cycles/kernel/kernel_path_volume.h

/*
 * 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

#ifdef __VOLUME_SCATTER__

ccl_device_inline void kernel_path_volume_connect_light(KernelGlobals *kg,
                                                        ShaderData *sd,
                                                        ShaderData *emission_sd,
                                                        float3 throughput,
                                                        ccl_addr_space PathState *state,
                                                        PathRadiance *L)
{
#  ifdef __EMISSION__
  /* sample illumination from lights to find path contribution */
  Ray light_ray ccl_optional_struct_init;
  BsdfEval L_light ccl_optional_struct_init;
  bool is_lamp = false;
  bool has_emission = false;

  light_ray.t = 0.0f;
#    ifdef __OBJECT_MOTION__
  /* connect to light from given point where shader has been evaluated */
  light_ray.time = sd->time;
#    endif

  if (kernel_data.integrator.use_direct_light) {
    float light_u, light_v;
    path_state_rng_2D(kg, state, PRNG_LIGHT_U, &light_u, &light_v);

    LightSample ls ccl_optional_struct_init;
    if (light_sample(kg, -1, light_u, light_v, sd->time, sd->P, state->bounce, &ls)) {
      float terminate = path_state_rng_light_termination(kg, state);
      has_emission = direct_emission(
          kg, sd, emission_sd, &ls, state, &light_ray, &L_light, &is_lamp, terminate);
    }
  }

  /* trace shadow ray */
  float3 shadow;

  const bool blocked = shadow_blocked(kg, sd, emission_sd, state, &light_ray, &shadow);

  if (has_emission && !blocked) {
    /* accumulate */
    path_radiance_accum_light(kg, L, state, throughput, &L_light, shadow, 1.0f, is_lamp);
  }
#  endif /* __EMISSION__ */
}

ccl_device_noinline_cpu bool kernel_path_volume_bounce(KernelGlobals *kg,
                                                       ShaderData *sd,
                                                       ccl_addr_space float3 *throughput,
                                                       ccl_addr_space PathState *state,
                                                       PathRadianceState *L_state,
                                                       ccl_addr_space Ray *ray)
{
  /* sample phase function */
  float phase_pdf;
  BsdfEval phase_eval ccl_optional_struct_init;
  float3 phase_omega_in ccl_optional_struct_init;
  differential3 phase_domega_in ccl_optional_struct_init;
  float phase_u, phase_v;
  path_state_rng_2D(kg, state, PRNG_BSDF_U, &phase_u, &phase_v);
  int label;

  label = shader_volume_phase_sample(
      kg, sd, phase_u, phase_v, &phase_eval, &phase_omega_in, &phase_domega_in, &phase_pdf);

  if (phase_pdf == 0.0f || bsdf_eval_is_zero(&phase_eval))
    return false;

  /* modify throughput */
  path_radiance_bsdf_bounce(kg, L_state, throughput, &phase_eval, phase_pdf, state->bounce, label);

  /* set labels */
  state->ray_pdf = phase_pdf;
#  ifdef __LAMP_MIS__
  state->ray_t = 0.0f;
#  endif
  state->min_ray_pdf = fminf(phase_pdf, state->min_ray_pdf);

  /* update path state */
  path_state_next(kg, state, label);

  /* Russian roulette termination of volume ray scattering. */
  float probability = path_state_continuation_probability(kg, state, *throughput);

  if (probability == 0.0f) {
    return false;
  }
  else if (probability != 1.0f) {
    /* Use dimension from the previous bounce, has not been used yet. */
    float terminate = path_state_rng_1D(kg, state, PRNG_TERMINATE - PRNG_BOUNCE_NUM);

    if (terminate >= probability) {
      return false;
    }

    *throughput /= probability;
  }

  /* setup ray */
  ray->P = sd->P;
  ray->D = phase_omega_in;
  ray->t = FLT_MAX;

#  ifdef __RAY_DIFFERENTIALS__
  ray->dP = sd->dP;
  ray->dD = phase_domega_in;
#  endif

  return true;
}

#  if !defined(__SPLIT_KERNEL__) && (defined(__BRANCHED_PATH__) || defined(__VOLUME_DECOUPLED__))
ccl_device void kernel_branched_path_volume_connect_light(KernelGlobals *kg,
                                                          ShaderData *sd,
                                                          ShaderData *emission_sd,
                                                          float3 throughput,
                                                          ccl_addr_space PathState *state,
                                                          PathRadiance *L,
                                                          bool sample_all_lights,
                                                          Ray *ray,
                                                          const VolumeSegment *segment)
{
#    ifdef __EMISSION__
  BsdfEval L_light ccl_optional_struct_init;

  int num_lights = 1;
  if (sample_all_lights) {
    num_lights = kernel_data.integrator.num_all_lights;
    if (kernel_data.integrator.pdf_triangles != 0.0f) {
      num_lights += 1;
    }
  }

  for (int i = 0; i < num_lights; ++i) {
    /* sample one light at random */
    int num_samples = 1;
    int num_all_lights = 1;
    uint lamp_rng_hash = state->rng_hash;
    bool double_pdf = false;
    bool is_mesh_light = false;
    bool is_lamp = false;

    if (sample_all_lights) {
      /* lamp sampling */
      is_lamp = i < kernel_data.integrator.num_all_lights;
      if (is_lamp) {
        if (UNLIKELY(light_select_reached_max_bounces(kg, i, state->bounce))) {
          continue;
        }
        num_samples = light_select_num_samples(kg, i);
        num_all_lights = kernel_data.integrator.num_all_lights;
        lamp_rng_hash = cmj_hash(state->rng_hash, i);
        double_pdf = kernel_data.integrator.pdf_triangles != 0.0f;
      }
      /* mesh light sampling */
      else {
        num_samples = kernel_data.integrator.mesh_light_samples;
        double_pdf = kernel_data.integrator.num_all_lights != 0;
        is_mesh_light = true;
      }
    }

    float num_samples_inv = 1.0f / (num_samples * num_all_lights);

    for (int j = 0; j < num_samples; j++) {
      Ray light_ray ccl_optional_struct_init;
      light_ray.t = 0.0f; /* reset ray */
#      ifdef __OBJECT_MOTION__
      light_ray.time = sd->time;
#      endif
      bool has_emission = false;

      float3 tp = throughput;

      if (kernel_data.integrator.use_direct_light) {
        /* sample random position on random light/triangle */
        float light_u, light_v;
        path_branched_rng_2D(
            kg, lamp_rng_hash, state, j, num_samples, PRNG_LIGHT_U, &light_u, &light_v);

        /* only sample triangle lights */
        if (is_mesh_light && double_pdf) {
          light_u = 0.5f * light_u;
        }

        LightSample ls ccl_optional_struct_init;
        const int lamp = is_lamp ? i : -1;
        light_sample(kg, lamp, light_u, light_v, sd->time, ray->P, state->bounce, &ls);

        /* sample position on volume segment */
        float rphase = path_branched_rng_1D(
            kg, state->rng_hash, state, j, num_samples, PRNG_PHASE_CHANNEL);
        float rscatter = path_branched_rng_1D(
            kg, state->rng_hash, state, j, num_samples, PRNG_SCATTER_DISTANCE);

        VolumeIntegrateResult result = kernel_volume_decoupled_scatter(kg,
                                                                       state,
                                                                       ray,
                                                                       sd,
                                                                       &tp,
                                                                       rphase,
                                                                       rscatter,
                                                                       segment,
                                                                       (ls.t != FLT_MAX) ? &ls.P :
                                                                                           NULL,
                                                                       false);

        if (result == VOLUME_PATH_SCATTERED) {
          /* todo: split up light_sample so we don't have to call it again with new position */
          if (light_sample(kg, lamp, light_u, light_v, sd->time, sd->P, state->bounce, &ls)) {
            if (double_pdf) {
              ls.pdf *= 2.0f;
            }

            /* sample random light */
            float terminate = path_branched_rng_light_termination(
                kg, state->rng_hash, state, j, num_samples);
            has_emission = direct_emission(
                kg, sd, emission_sd, &ls, state, &light_ray, &L_light, &is_lamp, terminate);
          }
        }
      }

      /* trace shadow ray */
      float3 shadow;

      const bool blocked = shadow_blocked(kg, sd, emission_sd, state, &light_ray, &shadow);

      if (has_emission && !blocked) {
        /* accumulate */
        path_radiance_accum_light(
            kg, L, state, tp * num_samples_inv, &L_light, shadow, num_samples_inv, is_lamp);
      }
    }
  }
#    endif /* __EMISSION__ */
}
#  endif /* __SPLIT_KERNEL__ */

#endif /* __VOLUME_SCATTER__ */

CCL_NAMESPACE_END