kg, stack_sd, isect, &volume_ray);
if(stack_sd->flag & SD_BACKFACING) {
bool need_add = true;
for(int i = 0; i < enclosed_index && need_add; ++i) {
/* If ray exited the volume and never entered to that volume
* it means that camera is inside such a volume.
*/
if(enclosed_volumes[i] == stack_sd->object) {
need_add = false;
}
}
for(int i = 0; i < stack_index && need_add; ++i) {
/* Don't add intersections twice. */
if(stack[i].object == stack_sd->object) {
need_add = false;
break;
}
}
if(need_add) {
stack[stack_index].object = stack_sd->object;
stack[stack_index].shader = stack_sd->shader;
++stack_index;
}
}
else {
/* If ray from camera enters the volume, this volume shouldn't
* be added to the stack on exit.
*/
enclosed_volumes[enclosed_index++] = stack_sd->object;
}
}
}
#else
int enclosed_volumes[VOLUME_STACK_SIZE];
int step = 0;
while(stack_index < VOLUME_STACK_SIZE - 1 &&
enclosed_index < VOLUME_STACK_SIZE - 1 &&
step < 2 * VOLUME_STACK_SIZE)
{
Intersection isect;
if(!scene_intersect_volume(kg, &volume_ray, &isect, visibility)) {
break;
}
shader_setup_from_ray(kg, stack_sd, &isect, &volume_ray);
if(stack_sd->flag & SD_BACKFACING) {
/* If ray exited the volume and never entered to that volume
* it means that camera is inside such a volume.
*/
bool need_add = true;
for(int i = 0; i < enclosed_index && need_add; ++i) {
/* If ray exited the volume and never entered to that volume
* it means that camera is inside such a volume.
*/
if(enclosed_volumes[i] == stack_sd->object) {
need_add = false;
}
}
for(int i = 0; i < stack_index && need_add; ++i) {
/* Don't add intersections twice. */
if(stack[i].object == stack_sd->object) {
need_add = false;
break;
}
}
if(need_add) {
stack[stack_index].object = stack_sd->object;
stack[stack_index].shader = stack_sd->shader;
++stack_index;
}
}
else {
/* If ray from camera enters the volume, this volume shouldn't
* be added to the stack on exit.
*/
enclosed_volumes[enclosed_index++] = stack_sd->object;
}
/* Move ray forward. */
volume_ray.P = ray_offset(stack_sd->P, -stack_sd->Ng);
++step;
}
#endif
/* stack_index of 0 means quick checks outside of the kernel gave false
* positive, nothing to worry about, just we've wasted quite a few of
* ticks just to come into conclusion that camera is in the air.
*
* In this case we're doing the same above -- check whether background has
* volume.
*/
if(stack_index == 0 && kernel_data.background.volume_shader == SHADER_NONE) {
stack[0].shader = kernel_data.background.volume_shader;
stack[0].object = PRIM_NONE;
stack[1].shader = SHADER_NONE;
}
else {
stack[stack_index].shader = SHADER_NONE;
}
}
ccl_device void kernel_volume_stack_enter_exit(KernelGlobals *kg, ShaderData *sd, ccl_addr_space 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;
}
}
#ifdef __SUBSURFACE__
ccl_device void kernel_volume_stack_update_for_subsurface(KernelGlobals *kg,
ShaderData *stack_sd,
Ray *ray,
ccl_addr_space VolumeStack *stack)
{
kernel_assert(kernel_data.integrator.use_volumes);
Ray volume_ray = *ray;
# ifdef __VOLUME_RECORD_ALL__
Intersection hits[2*VOLUME_STACK_SIZE + 1];
uint num_hits = scene_intersect_volume_all(kg,
&volume_ray,
hits,
2*VOLUME_STACK_SIZE,
PATH_RAY_ALL_VISIBILITY);
if(num_hits > 0) {
Intersection *isect = hits;
qsort(hits, num_hits, sizeof(Intersection), intersections_compare);
for(uint hit = 0; hit < num_hits; ++hit, ++isect) {
shader_setup_from_ray(kg, stack_sd, isect, &volume_ray);
kernel_volume_stack_enter_exit(kg, stack_sd, stack);
}
}
# else
Intersection isect;
int step = 0;
float3 Pend = ray->P + ray->D*ray->t;
while(step < 2 * VOLUME_STACK_SIZE &&
scene_intersect_volume(kg,
&volume_ray,
&isect,
PATH_RAY_ALL_VISIBILITY))
{
shader_setup_from_ray(kg, stack_sd, &isect, &volume_ray);
kernel_volume_stack_enter_exit(kg, stack_sd, stack);
/* Move ray forward. */
volume_ray.P = ray_offset(stack_sd->P, -stack_sd->Ng);
if(volume_ray.t != FLT_MAX) {
volume_ray.D = normalize_len(Pend - volume_ray.P, &volume_ray.t);
}
++step;
}
# endif
}
#endif
/* Clean stack after the last bounce.
*
* It is expected that all volumes are closed manifolds, so at the time when ray
* hits nothing (for example, it is a last bounce which goes to environment) the
* only expected volume in the stack is the world's one. All the rest volume
* entries should have been exited already.
*
* This isn't always true because of ray intersection precision issues, which
* could lead us to an infinite non-world volume in the stack, causing render
* artifacts.
*
* Use this function after the last bounce to get rid of all volumes apart from
* the world's one after the last bounce to avoid render artifacts.
*/
ccl_device_inline void kernel_volume_clean_stack(KernelGlobals *kg,
ccl_addr_space VolumeStack *volume_stack)
{
if(kernel_data.background.volume_shader != SHADER_NONE) {
/* Keep the world's volume in stack. */
volume_stack[1].shader = SHADER_NONE;
}
else {
volume_stack[0].shader = SHADER_NONE;
}
}
CCL_NAMESPACE_END
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