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Cycles: improve sample stratification on area lights for path tracing.

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Previously we used a 1D sequence to select a light, and another 2D sequence
to sample a point on the light. For multiple lights this meant each light
would get a random subset of a 2D stratified sequence, which is not
guaranteed to be stratified anymore.

Now we use only a 2D sequence, split into segments along the X axis, one for
each light. The samples that fall within a segment then each are a stratified
sequence, at least in the limit. So for example for two lights, we split up
the unit square into two segments [0,0.5[ x [0,1[ and [0.5,1[ x [0,1[.

This doesn't make much difference in most scenes, mainly helps if you have a
few large area lights or some types of HDR backgrounds.
This commit is contained in:
Brecht Van Lommel
2017-09-08 01:42:14 +02:00
parent d454a44e96
commit cd6c9e9e5f
5 changed files with 29 additions and 28 deletions
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26
intern/cycles/kernel/kernel_light.h
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@@ -1013,20 +1013,21 @@ ccl_device_forceinline void triangle_light_sample(KernelGlobals *kg, int prim, i
/* Light Distribution */ /* Light Distribution */
ccl_device int light_distribution_sample(KernelGlobals *kg, float randt) ccl_device int light_distribution_sample(KernelGlobals *kg, float *randu)
{ {
/* this is basically std::upper_bound as used by pbrt, to find a point light or /* This is basically std::upper_bound as used by pbrt, to find a point light or
* triangle to emit from, proportional to area. a good improvement would be to * triangle to emit from, proportional to area. a good improvement would be to
* also sample proportional to power, though it's not so well defined with * also sample proportional to power, though it's not so well defined with
* OSL shaders. */ * arbitrary shaders. */
int first = 0; int first = 0;
int len = kernel_data.integrator.num_distribution + 1; int len = kernel_data.integrator.num_distribution + 1;
float r = *randu;
while(len > 0) { while(len > 0) {
int half_len = len >> 1; int half_len = len >> 1;
int middle = first + half_len; int middle = first + half_len;
if(randt < kernel_tex_fetch(__light_distribution, middle).x) { if(r < kernel_tex_fetch(__light_distribution, middle).x) {
len = half_len; len = half_len;
} }
else { else {
@@ -1035,9 +1036,17 @@ ccl_device int light_distribution_sample(KernelGlobals *kg, float randt)
} }
} }
/* clamping should not be needed but float rounding errors seem to /* Clamping should not be needed but float rounding errors seem to
* make this fail on rare occasions */ * make this fail on rare occasions. */
return clamp(first-1, 0, kernel_data.integrator.num_distribution-1); int index = clamp(first-1, 0, kernel_data.integrator.num_distribution-1);
/* Rescale to reuse random number. this helps the 2D samples within
* each area light be stratified as well. */
float distr_min = kernel_tex_fetch(__light_distribution, index).x;
float distr_max = kernel_tex_fetch(__light_distribution, index+1).x;
*randu = (r - distr_min)/(distr_max - distr_min);
return index;
} }
/* Generic Light */ /* Generic Light */
@@ -1049,7 +1058,6 @@ ccl_device bool light_select_reached_max_bounces(KernelGlobals *kg, int index, i
} }
ccl_device_noinline bool light_sample(KernelGlobals *kg, ccl_device_noinline bool light_sample(KernelGlobals *kg,
float randt,
float randu, float randu,
float randv, float randv,
float time, float time,
@@ -1058,7 +1066,7 @@ ccl_device_noinline bool light_sample(KernelGlobals *kg,
LightSample *ls) LightSample *ls)
{ {
/* sample index */ /* sample index */
int index = light_distribution_sample(kg, randt); int index = light_distribution_sample(kg, &randu);
/* fetch light data */ /* fetch light data */
float4 l = kernel_tex_fetch(__light_distribution, index); float4 l = kernel_tex_fetch(__light_distribution, index);

11
intern/cycles/kernel/kernel_path_surface.h
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@@ -85,17 +85,16 @@ ccl_device_noinline void kernel_branched_path_surface_connect_light(
float num_samples_inv = num_samples_adjust/num_samples; float num_samples_inv = num_samples_adjust/num_samples;
for(int j = 0; j < num_samples; j++) { for(int j = 0; j < num_samples; j++) {
float light_t = path_branched_rng_1D(kg, state->rng_hash, state, j, num_samples, PRNG_LIGHT);
float light_u, light_v; float light_u, light_v;
path_branched_rng_2D(kg, state->rng_hash, state, j, num_samples, PRNG_LIGHT_U, &light_u, &light_v); path_branched_rng_2D(kg, state->rng_hash, state, j, num_samples, PRNG_LIGHT_U, &light_u, &light_v);
float terminate = path_branched_rng_light_termination(kg, state->rng_hash, state, j, num_samples); float terminate = path_branched_rng_light_termination(kg, state->rng_hash, state, j, num_samples);
/* only sample triangle lights */ /* only sample triangle lights */
if(kernel_data.integrator.num_all_lights) if(kernel_data.integrator.num_all_lights)
light_t = 0.5f*light_t; light_u = 0.5f*light_u;
LightSample ls; LightSample ls;
if(light_sample(kg, light_t, light_u, light_v, sd->time, sd->P, state->bounce, &ls)) { if(light_sample(kg, light_u, light_v, sd->time, sd->P, state->bounce, &ls)) {
/* Same as above, probability needs to be corrected since the sampling was forced to select a mesh light. */ /* Same as above, probability needs to be corrected since the sampling was forced to select a mesh light. */
if(kernel_data.integrator.num_all_lights) if(kernel_data.integrator.num_all_lights)
ls.pdf *= 2.0f; ls.pdf *= 2.0f;
@@ -118,13 +117,12 @@ ccl_device_noinline void kernel_branched_path_surface_connect_light(
} }
else { else {
/* sample one light at random */ /* sample one light at random */
float light_t = path_state_rng_1D(kg, state, PRNG_LIGHT);
float light_u, light_v; float light_u, light_v;
path_state_rng_2D(kg, state, PRNG_LIGHT_U, &light_u, &light_v); path_state_rng_2D(kg, state, PRNG_LIGHT_U, &light_u, &light_v);
float terminate = path_state_rng_light_termination(kg, state); float terminate = path_state_rng_light_termination(kg, state);
LightSample ls; LightSample ls;
if(light_sample(kg, light_t, light_u, light_v, sd->time, sd->P, state->bounce, &ls)) { if(light_sample(kg, light_u, light_v, sd->time, sd->P, state->bounce, &ls)) {
/* sample random light */ /* sample random light */
if(direct_emission(kg, sd, emission_sd, &ls, state, &light_ray, &L_light, &is_lamp, terminate)) { if(direct_emission(kg, sd, emission_sd, &ls, state, &light_ray, &L_light, &is_lamp, terminate)) {
/* trace shadow ray */ /* trace shadow ray */
@@ -238,7 +236,6 @@ ccl_device_inline void kernel_path_surface_connect_light(KernelGlobals *kg,
#endif #endif
/* sample illumination from lights to find path contribution */ /* sample illumination from lights to find path contribution */
float light_t = path_state_rng_1D(kg, state, PRNG_LIGHT);
float light_u, light_v; float light_u, light_v;
path_state_rng_2D(kg, state, PRNG_LIGHT_U, &light_u, &light_v); path_state_rng_2D(kg, state, PRNG_LIGHT_U, &light_u, &light_v);
@@ -251,7 +248,7 @@ ccl_device_inline void kernel_path_surface_connect_light(KernelGlobals *kg,
#endif #endif
LightSample ls; LightSample ls;
if(light_sample(kg, light_t, light_u, light_v, sd->time, sd->P, state->bounce, &ls)) { if(light_sample(kg, light_u, light_v, sd->time, sd->P, state->bounce, &ls)) {
float terminate = path_state_rng_light_termination(kg, state); float terminate = path_state_rng_light_termination(kg, state);
if(direct_emission(kg, sd, emission_sd, &ls, state, &light_ray, &L_light, &is_lamp, terminate)) { if(direct_emission(kg, sd, emission_sd, &ls, state, &light_ray, &L_light, &is_lamp, terminate)) {
/* trace shadow ray */ /* trace shadow ray */

15
intern/cycles/kernel/kernel_path_volume.h
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@@ -31,7 +31,6 @@ ccl_device_inline void kernel_path_volume_connect_light(
return; return;
/* sample illumination from lights to find path contribution */ /* sample illumination from lights to find path contribution */
float light_t = path_state_rng_1D(kg, state, PRNG_LIGHT);
float light_u, light_v; float light_u, light_v;
path_state_rng_2D(kg, state, PRNG_LIGHT_U, &light_u, &light_v); path_state_rng_2D(kg, state, PRNG_LIGHT_U, &light_u, &light_v);
@@ -45,7 +44,7 @@ ccl_device_inline void kernel_path_volume_connect_light(
light_ray.time = sd->time; light_ray.time = sd->time;
# endif # endif
if(light_sample(kg, light_t, light_u, light_v, sd->time, sd->P, state->bounce, &ls)) if(light_sample(kg, light_u, light_v, sd->time, sd->P, state->bounce, &ls))
{ {
float terminate = path_state_rng_light_termination(kg, state); float terminate = path_state_rng_light_termination(kg, state);
if(direct_emission(kg, sd, emission_sd, &ls, state, &light_ray, &L_light, &is_lamp, terminate)) { if(direct_emission(kg, sd, emission_sd, &ls, state, &light_ray, &L_light, &is_lamp, terminate)) {
@@ -195,16 +194,15 @@ ccl_device void kernel_branched_path_volume_connect_light(
for(int j = 0; j < num_samples; j++) { for(int j = 0; j < num_samples; j++) {
/* sample random position on random triangle */ /* sample random position on random triangle */
float light_t = path_branched_rng_1D_for_decision(kg, state->rng_hash, state, j, num_samples, PRNG_LIGHT);
float light_u, light_v; float light_u, light_v;
path_branched_rng_2D(kg, state->rng_hash, state, j, num_samples, PRNG_LIGHT_U, &light_u, &light_v); path_branched_rng_2D(kg, state->rng_hash, state, j, num_samples, PRNG_LIGHT_U, &light_u, &light_v);
/* only sample triangle lights */ /* only sample triangle lights */
if(kernel_data.integrator.num_all_lights) if(kernel_data.integrator.num_all_lights)
light_t = 0.5f*light_t; light_u = 0.5f*light_u;
LightSample ls; LightSample ls;
light_sample(kg, light_t, light_u, light_v, sd->time, ray->P, state->bounce, &ls); light_sample(kg, light_u, light_v, sd->time, ray->P, state->bounce, &ls);
float3 tp = throughput; float3 tp = throughput;
@@ -219,7 +217,7 @@ ccl_device void kernel_branched_path_volume_connect_light(
kernel_assert(result == VOLUME_PATH_SCATTERED); kernel_assert(result == VOLUME_PATH_SCATTERED);
/* todo: split up light_sample so we don't have to call it again with new position */ /* todo: split up light_sample so we don't have to call it again with new position */
if(light_sample(kg, light_t, light_u, light_v, sd->time, sd->P, state->bounce, &ls)) { if(light_sample(kg, light_u, light_v, sd->time, sd->P, state->bounce, &ls)) {
if(kernel_data.integrator.num_all_lights) if(kernel_data.integrator.num_all_lights)
ls.pdf *= 2.0f; ls.pdf *= 2.0f;
@@ -239,12 +237,11 @@ ccl_device void kernel_branched_path_volume_connect_light(
} }
else { else {
/* sample random position on random light */ /* sample random position on random light */
float light_t = path_state_rng_1D(kg, state, PRNG_LIGHT);
float light_u, light_v; float light_u, light_v;
path_state_rng_2D(kg, state, PRNG_LIGHT_U, &light_u, &light_v); path_state_rng_2D(kg, state, PRNG_LIGHT_U, &light_u, &light_v);
LightSample ls; LightSample ls;
light_sample(kg, light_t, light_u, light_v, sd->time, ray->P, state->bounce, &ls); light_sample(kg, light_u, light_v, sd->time, ray->P, state->bounce, &ls);
float3 tp = throughput; float3 tp = throughput;
@@ -259,7 +256,7 @@ ccl_device void kernel_branched_path_volume_connect_light(
kernel_assert(result == VOLUME_PATH_SCATTERED); kernel_assert(result == VOLUME_PATH_SCATTERED);
/* todo: split up light_sample so we don't have to call it again with new position */ /* todo: split up light_sample so we don't have to call it again with new position */
if(light_sample(kg, light_t, light_u, light_v, sd->time, sd->P, state->bounce, &ls)) { if(light_sample(kg, light_u, light_v, sd->time, sd->P, state->bounce, &ls)) {
/* sample random light */ /* sample random light */
float terminate = path_state_rng_light_termination(kg, state); float terminate = path_state_rng_light_termination(kg, state);
if(direct_emission(kg, sd, emission_sd, &ls, state, &light_ray, &L_light, &is_lamp, terminate)) { if(direct_emission(kg, sd, emission_sd, &ls, state, &light_ray, &L_light, &is_lamp, terminate)) {

2
intern/cycles/kernel/kernel_types.h
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@@ -292,7 +292,7 @@ enum PathTraceDimension {
PRNG_BSDF_U = 0, PRNG_BSDF_U = 0,
PRNG_BSDF_V = 1, PRNG_BSDF_V = 1,
PRNG_BSDF = 2, PRNG_BSDF = 2,
PRNG_LIGHT = 3, PRNG_UNUSED3 = 3,
PRNG_LIGHT_U = 4, PRNG_LIGHT_U = 4,
PRNG_LIGHT_V = 5, PRNG_LIGHT_V = 5,
PRNG_LIGHT_TERMINATE = 6, PRNG_LIGHT_TERMINATE = 6,

3
intern/cycles/kernel/split/kernel_direct_lighting.h
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@@ -81,14 +81,13 @@ ccl_device void kernel_direct_lighting(KernelGlobals *kg,
if(flag) { if(flag) {
/* Sample illumination from lights to find path contribution. */ /* Sample illumination from lights to find path contribution. */
float light_t = path_state_rng_1D(kg, state, PRNG_LIGHT);
float light_u, light_v; float light_u, light_v;
path_state_rng_2D(kg, state, PRNG_LIGHT_U, &light_u, &light_v); path_state_rng_2D(kg, state, PRNG_LIGHT_U, &light_u, &light_v);
float terminate = path_state_rng_light_termination(kg, state); float terminate = path_state_rng_light_termination(kg, state);
LightSample ls; LightSample ls;
if(light_sample(kg, if(light_sample(kg,
light_t, light_u, light_v, light_u, light_v,
sd->time, sd->time,
sd->P, sd->P,
state->bounce, state->bounce,