volume_draine.h

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/* SPDX-FileCopyrightText: 2011-2024 Blender Foundation
 *
 * SPDX-License-Identifier: Apache-2.0 */
 
#pragma once
 
#include "kernel/types.h"
 
#include "kernel/closure/volume_util.h"
 
CCL_NAMESPACE_BEGIN
 
/* DRAINE CLOSURE */
 
struct DraineVolume {
  SHADER_CLOSURE_VOLUME_BASE;
 
  float g;
  float alpha;
};
static_assert(sizeof(ShaderVolumeClosure) >= sizeof(DraineVolume), "DraineVolume is too large!");
 
ccl_device int volume_draine_setup(ccl_private DraineVolume *volume)
{
  volume->type = CLOSURE_VOLUME_DRAINE_ID;
  /* clamp anisotropy */
  volume->g = signf(volume->g) * min(fabsf(volume->g), 1.0f - 1e-3f);
 
  return SD_SCATTER;
}
 
ccl_device Spectrum volume_draine_eval(const ccl_private ShaderData *sd,
                                       const ccl_private ShaderVolumeClosure *svc,
                                       const float3 wo,
                                       ccl_private float *pdf)
{
  const ccl_private DraineVolume *volume = (const ccl_private DraineVolume *)svc;
 
  /* note that wi points towards the viewer */
  const float cos_theta = dot(-sd->wi, wo);
  *pdf = phase_draine(cos_theta, volume->g, volume->alpha);
 
  return make_spectrum(*pdf);
}
 
ccl_device int volume_draine_sample(const ccl_private ShaderData *sd,
                                    const ccl_private ShaderVolumeClosure *svc,
                                    const float2 rand,
                                    ccl_private Spectrum *eval,
                                    ccl_private float3 *wo,
                                    ccl_private float *pdf)
{
  const ccl_private DraineVolume *volume = (const ccl_private DraineVolume *)svc;
 
  /* note that wi points towards the viewer and so is used negated */
  *wo = phase_draine_sample(-sd->wi, volume->g, volume->alpha, rand, pdf);
  *eval = make_spectrum(*pdf); /* perfect importance sampling */
 
  return LABEL_VOLUME_SCATTER;
}
 
CCL_NAMESPACE_END