intern/cycles/kernel/kernel_volume.h

   1 /*
   2  * Copyright 2011-2013 Blender Foundation
   3  *
   4  * Licensed under the Apache License, Version 2.0 (the "License");
   5  * you may not use this file except in compliance with the License.
   6  * You may obtain a copy of the License at
   7  *
   8  * http://www.apache.org/licenses/LICENSE-2.0
   9  *
  10  * Unless required by applicable law or agreed to in writing, software
  11  * distributed under the License is distributed on an "AS IS" BASIS,
  12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  13  * See the License for the specific language governing permissions and
  14  * limitations under the License.
  15  */
  16
  17 CCL_NAMESPACE_BEGIN
  18
  19 /* Events for probalistic scattering */
  20
  21 typedef enum VolumeIntegrateResult {
  22         VOLUME_PATH_SCATTERED = 0,
  23         VOLUME_PATH_ATTENUATED = 1,
  24         VOLUME_PATH_MISSED = 2
  25 } VolumeIntegrateResult;
  26
  27 /* Volume shader properties
  28  *
  29  * extinction coefficient = absorption coefficient + scattering coefficient
  30  * sigma_t = sigma_a + sigma_s */
  31
  32 typedef struct VolumeShaderCoefficients {
  33         float3 sigma_a;
  34         float3 sigma_s;
  35         float3 emission;
  36 } VolumeShaderCoefficients;
  37
  38 /* evaluate shader to get extinction coefficient at P */
  39 ccl_device bool volume_shader_extinction_sample(KernelGlobals *kg, ShaderData *sd, PathState *state, float3 P, float3 *extinction)
  40 {
  41         sd->P = P;
  42         shader_eval_volume(kg, sd, state, state->volume_stack, PATH_RAY_SHADOW, SHADER_CONTEXT_SHADOW);
  43
  44         if(!(sd->flag & (SD_ABSORPTION|SD_SCATTER)))
  45                 return false;
  46
  47         float3 sigma_t = make_float3(0.0f, 0.0f, 0.0f);
  48
  49         for(int i = 0; i < sd->num_closure; i++) {
  50                 const ShaderClosure *sc = &sd->closure[i];
  51
  52                 if(CLOSURE_IS_VOLUME(sc->type))
  53                         sigma_t += sc->weight;
  54         }
  55
  56         *extinction = sigma_t;
  57         return true;
  58 }
  59
  60 /* evaluate shader to get absorption, scattering and emission at P */
  61 ccl_device bool volume_shader_sample(KernelGlobals *kg, ShaderData *sd, PathState *state, float3 P, VolumeShaderCoefficients *coeff)
  62 {
  63         sd->P = P;
  64         shader_eval_volume(kg, sd, state, state->volume_stack, state->flag, SHADER_CONTEXT_VOLUME);
  65
  66         if(!(sd->flag & (SD_ABSORPTION|SD_SCATTER|SD_EMISSION)))
  67                 return false;
  68
  69         coeff->sigma_a = make_float3(0.0f, 0.0f, 0.0f);
  70         coeff->sigma_s = make_float3(0.0f, 0.0f, 0.0f);
  71         coeff->emission = make_float3(0.0f, 0.0f, 0.0f);
  72
  73         for(int i = 0; i < sd->num_closure; i++) {
  74                 const ShaderClosure *sc = &sd->closure[i];
  75
  76                 if(sc->type == CLOSURE_VOLUME_ABSORPTION_ID)
  77                         coeff->sigma_a += sc->weight;
  78                 else if(sc->type == CLOSURE_EMISSION_ID)
  79                         coeff->emission += sc->weight;
  80                 else if(CLOSURE_IS_VOLUME(sc->type))
  81                         coeff->sigma_s += sc->weight;
  82         }
  83
  84         /* when at the max number of bounces, treat scattering as absorption */
  85         if(sd->flag & SD_SCATTER) {
  86                 if(state->volume_bounce >= kernel_data.integrator.max_volume_bounce) {
  87                         coeff->sigma_a += coeff->sigma_s;
  88                         coeff->sigma_s = make_float3(0.0f, 0.0f, 0.0f);
  89                         sd->flag &= ~SD_SCATTER;
  90                         sd->flag |= SD_ABSORPTION;
  91                 }
  92         }
  93
  94         return true;
  95 }
  96
  97 ccl_device float3 volume_color_transmittance(float3 sigma, float t)
  98 {
  99         return make_float3(expf(-sigma.x * t), expf(-sigma.y * t), expf(-sigma.z * t));
 100 }
 101
 102 ccl_device float kernel_volume_channel_get(float3 value, int channel)
 103 {
 104         return (channel == 0)? value.x: ((channel == 1)? value.y: value.z);
 105 }
 106
 107 ccl_device bool volume_stack_is_heterogeneous(KernelGlobals *kg, VolumeStack *stack)
 108 {
 109         for(int i = 0; stack[i].shader != SHADER_NONE; i++) {
 110                 int shader_flag = kernel_tex_fetch(__shader_flag, (stack[i].shader & SHADER_MASK)*2);
 111
 112                 if(shader_flag & SD_HETEROGENEOUS_VOLUME)
 113                         return true;
 114         }
 115
 116         return false;
 117 }
 118
 119 ccl_device int volume_stack_sampling_method(KernelGlobals *kg, VolumeStack *stack)
 120 {
 121         if(kernel_data.integrator.num_all_lights == 0)
 122                 return 0;
 123
 124         int method = -1;
 125
 126         for(int i = 0; stack[i].shader != SHADER_NONE; i++) {
 127                 int shader_flag = kernel_tex_fetch(__shader_flag, (stack[i].shader & SHADER_MASK)*2);
 128
 129                 if(shader_flag & SD_VOLUME_MIS) {
 130                         return SD_VOLUME_MIS;
 131                 }
 132                 else if(shader_flag & SD_VOLUME_EQUIANGULAR) {
 133                         if(method == 0)
 134                                 return SD_VOLUME_MIS;
 135
 136                         method = SD_VOLUME_EQUIANGULAR;
 137                 }
 138                 else {
 139                         if(method == SD_VOLUME_EQUIANGULAR)
 140                                 return SD_VOLUME_MIS;
 141
 142                         method = 0;
 143                 }
 144         }
 145
 146         return method;
 147 }
 148
 149 /* Volume Shadows
 150  *
 151  * These functions are used to attenuate shadow rays to lights. Both absorption
 152  * and scattering will block light, represented by the extinction coefficient. */
 153
 154 /* homogeneous volume: assume shader evaluation at the starts gives
 155  * the extinction coefficient for the entire line segment */
 156 ccl_device void kernel_volume_shadow_homogeneous(KernelGlobals *kg, PathState *state, Ray *ray, ShaderData *sd, float3 *throughput)
 157 {
 158         float3 sigma_t;
 159
 160         if(volume_shader_extinction_sample(kg, sd, state, ray->P, &sigma_t))
 161                 *throughput *= volume_color_transmittance(sigma_t, ray->t);
 162 }
 163
 164 /* heterogeneous volume: integrate stepping through the volume until we
 165  * reach the end, get absorbed entirely, or run out of iterations */
 166 ccl_device void kernel_volume_shadow_heterogeneous(KernelGlobals *kg, PathState *state, Ray *ray, ShaderData *sd, float3 *throughput)
 167 {
 168         float3 tp = *throughput;
 169         const float tp_eps = 1e-6f; /* todo: this is likely not the right value */
 170
 171         /* prepare for stepping */
 172         int max_steps = kernel_data.integrator.volume_max_steps;
 173         float step = kernel_data.integrator.volume_step_size;
 174         float random_jitter_offset = lcg_step_float(&state->rng_congruential) * step;
 175
 176         /* compute extinction at the start */
 177         float t = 0.0f;
 178
 179         float3 sum = make_float3(0.0f, 0.0f, 0.0f);
 180
 181         for(int i = 0; i < max_steps; i++) {
 182                 /* advance to new position */
 183                 float new_t = min(ray->t, (i+1) * step);
 184                 float dt = new_t - t;
 185
 186                 /* use random position inside this segment to sample shader */
 187                 if(new_t == ray->t)
 188                         random_jitter_offset = lcg_step_float(&state->rng_congruential) * dt;
 189
 190                 float3 new_P = ray->P + ray->D * (t + random_jitter_offset);
 191                 float3 sigma_t;
 192
 193                 /* compute attenuation over segment */
 194                 if(volume_shader_extinction_sample(kg, sd, state, new_P, &sigma_t)) {
 195                         /* Compute expf() only for every Nth step, to save some calculations
 196                          * because exp(a)*exp(b) = exp(a+b), also do a quick tp_eps check then. */
 197
 198                         sum += (-sigma_t * (new_t - t));
 199                         if((i & 0x07) == 0) { /* ToDo: Other interval? */
 200                                 tp = *throughput * make_float3(expf(sum.x), expf(sum.y), expf(sum.z));
 201
 202                                 /* stop if nearly all light is blocked */
 203                                 if(tp.x < tp_eps && tp.y < tp_eps && tp.z < tp_eps)
 204                                         break;
 205                         }
 206                 }
 207
 208                 /* stop if at the end of the volume */
 209                 t = new_t;
 210                 if(t == ray->t) {
 211                         /* Update throughput in case we haven't done it above */
 212                         tp = *throughput * make_float3(expf(sum.x), expf(sum.y), expf(sum.z));
 213                         break;
 214                 }
 215         }
 216
 217         *throughput = tp;
 218 }
 219
 220 /* get the volume attenuation over line segment defined by ray, with the
 221  * assumption that there are no surfaces blocking light between the endpoints */
 222 ccl_device_noinline void kernel_volume_shadow(KernelGlobals *kg, PathState *state, Ray *ray, float3 *throughput)
 223 {
 224         ShaderData sd;
 225         shader_setup_from_volume(kg, &sd, ray);
 226
 227         if(volume_stack_is_heterogeneous(kg, state->volume_stack))
 228                 kernel_volume_shadow_heterogeneous(kg, state, ray, &sd, throughput);
 229         else
 230                 kernel_volume_shadow_homogeneous(kg, state, ray, &sd, throughput);
 231 }
 232
 233 /* Equi-angular sampling as in:
 234  * "Importance Sampling Techniques for Path Tracing in Participating Media" */
 235
 236 ccl_device float kernel_volume_equiangular_sample(Ray *ray, float3 light_P, float xi, float *pdf)
 237 {
 238         float t = ray->t;
 239
 240         float delta = dot((light_P - ray->P) , ray->D);
 241         float D = sqrtf(len_squared(light_P - ray->P) - delta * delta);
 242         float theta_a = -atan2f(delta, D);
 243         float theta_b = atan2f(t - delta, D);
 244         float t_ = D * tanf((xi * theta_b) + (1 - xi) * theta_a);
 245
 246         *pdf = D / ((theta_b - theta_a) * (D * D + t_ * t_));
 247
 248         return min(t, delta + t_); /* min is only for float precision errors */
 249 }
 250
 251 ccl_device float kernel_volume_equiangular_pdf(Ray *ray, float3 light_P, float sample_t)
 252 {
 253         float delta = dot((light_P - ray->P) , ray->D);
 254         float D = sqrtf(len_squared(light_P - ray->P) - delta * delta);
 255
 256         float t = ray->t;
 257         float t_ = sample_t - delta;
 258
 259         float theta_a = -atan2f(delta, D);
 260         float theta_b = atan2f(t - delta, D);
 261
 262         float pdf = D / ((theta_b - theta_a) * (D * D + t_ * t_));
 263
 264         return pdf;
 265 }
 266
 267 /* Distance sampling */
 268
 269 ccl_device float kernel_volume_distance_sample(float max_t, float3 sigma_t, int channel, float xi, float3 *transmittance, float3 *pdf)
 270 {
 271         /* xi is [0, 1[ so log(0) should never happen, division by zero is
 272          * avoided because sample_sigma_t > 0 when SD_SCATTER is set */
 273         float sample_sigma_t = kernel_volume_channel_get(sigma_t, channel);
 274         float3 full_transmittance = volume_color_transmittance(sigma_t, max_t);
 275         float sample_transmittance = kernel_volume_channel_get(full_transmittance, channel);
 276
 277         float sample_t = min(max_t, -logf(1.0f - xi*(1.0f - sample_transmittance))/sample_sigma_t);
 278
 279         *transmittance = volume_color_transmittance(sigma_t, sample_t);
 280         *pdf = (sigma_t * *transmittance)/(make_float3(1.0f, 1.0f, 1.0f) - full_transmittance);
 281
 282         /* todo: optimization: when taken together with hit/miss decision,
 283          * the full_transmittance cancels out drops out and xi does not
 284          * need to be remapped */
 285
 286         return sample_t;
 287 }
 288
 289 ccl_device float3 kernel_volume_distance_pdf(float max_t, float3 sigma_t, float sample_t)
 290 {
 291         float3 full_transmittance = volume_color_transmittance(sigma_t, max_t);
 292         float3 transmittance = volume_color_transmittance(sigma_t, sample_t);
 293
 294         return (sigma_t * transmittance)/(make_float3(1.0f, 1.0f, 1.0f) - full_transmittance);
 295 }
 296
 297 /* Emission */
 298
 299 ccl_device float3 kernel_volume_emission_integrate(VolumeShaderCoefficients *coeff, int closure_flag, float3 transmittance, float t)
 300 {
 301         /* integral E * exp(-sigma_t * t) from 0 to t = E * (1 - exp(-sigma_t * t))/sigma_t
 302          * this goes to E * t as sigma_t goes to zero
 303          *
 304          * todo: we should use an epsilon to avoid precision issues near zero sigma_t */
 305         float3 emission = coeff->emission;
 306
 307         if(closure_flag & SD_ABSORPTION) {
 308                 float3 sigma_t = coeff->sigma_a + coeff->sigma_s;
 309
 310                 emission.x *= (sigma_t.x > 0.0f)? (1.0f - transmittance.x)/sigma_t.x: t;
 311                 emission.y *= (sigma_t.y > 0.0f)? (1.0f - transmittance.y)/sigma_t.y: t;
 312                 emission.z *= (sigma_t.z > 0.0f)? (1.0f - transmittance.z)/sigma_t.z: t;
 313         }
 314         else
 315                 emission *= t;
 316
 317         return emission;
 318 }
 319
 320 /* Volume Path */
 321
 322 /* homogeneous volume: assume shader evaluation at the start gives
 323  * the volume shading coefficient for the entire line segment */
 324 ccl_device VolumeIntegrateResult kernel_volume_integrate_homogeneous(KernelGlobals *kg,
 325         PathState *state, Ray *ray, ShaderData *sd, PathRadiance *L, float3 *throughput,
 326         RNG *rng, bool probalistic_scatter)
 327 {
 328         VolumeShaderCoefficients coeff;
 329
 330         if(!volume_shader_sample(kg, sd, state, ray->P, &coeff))
 331                 return VOLUME_PATH_MISSED;
 332
 333         int closure_flag = sd->flag;
 334         float t = ray->t;
 335         float3 new_tp;
 336
 337 #ifdef __VOLUME_SCATTER__
 338         /* randomly scatter, and if we do t is shortened */
 339         if(closure_flag & SD_SCATTER) {
 340                 /* extinction coefficient */
 341                 float3 sigma_t = coeff.sigma_a + coeff.sigma_s;
 342
 343                 /* pick random color channel, we use the Veach one-sample
 344                  * model with balance heuristic for the channels */
 345                 float rphase = path_state_rng_1D_for_decision(kg, rng, state, PRNG_PHASE);
 346                 int channel = (int)(rphase*3.0f);
 347                 sd->randb_closure = rphase*3.0f - channel;
 348
 349                 /* decide if we will hit or miss */
 350                 bool scatter = true;
 351                 float xi = path_state_rng_1D_for_decision(kg, rng, state, PRNG_SCATTER_DISTANCE);
 352
 353                 if(probalistic_scatter) {
 354                         float sample_sigma_t = kernel_volume_channel_get(sigma_t, channel);
 355                         float sample_transmittance = expf(-sample_sigma_t * t);
 356
 357                         if(1.0f - xi >= sample_transmittance) {
 358                                 scatter = true;
 359
 360                                 /* rescale random number so we can reuse it */
 361                                 xi = 1.0f - (1.0f - xi - sample_transmittance)/(1.0f - sample_transmittance);
 362
 363                         }
 364                         else
 365                                 scatter = false;
 366                 }
 367
 368                 if(scatter) {
 369                         /* scattering */
 370                         float3 pdf;
 371                         float3 transmittance;
 372                         float sample_t;
 373
 374                         /* distance sampling */
 375                         sample_t = kernel_volume_distance_sample(ray->t, sigma_t, channel, xi, &transmittance, &pdf);
 376
 377                         /* modify pdf for hit/miss decision */
 378                         if(probalistic_scatter)
 379                                 pdf *= make_float3(1.0f, 1.0f, 1.0f) - volume_color_transmittance(sigma_t, t);
 380
 381                         new_tp = *throughput * coeff.sigma_s * transmittance / average(pdf);
 382                         t = sample_t;
 383                 }
 384                 else {
 385                         /* no scattering */
 386                         float3 transmittance = volume_color_transmittance(sigma_t, t);
 387                         float pdf = average(transmittance);
 388                         new_tp = *throughput * transmittance / pdf;
 389                 }
 390         }
 391         else
 392 #endif
 393         if(closure_flag & SD_ABSORPTION) {
 394                 /* absorption only, no sampling needed */
 395                 float3 transmittance = volume_color_transmittance(coeff.sigma_a, t);
 396                 new_tp = *throughput * transmittance;
 397         }
 398
 399         /* integrate emission attenuated by extinction */
 400         if(L && (closure_flag & SD_EMISSION)) {
 401                 float3 sigma_t = coeff.sigma_a + coeff.sigma_s;
 402                 float3 transmittance = volume_color_transmittance(sigma_t, ray->t);
 403                 float3 emission = kernel_volume_emission_integrate(&coeff, closure_flag, transmittance, ray->t);
 404                 path_radiance_accum_emission(L, *throughput, emission, state->bounce);
 405         }
 406
 407         /* modify throughput */
 408         if(closure_flag & (SD_ABSORPTION|SD_SCATTER)) {
 409                 *throughput = new_tp;
 410
 411                 /* prepare to scatter to new direction */
 412                 if(t < ray->t) {
 413                         /* adjust throughput and move to new location */
 414                         sd->P = ray->P + t*ray->D;
 415
 416                         return VOLUME_PATH_SCATTERED;
 417                 }
 418         }
 419
 420         return VOLUME_PATH_ATTENUATED;
 421 }
 422
 423 /* heterogeneous volume distance sampling: integrate stepping through the
 424  * volume until we reach the end, get absorbed entirely, or run out of
 425  * iterations. this does probabilistically scatter or get transmitted through
 426  * for path tracing where we don't want to branch. */
 427 ccl_device VolumeIntegrateResult kernel_volume_integrate_heterogeneous_distance(KernelGlobals *kg,
 428         PathState *state, Ray *ray, ShaderData *sd, PathRadiance *L, float3 *throughput, RNG *rng)
 429 {
 430         float3 tp = *throughput;
 431         const float tp_eps = 1e-6f; /* todo: this is likely not the right value */
 432
 433         /* prepare for stepping */
 434         int max_steps = kernel_data.integrator.volume_max_steps;
 435         float step_size = kernel_data.integrator.volume_step_size;
 436         float random_jitter_offset = lcg_step_float(&state->rng_congruential) * step_size;
 437
 438         /* compute coefficients at the start */
 439         float t = 0.0f;
 440         float3 accum_transmittance = make_float3(1.0f, 1.0f, 1.0f);
 441
 442         /* pick random color channel, we use the Veach one-sample
 443          * model with balance heuristic for the channels */
 444         float xi = path_state_rng_1D_for_decision(kg, rng, state, PRNG_SCATTER_DISTANCE);
 445         float rphase = path_state_rng_1D_for_decision(kg, rng, state, PRNG_PHASE);
 446         int channel = (int)(rphase*3.0f);
 447         sd->randb_closure = rphase*3.0f - channel;
 448         bool has_scatter = false;
 449
 450         for(int i = 0; i < max_steps; i++) {
 451                 /* advance to new position */
 452                 float new_t = min(ray->t, (i+1) * step_size);
 453                 float dt = new_t - t;
 454
 455                 /* use random position inside this segment to sample shader */
 456                 if(new_t == ray->t)
 457                         random_jitter_offset = lcg_step_float(&state->rng_congruential) * dt;
 458
 459                 float3 new_P = ray->P + ray->D * (t + random_jitter_offset);
 460                 VolumeShaderCoefficients coeff;
 461
 462                 /* compute segment */
 463                 if(volume_shader_sample(kg, sd, state, new_P, &coeff)) {
 464                         int closure_flag = sd->flag;
 465                         float3 new_tp;
 466                         float3 transmittance;
 467                         bool scatter = false;
 468
 469                         /* distance sampling */
 470 #ifdef __VOLUME_SCATTER__
 471                         if((closure_flag & SD_SCATTER) || (has_scatter && (closure_flag & SD_ABSORPTION))) {
 472                                 has_scatter = true;
 473
 474                                 float3 sigma_t = coeff.sigma_a + coeff.sigma_s;
 475                                 float3 sigma_s = coeff.sigma_s;
 476
 477                                 /* compute transmittance over full step */
 478                                 transmittance = volume_color_transmittance(sigma_t, dt);
 479
 480                                 /* decide if we will scatter or continue */
 481                                 float sample_transmittance = kernel_volume_channel_get(transmittance, channel);
 482
 483                                 if(1.0f - xi >= sample_transmittance) {
 484                                         /* compute sampling distance */
 485                                         float sample_sigma_t = kernel_volume_channel_get(sigma_t, channel);
 486                                         float new_dt = -logf(1.0f - xi)/sample_sigma_t;
 487                                         new_t = t + new_dt;
 488
 489                                         /* transmittance and pdf */
 490                                         float3 new_transmittance = volume_color_transmittance(sigma_t, new_dt);
 491                                         float3 pdf = sigma_t * new_transmittance;
 492
 493                                         /* throughput */
 494                                         new_tp = tp * sigma_s * new_transmittance / average(pdf);
 495                                         scatter = true;
 496                                 }
 497                                 else {
 498                                         /* throughput */
 499                                         float pdf = average(transmittance);
 500                                         new_tp = tp * transmittance / pdf;
 501
 502                                         /* remap xi so we can reuse it and keep thing stratified */
 503                                         xi = 1.0f - (1.0f - xi)/sample_transmittance;
 504                                 }
 505                         }
 506                         else
 507 #endif
 508                         if(closure_flag & SD_ABSORPTION) {
 509                                 /* absorption only, no sampling needed */
 510                                 float3 sigma_a = coeff.sigma_a;
 511
 512                                 transmittance = volume_color_transmittance(sigma_a, dt);
 513                                 new_tp = tp * transmittance;
 514                         }
 515
 516                         /* integrate emission attenuated by absorption */
 517                         if(L && (closure_flag & SD_EMISSION)) {
 518                                 float3 emission = kernel_volume_emission_integrate(&coeff, closure_flag, transmittance, dt);
 519                                 path_radiance_accum_emission(L, tp, emission, state->bounce);
 520                         }
 521
 522                         /* modify throughput */
 523                         if(closure_flag & (SD_ABSORPTION|SD_SCATTER)) {
 524                                 tp = new_tp;
 525
 526                                 /* stop if nearly all light blocked */
 527                                 if(tp.x < tp_eps && tp.y < tp_eps && tp.z < tp_eps) {
 528                                         tp = make_float3(0.0f, 0.0f, 0.0f);
 529                                         break;
 530                                 }
 531                         }
 532
 533                         /* prepare to scatter to new direction */
 534                         if(scatter) {
 535                                 /* adjust throughput and move to new location */
 536                                 sd->P = ray->P + new_t*ray->D;
 537                                 *throughput = tp;
 538
 539                                 return VOLUME_PATH_SCATTERED;
 540                         }
 541                         else {
 542                                 /* accumulate transmittance */
 543                                 accum_transmittance *= transmittance;
 544                         }
 545                 }
 546
 547                 /* stop if at the end of the volume */
 548                 t = new_t;
 549                 if(t == ray->t)
 550                         break;
 551         }
 552
 553         *throughput = tp;
 554
 555         return VOLUME_PATH_ATTENUATED;
 556 }
 557
 558 /* get the volume attenuation and emission over line segment defined by
 559  * ray, with the assumption that there are no surfaces blocking light
 560  * between the endpoints. distance sampling is used to decide if we will
 561  * scatter or not. */
 562 ccl_device_noinline VolumeIntegrateResult kernel_volume_integrate(KernelGlobals *kg,
 563         PathState *state, ShaderData *sd, Ray *ray, PathRadiance *L, float3 *throughput, RNG *rng, bool heterogeneous)
 564 {
 565         /* workaround to fix correlation bug in T38710, can find better solution
 566          * in random number generator later, for now this is done here to not impact
 567          * performance of rendering without volumes */
 568         RNG tmp_rng = cmj_hash(*rng, state->rng_offset);
 569
 570         shader_setup_from_volume(kg, sd, ray);
 571
 572         if(heterogeneous)
 573                 return kernel_volume_integrate_heterogeneous_distance(kg, state, ray, sd, L, throughput, &tmp_rng);
 574         else
 575                 return kernel_volume_integrate_homogeneous(kg, state, ray, sd, L, throughput, &tmp_rng, true);
 576 }
 577
 578 /* Decoupled Volume Sampling
 579  *
 580  * VolumeSegment is list of coefficients and transmittance stored at all steps
 581  * through a volume. This can then later be used for decoupled sampling as in:
 582  * "Importance Sampling Techniques for Path Tracing in Participating Media"
 583  *
 584  * On the GPU this is only supported (but currently not enabled)
 585  * for homogeneous volumes (1 step), due to
 586  * no support for malloc/free and too much stack usage with a fix size array. */
 587
 588 typedef struct VolumeStep {
 589         float3 sigma_s;                         /* scatter coefficient */
 590         float3 sigma_t;                         /* extinction coefficient */
 591         float3 accum_transmittance;     /* accumulated transmittance including this step */
 592         float3 cdf_distance;            /* cumulative density function for distance sampling */
 593         float t;                                        /* distance at end of this step */
 594         float shade_t;                          /* jittered distance where shading was done in step */
 595         int closure_flag;                       /* shader evaluation closure flags */
 596 } VolumeStep;
 597
 598 typedef struct VolumeSegment {
 599         VolumeStep stack_step;      /* stack storage for homogeneous step, to avoid malloc */
 600         VolumeStep *steps;                      /* recorded steps */
 601         int numsteps;                           /* number of steps */
 602         int closure_flag;                       /* accumulated closure flags from all steps */
 603
 604         float3 accum_emission;          /* accumulated emission at end of segment */
 605         float3 accum_transmittance;     /* accumulated transmittance at end of segment */
 606
 607         int sampling_method;            /* volume sampling method */
 608 } VolumeSegment;
 609
 610 /* record volume steps to the end of the volume.
 611  *
 612  * it would be nice if we could only record up to the point that we need to scatter,
 613  * but the entire segment is needed to do always scattering, rather than probabilistically
 614  * hitting or missing the volume. if we don't know the transmittance at the end of the
 615  * volume we can't generate stratified distance samples up to that transmittance */
 616 ccl_device void kernel_volume_decoupled_record(KernelGlobals *kg, PathState *state,
 617         Ray *ray, ShaderData *sd, VolumeSegment *segment, bool heterogeneous)
 618 {
 619         const float tp_eps = 1e-6f; /* todo: this is likely not the right value */
 620
 621         /* prepare for volume stepping */
 622         int max_steps;
 623         float step_size, random_jitter_offset;
 624
 625         if(heterogeneous) {
 626                 const int global_max_steps = kernel_data.integrator.volume_max_steps;
 627                 step_size = kernel_data.integrator.volume_step_size;
 628                 /* compute exact steps in advance for malloc */
 629                 max_steps = max((int)ceilf(ray->t/step_size), 1);
 630                 if(max_steps > global_max_steps) {
 631                         max_steps = global_max_steps;
 632                         step_size = ray->t / (float)max_steps;
 633                 }
 634                 segment->steps = (VolumeStep*)malloc(sizeof(VolumeStep)*max_steps);
 635                 random_jitter_offset = lcg_step_float(&state->rng_congruential) * step_size;
 636         }
 637         else {
 638                 max_steps = 1;
 639                 step_size = ray->t;
 640                 random_jitter_offset = 0.0f;
 641                 segment->steps = &segment->stack_step;
 642         }
 643
 644         /* init accumulation variables */
 645         float3 accum_emission = make_float3(0.0f, 0.0f, 0.0f);
 646         float3 accum_transmittance = make_float3(1.0f, 1.0f, 1.0f);
 647         float3 cdf_distance = make_float3(0.0f, 0.0f, 0.0f);
 648         float t = 0.0f;
 649
 650         segment->numsteps = 0;
 651         segment->closure_flag = 0;
 652         bool is_last_step_empty = false;
 653
 654         VolumeStep *step = segment->steps;
 655
 656         for(int i = 0; i < max_steps; i++, step++) {
 657                 /* advance to new position */
 658                 float new_t = min(ray->t, (i+1) * step_size);
 659                 float dt = new_t - t;
 660
 661                 /* use random position inside this segment to sample shader */
 662                 if(heterogeneous && new_t == ray->t)
 663                         random_jitter_offset = lcg_step_float(&state->rng_congruential) * dt;
 664
 665                 float3 new_P = ray->P + ray->D * (t + random_jitter_offset);
 666                 VolumeShaderCoefficients coeff;
 667
 668                 /* compute segment */
 669                 if(volume_shader_sample(kg, sd, state, new_P, &coeff)) {
 670                         int closure_flag = sd->flag;
 671                         float3 sigma_t = coeff.sigma_a + coeff.sigma_s;
 672
 673                         /* compute accumulated transmittance */
 674                         float3 transmittance = volume_color_transmittance(sigma_t, dt);
 675
 676                         /* compute emission attenuated by absorption */
 677                         if(closure_flag & SD_EMISSION) {
 678                                 float3 emission = kernel_volume_emission_integrate(&coeff, closure_flag, transmittance, dt);
 679                                 accum_emission += accum_transmittance * emission;
 680                         }
 681
 682                         accum_transmittance *= transmittance;
 683
 684                         /* compute pdf for distance sampling */
 685                         float3 pdf_distance = dt * accum_transmittance * coeff.sigma_s;
 686                         cdf_distance = cdf_distance + pdf_distance;
 687
 688                         /* write step data */
 689                         step->sigma_t = sigma_t;
 690                         step->sigma_s = coeff.sigma_s;
 691                         step->closure_flag = closure_flag;
 692
 693                         segment->closure_flag |= closure_flag;
 694
 695                         is_last_step_empty = false;
 696                         segment->numsteps++;
 697                 }
 698                 else {
 699                         if(is_last_step_empty) {
 700                                 /* consecutive empty step, merge */
 701                                 step--;
 702                         }
 703                         else {
 704                                 /* store empty step */
 705                                 step->sigma_t = make_float3(0.0f, 0.0f, 0.0f);
 706                                 step->sigma_s = make_float3(0.0f, 0.0f, 0.0f);
 707                                 step->closure_flag = 0;
 708
 709                                 segment->numsteps++;
 710                                 is_last_step_empty = true;
 711                         }
 712                 }
 713
 714                 step->accum_transmittance = accum_transmittance;
 715                 step->cdf_distance = cdf_distance;
 716                 step->t = new_t;
 717                 step->shade_t = t + random_jitter_offset;
 718
 719                 /* stop if at the end of the volume */
 720                 t = new_t;
 721                 if(t == ray->t)
 722                         break;
 723
 724                 /* stop if nearly all light blocked */
 725                 if(accum_transmittance.x < tp_eps && accum_transmittance.y < tp_eps && accum_transmittance.z < tp_eps)
 726                         break;
 727         }
 728
 729         /* store total emission and transmittance */
 730         segment->accum_emission = accum_emission;
 731         segment->accum_transmittance = accum_transmittance;
 732
 733         /* normalize cumulative density function for distance sampling */
 734         VolumeStep *last_step = segment->steps + segment->numsteps - 1;
 735
 736         if(!is_zero(last_step->cdf_distance)) {
 737                 VolumeStep *step = &segment->steps[0];
 738                 int numsteps = segment->numsteps;
 739                 float3 inv_cdf_distance_sum = safe_invert_color(last_step->cdf_distance);
 740
 741                 for(int i = 0; i < numsteps; i++, step++)
 742                         step->cdf_distance *= inv_cdf_distance_sum;
 743         }
 744 }
 745
 746 ccl_device void kernel_volume_decoupled_free(KernelGlobals *kg, VolumeSegment *segment)
 747 {
 748         if(segment->steps != &segment->stack_step)
 749                 free(segment->steps);
 750 }
 751
 752 /* scattering for homogeneous and heterogeneous volumes, using decoupled ray
 753  * marching.
 754  *
 755  * function is expected to return VOLUME_PATH_SCATTERED when probalistic_scatter is false */
 756 ccl_device VolumeIntegrateResult kernel_volume_decoupled_scatter(
 757         KernelGlobals *kg, PathState *state, Ray *ray, ShaderData *sd,
 758         float3 *throughput, float rphase, float rscatter,
 759         const VolumeSegment *segment, const float3 *light_P, bool probalistic_scatter)
 760 {
 761         kernel_assert(segment->closure_flag & SD_SCATTER);
 762
 763         /* pick random color channel, we use the Veach one-sample
 764          * model with balance heuristic for the channels */
 765         int channel = (int)(rphase*3.0f);
 766         sd->randb_closure = rphase*3.0f - channel;
 767         float xi = rscatter;
 768
 769         /* probabilistic scattering decision based on transmittance */
 770         if(probalistic_scatter) {
 771                 float sample_transmittance = kernel_volume_channel_get(segment->accum_transmittance, channel);
 772
 773                 if(1.0f - xi >= sample_transmittance) {
 774                         /* rescale random number so we can reuse it */
 775                         xi = 1.0f - (1.0f - xi - sample_transmittance)/(1.0f - sample_transmittance);
 776                 }
 777                 else {
 778                         *throughput /= sample_transmittance;
 779                         return VOLUME_PATH_MISSED;
 780                 }
 781         }
 782
 783         VolumeStep *step;
 784         float3 transmittance;
 785         float pdf, sample_t;
 786         float mis_weight = 1.0f;
 787         bool distance_sample = true;
 788         bool use_mis = false;
 789
 790         if(segment->sampling_method && light_P) {
 791                 if(segment->sampling_method == SD_VOLUME_MIS) {
 792                         /* multiple importance sample: randomly pick between
 793                          * equiangular and distance sampling strategy */
 794                         if(xi < 0.5f) {
 795                                 xi *= 2.0f;
 796                         }
 797                         else {
 798                                 xi = (xi - 0.5f)*2.0f;
 799                                 distance_sample = false;
 800                         }
 801
 802                         use_mis = true;
 803                 }
 804                 else {
 805                         /* only equiangular sampling */
 806                         distance_sample = false;
 807                 }
 808         }
 809
 810         /* distance sampling */
 811         if(distance_sample) {
 812                 /* find step in cdf */
 813                 step = segment->steps;
 814
 815                 float prev_t = 0.0f;
 816                 float3 step_pdf_distance = make_float3(1.0f, 1.0f, 1.0f);
 817
 818                 if(segment->numsteps > 1) {
 819                         float prev_cdf = 0.0f;
 820                         float step_cdf = 1.0f;
 821                         float3 prev_cdf_distance = make_float3(0.0f, 0.0f, 0.0f);
 822
 823                         for(int i = 0; ; i++, step++) {
 824                                 /* todo: optimize using binary search */
 825                                 step_cdf = kernel_volume_channel_get(step->cdf_distance, channel);
 826
 827                                 if(xi < step_cdf || i == segment->numsteps-1)
 828                                         break;
 829
 830                                 prev_cdf = step_cdf;
 831                                 prev_t = step->t;
 832                                 prev_cdf_distance = step->cdf_distance;
 833                         }
 834
 835                         /* remap xi so we can reuse it */
 836                         xi = (xi - prev_cdf)/(step_cdf - prev_cdf);
 837
 838                         /* pdf for picking step */
 839                         step_pdf_distance = step->cdf_distance - prev_cdf_distance;
 840                 }
 841
 842                 /* determine range in which we will sample */
 843                 float step_t = step->t - prev_t;
 844
 845                 /* sample distance and compute transmittance */
 846                 float3 distance_pdf;
 847                 sample_t = prev_t + kernel_volume_distance_sample(step_t, step->sigma_t, channel, xi, &transmittance, &distance_pdf);
 848
 849                 /* modify pdf for hit/miss decision */
 850                 if(probalistic_scatter)
 851                         distance_pdf *= make_float3(1.0f, 1.0f, 1.0f) - segment->accum_transmittance;
 852
 853                 pdf = average(distance_pdf * step_pdf_distance);
 854
 855                 /* multiple importance sampling */
 856                 if(use_mis) {
 857                         float equi_pdf = kernel_volume_equiangular_pdf(ray, *light_P, sample_t);
 858                         mis_weight = 2.0f*power_heuristic(pdf, equi_pdf);
 859                 }
 860         }
 861         /* equi-angular sampling */
 862         else {
 863                 /* sample distance */
 864                 sample_t = kernel_volume_equiangular_sample(ray, *light_P, xi, &pdf);
 865
 866                 /* find step in which sampled distance is located */
 867                 step = segment->steps;
 868
 869                 float prev_t = 0.0f;
 870                 float3 step_pdf_distance = make_float3(1.0f, 1.0f, 1.0f);
 871
 872                 if(segment->numsteps > 1) {
 873                         float3 prev_cdf_distance = make_float3(0.0f, 0.0f, 0.0f);
 874
 875                         int numsteps = segment->numsteps;
 876                         int high = numsteps - 1;
 877                         int low = 0;
 878                         int mid;
 879
 880                         while(low < high) {
 881                                 mid = (low + high) >> 1;
 882
 883                                 if(sample_t < step[mid].t)
 884                                         high = mid;
 885                                 else if(sample_t >= step[mid + 1].t)
 886                                         low = mid + 1;
 887                                 else {
 888                                         /* found our interval in step[mid] .. step[mid+1] */
 889                                         prev_t = step[mid].t;
 890                                         prev_cdf_distance = step[mid].cdf_distance;
 891                                         step += mid+1;
 892                                         break;
 893                                 }
 894                         }
 895
 896                         if(low >= numsteps - 1) {
 897                                 prev_t = step[numsteps - 1].t;
 898                                 prev_cdf_distance = step[numsteps-1].cdf_distance;
 899                                 step += numsteps - 1;
 900                         }
 901
 902                         /* pdf for picking step with distance sampling */
 903                         step_pdf_distance = step->cdf_distance - prev_cdf_distance;
 904                 }
 905
 906                 /* determine range in which we will sample */
 907                 float step_t = step->t - prev_t;
 908                 float step_sample_t = sample_t - prev_t;
 909
 910                 /* compute transmittance */
 911                 transmittance = volume_color_transmittance(step->sigma_t, step_sample_t);
 912
 913                 /* multiple importance sampling */
 914                 if(use_mis) {
 915                         float3 distance_pdf3 = kernel_volume_distance_pdf(step_t, step->sigma_t, step_sample_t);
 916                         float distance_pdf = average(distance_pdf3 * step_pdf_distance);
 917                         mis_weight = 2.0f*power_heuristic(pdf, distance_pdf);
 918                 }
 919         }
 920
 921         /* compute transmittance up to this step */
 922         if(step != segment->steps)
 923                 transmittance *= (step-1)->accum_transmittance;
 924
 925         /* modify throughput */
 926         *throughput *= step->sigma_s * transmittance * (mis_weight / pdf);
 927
 928         /* evaluate shader to create closures at shading point */
 929         if(segment->numsteps > 1) {
 930                 sd->P = ray->P + step->shade_t*ray->D;
 931
 932                 VolumeShaderCoefficients coeff;
 933                 volume_shader_sample(kg, sd, state, sd->P, &coeff);
 934         }
 935
 936         /* move to new position */
 937         sd->P = ray->P + sample_t*ray->D;
 938
 939         return VOLUME_PATH_SCATTERED;
 940 }
 941
 942 /* decide if we need to use decoupled or not */
 943 ccl_device bool kernel_volume_use_decoupled(KernelGlobals *kg, bool heterogeneous, bool direct, int sampling_method)
 944 {
 945         /* decoupled ray marching for heterogeneous volumes not supported on the GPU,
 946          * which also means equiangular and multiple importance sampling is not
 947          * support for that case */
 948 #ifdef __KERNEL_GPU__
 949         if(heterogeneous)
 950                 return false;
 951 #endif
 952
 953         /* equiangular and multiple importance sampling only implemented for decoupled */
 954         if(sampling_method != 0)
 955                 return true;
 956
 957         /* for all light sampling use decoupled, reusing shader evaluations is
 958          * typically faster in that case */
 959         if(direct)
 960                 return kernel_data.integrator.sample_all_lights_direct;
 961         else
 962                 return kernel_data.integrator.sample_all_lights_indirect;
 963 }
 964
 965 /* Volume Stack
 966  *
 967  * This is an array of object/shared ID's that the current segment of the path
 968  * is inside of. */
 969
 970 ccl_device void kernel_volume_stack_init(KernelGlobals *kg,
 971                                          Ray *ray,
 972                                          VolumeStack *stack)
 973 {
 974         /* NULL ray happens in the baker, does it need proper initialization of
 975          * camera in volume?
 976          */
 977         if(!kernel_data.cam.is_inside_volume || ray == NULL) {
 978                 /* Camera is guaranteed to be in the air, only take background volume
 979                  * into account in this case.
 980                  */
 981                 if(kernel_data.background.volume_shader != SHADER_NONE) {
 982                         stack[0].shader = kernel_data.background.volume_shader;
 983                         stack[0].object = PRIM_NONE;
 984                         stack[1].shader = SHADER_NONE;
 985                 }
 986                 else {
 987                         stack[0].shader = SHADER_NONE;
 988                 }
 989                 return;
 990         }
 991
 992         Ray volume_ray = *ray;
 993         volume_ray.t = FLT_MAX;
 994
 995         int stack_index = 0, enclosed_index = 0;
 996
 997         const uint visibility = PATH_RAY_ALL_VISIBILITY | kernel_data.integrator.layer_flag;
 998 #ifdef __VOLUME_RECORD_ALL__
 999         Intersection hits[2*VOLUME_STACK_SIZE];
1000         uint num_hits = scene_intersect_volume_all(kg,
1001                                                    &volume_ray,
1002                                                    hits,
1003                                                    2*VOLUME_STACK_SIZE,
1004                                                    visibility);
1005         if(num_hits > 0) {
1006                 int enclosed_volumes[VOLUME_STACK_SIZE];
1007                 Intersection *isect = hits;
1008
1009                 qsort(hits, num_hits, sizeof(Intersection), intersections_compare);
1010
1011                 for(uint hit = 0; hit < num_hits; ++hit, ++isect) {
1012                         ShaderData sd;
1013                         shader_setup_from_ray(kg, &sd, isect, &volume_ray);
1014                         if(sd.flag & SD_BACKFACING) {
1015                                 bool need_add = true;
1016                                 for(int i = 0; i < enclosed_index && need_add; ++i) {
1017                                         /* If ray exited the volume and never entered to that volume
1018                                          * it means that camera is inside such a volume.
1019                                          */
1020                                         if(enclosed_volumes[i] == sd.object) {
1021                                                 need_add = false;
1022                                         }
1023                                 }
1024                                 for(int i = 0; i < stack_index && need_add; ++i) {
1025                                         /* Don't add intersections twice. */
1026                                         if(stack[i].object == sd.object) {
1027                                                 need_add = false;
1028                                                 break;
1029                                         }
1030                                 }
1031                                 if(need_add) {
1032                                         stack[stack_index].object = sd.object;
1033                                         stack[stack_index].shader = sd.shader;
1034                                         ++stack_index;
1035                                 }
1036                         }
1037                         else {
1038                                 /* If ray from camera enters the volume, this volume shouldn't
1039                                  * be added to the stack on exit.
1040                                  */
1041                                 enclosed_volumes[enclosed_index++] = sd.object;
1042                         }
1043                 }
1044         }
1045 #else
1046         int enclosed_volumes[VOLUME_STACK_SIZE];
1047         int step = 0;
1048
1049         while(stack_index < VOLUME_STACK_SIZE - 1 &&
1050               enclosed_index < VOLUME_STACK_SIZE - 1 &&
1051               step < 2 * VOLUME_STACK_SIZE)
1052         {
1053                 Intersection isect;
1054                 if(!scene_intersect_volume(kg, &volume_ray, &isect, visibility)) {
1055                         break;
1056                 }
1057
1058                 ShaderData sd;
1059                 shader_setup_from_ray(kg, &sd, &isect, &volume_ray);
1060                 if(sd.flag & SD_BACKFACING) {
1061                         /* If ray exited the volume and never entered to that volume
1062                          * it means that camera is inside such a volume.
1063                          */
1064                         bool need_add = true;
1065                         for(int i = 0; i < enclosed_index && need_add; ++i) {
1066                                 /* If ray exited the volume and never entered to that volume
1067                                  * it means that camera is inside such a volume.
1068                                  */
1069                                 if(enclosed_volumes[i] == sd.object) {
1070                                         need_add = false;
1071                                 }
1072                         }
1073                         for(int i = 0; i < stack_index && need_add; ++i) {
1074                                 /* Don't add intersections twice. */
1075                                 if(stack[i].object == sd.object) {
1076                                         need_add = false;
1077                                         break;
1078                                 }
1079                         }
1080                         if(need_add) {
1081                                 stack[stack_index].object = sd.object;
1082                                 stack[stack_index].shader = sd.shader;
1083                                 ++stack_index;
1084                         }
1085                 }
1086                 else {
1087                         /* If ray from camera enters the volume, this volume shouldn't
1088                          * be added to the stack on exit.
1089                          */
1090                         enclosed_volumes[enclosed_index++] = sd.object;
1091                 }
1092
1093                 /* Move ray forward. */
1094                 volume_ray.P = ray_offset(sd.P, -sd.Ng);
1095                 ++step;
1096         }
1097 #endif
1098         /* stack_index of 0 means quick checks outside of the kernel gave false
1099          * positive, nothing to worry about, just we've wasted quite a few of
1100          * ticks just to come into conclusion that camera is in the air.
1101          *
1102          * In this case we're doing the same above -- check whether background has
1103          * volume.
1104          */
1105         if(stack_index == 0 && kernel_data.background.volume_shader == SHADER_NONE) {
1106                 stack[0].shader = kernel_data.background.volume_shader;
1107                 stack[0].object = PRIM_NONE;
1108                 stack[1].shader = SHADER_NONE;
1109         }
1110         else {
1111                 stack[stack_index].shader = SHADER_NONE;
1112         }
1113 }
1114
1115 ccl_device void kernel_volume_stack_enter_exit(KernelGlobals *kg, ShaderData *sd, VolumeStack *stack)
1116 {
1117         /* todo: we should have some way for objects to indicate if they want the
1118          * world shader to work inside them. excluding it by default is problematic
1119          * because non-volume objects can't be assumed to be closed manifolds */
1120
1121         if(!(sd->flag & SD_HAS_VOLUME))
1122                 return;
1123
1124         if(sd->flag & SD_BACKFACING) {
1125                 /* exit volume object: remove from stack */
1126                 for(int i = 0; stack[i].shader != SHADER_NONE; i++) {
1127                         if(stack[i].object == sd->object) {
1128                                 /* shift back next stack entries */
1129                                 do {
1130                                         stack[i] = stack[i+1];
1131                                         i++;
1132                                 }
1133                                 while(stack[i].shader != SHADER_NONE);
1134
1135                                 return;
1136                         }
1137                 }
1138         }
1139         else {
1140                 /* enter volume object: add to stack */
1141                 int i;
1142
1143                 for(i = 0; stack[i].shader != SHADER_NONE; i++) {
1144                         /* already in the stack? then we have nothing to do */
1145                         if(stack[i].object == sd->object)
1146                                 return;
1147                 }
1148
1149                 /* if we exceed the stack limit, ignore */
1150                 if(i >= VOLUME_STACK_SIZE-1)
1151                         return;
1152
1153                 /* add to the end of the stack */
1154                 stack[i].shader = sd->shader;
1155                 stack[i].object = sd->object;
1156                 stack[i+1].shader = SHADER_NONE;
1157         }
1158 }
1159
1160 #ifdef __SUBSURFACE__
1161 ccl_device void kernel_volume_stack_update_for_subsurface(KernelGlobals *kg,
1162                                                           Ray *ray,
1163                                                           VolumeStack *stack)
1164 {
1165         kernel_assert(kernel_data.integrator.use_volumes);
1166
1167         Ray volume_ray = *ray;
1168
1169 #  ifdef __VOLUME_RECORD_ALL__
1170         Intersection hits[2*VOLUME_STACK_SIZE];
1171         uint num_hits = scene_intersect_volume_all(kg,
1172                                                    &volume_ray,
1173                                                    hits,
1174                                                    2*VOLUME_STACK_SIZE,
1175                                                    PATH_RAY_ALL_VISIBILITY);
1176         if(num_hits > 0) {
1177                 Intersection *isect = hits;
1178
1179                 qsort(hits, num_hits, sizeof(Intersection), intersections_compare);
1180
1181                 for(uint hit = 0; hit < num_hits; ++hit, ++isect) {
1182                         ShaderData sd;
1183                         shader_setup_from_ray(kg, &sd, isect, &volume_ray);
1184                         kernel_volume_stack_enter_exit(kg, &sd, stack);
1185                 }
1186         }
1187 #  else
1188         Intersection isect;
1189         int step = 0;
1190         while(step < 2 * VOLUME_STACK_SIZE &&
1191               scene_intersect_volume(kg,
1192                                      &volume_ray,
1193                                      &isect,
1194                                      PATH_RAY_ALL_VISIBILITY))
1195         {
1196                 ShaderData sd;
1197                 shader_setup_from_ray(kg, &sd, &isect, &volume_ray);
1198                 kernel_volume_stack_enter_exit(kg, &sd, stack);
1199
1200                 /* Move ray forward. */
1201                 volume_ray.P = ray_offset(sd.P, -sd.Ng);
1202                 volume_ray.t -= sd.ray_length;
1203                 ++step;
1204         }
1205 #  endif
1206 }
1207 #endif
1208
1209 CCL_NAMESPACE_END