text: /*
      This file is part of Mitsuba, a physically based rendering system.
  
      Copyright (c) 2007-2014 by Wenzel Jakob and others.
  
      Mitsuba is free software; you can redistribute it and/or modify
      it under the terms of the GNU General Public License Version 3
      as published by the Free Software Foundation.
  
      Mitsuba is distributed in the hope that it will be useful,
      but WITHOUT ANY WARRANTY; without even the implied warranty of
      MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
      GNU General Public License for more details.
  
      You should have received a copy of the GNU General Public License
      along with this program. If not, see <http://www.gnu.org/licenses/>.
  */
  
  #include <mitsuba/bidir/vertex.h>
  #include <mitsuba/bidir/edge.h>
  #include "bdpt_proc.h"
  
  MTS_NAMESPACE_BEGIN
  
  /*!\plugin{bdpt}{Bidirectional path tracer}
   * \order{5}
   * \parameters{
   *     \parameter{maxDepth}{\Integer}{Specifies the longest path depth
   *         in the generated output image (where \code{-1} corresponds to $\infty$).
   *         A value of \code{1} will only render directly visible light sources.
   *         \code{2} will lead to single-bounce (direct-only) illumination,
   *         and so on. \default{\code{-1}}
   *     }
   *     \parameter{lightImage}{\Boolean}{Include sampling strategies that connect
   *        paths traced from emitters directly to the camera? (i.e. what \pluginref{ptracer} does)
   *        This improves the effectiveness of bidirectional path tracing
   *        but severely increases the local and remote communication
   *        overhead, since large \emph{light images} must be transferred between threads
   *        or over the network. See the text below for a more detailed explanation.
   *        \default{include these strategies, i.e. \code{true}}
   *     }
   *     \parameter{sampleDirect}{\Boolean}{Enable direct sampling strategies? This is a generalization
   *        of direct illumination sampling that works with both emitters and sensors. Usually a good idea.
   *        \default{use direct sampling, i.e. \code{true}}}
   *     \parameter{rrDepth}{\Integer}{Specifies the minimum path depth, after
   *        which the implementation will start to use the ``russian roulette''
   *        path termination criterion. \default{\code{5}}
   *     }
   * }
   *
   ** \renderings{
   *     \rendering{Path tracer, 32 samples/pixel}{integrator_bdpt_path}
   *     \rendering{Bidirectional path tracer, 32 samples/pixel}{integrator_bdpt_bdpt}
   *     \caption{
   *      The bidirectional path tracer finds light paths by generating partial
   *      paths starting at the emitters and the sensor and connecting them in every possible way.
   *      This works particularly well in closed scenes as the one shown above. Here, the unidirectional
   *      path tracer has severe difficulties finding some of the indirect illumination paths.
   *      Modeled after a scene by Eric Veach.
   *     }
   * }
   * \renderings{
   *    \includegraphics[width=12cm]{images/integrator_bdpt_sketch.pdf}\hfill\,
   *    \caption{The four different ways in which BDPT can create a direct illumination
   *    path (matching the first row on the next page): \textbf{(a)} Standard path
   *    tracing without direct illumination sampling, \textbf{(b)} path tracing with
   *    direct illumination sampling, \textbf{(c)} Particle tracing with recording of
   *    scattering events observed by the sensor, \textbf{(d)} Particle tracing with
   *    recording of particles that hit the sensor.}\vspace{-3mm}
   * }
   * \renderings{
   *     \unframedbigrendering{The individual sampling strategies that comprise BDPT, but
   *     \emph{without} multiple importance sampling. $s$ denotes the number of steps
   *     taken from the emitters, and $t$ denotes the number of steps from the sensor.
   *     Note how almost every strategy has deficiencies of some kind} {integrator_bdpt_unweighted.pdf}
   *     \unframedbigrendering{The same sampling strategies, but now weighted using multiple importance
   *     sampling---effectively ``turning off'' each strategy where it does not perform well.
   *     The final result is computed by summing all of these images.}{integrator_bdpt_weighted.pdf}
   * }
   *
   * This plugin implements a bidirectional path tracer (short: BDPT) with support for multiple
   * importance sampling, as proposed by Veach and Guibas \cite{Veach1994Bidirectional}.
   *
   * A bidirectional path tracer computes radiance estimates by starting two separate
   * random walks from an emitter and a sensor. The resulting \emph{subpaths} are
   * connected at every possible interaction vertex, creating a large number of complete paths
   * of different lengths. These paths are then used to estimate the amount of
   * radiance that is transferred from the emitter to a pixel on the sensor.
   *
   * Generally, some of the created paths will be undesirable, since they lead to
   * high-variance radiance estimates. To alleviate this situation, BDPT makes
   * use of \emph{multiple importance sampling} which, roughly speaking, weights
   * paths based on their predicted utility.
   *
   * The bidirectional path tracer in Mitsuba is a complete implementation of the
   * technique that handles all sampling strategies, including those that involve
   * direct interactions with the sensor. For this purpose, finite-aperture sensors
   * are explicitly represented by surfaces in the scene so that they can be
   * intersected by random walks started at emitters.
   *
   * Bidirectional path tracing is a relatively ``heavy'' rendering technique---for
   * the same number of samples per pixel, it is easily 3-4 times slower than regular
   * path tracing. However, it usually makes up for this by producing considerably
   * lower-variance radiance estimates (i.e. the output images have less noise).
   *
   * The code parallelizes over multiple cores and machines, but with one caveat:
   * some of the BDPT path sampling strategies are incompatble with the usual
   * approach of rendering an image tile by tile, since they can potentially
   * contribute to \emph{any} pixel on the screen. This means that each
   * rendering work unit must be associated with a full-sized image!
   * When network render nodes are involved or the resolution of this \emph{light image}
   * is very high, a bottleneck can arise where more work is spent  accumulating or
   * transmitting these images than actual rendering.
   *
   * There are two possible resorts should this situation arise: the first one
   * is to reduce the number of work units so that there is approximately one
   * unit per core (and hence one image to transmit per core). This can be done by
   * increasing the block size in the GUI preferences or passing the \code{-b}
   * parameter to the \code{mitsuba} executable. The second option is to simply
   * disable these sampling strategies at the cost of reducing the
   * effectiveness of bidirectional path tracing (particularly, when rendering
   * caustics). For this, set \code{lightImage} to \code{false}.
   * When rendering an image of a reasonable resolution without network nodes,
   * this is not a big concern, hence these strategies are enabled by default.
   *
   * \remarks{
   *    \item This integrator does not work with dipole-style subsurface
   *    scattering models.
   *    \item This integrator does not yet work with certain non-reciprocal BSDFs (i.e.
   *          \pluginref{bumpmap}, but this will be addressed in the future
   * }
   */
  class BDPTIntegrator : public Integrator {
  public:
      BDPTIntegrator(const Properties &props) : Integrator(props) {
          /* Load the parameters / defaults */
          m_config.maxDepth = props.getInteger("maxDepth", -1);
          m_config.rrDepth = props.getInteger("rrDepth", 5);
          m_config.lightImage = props.getBoolean("lightImage", true);
          m_config.sampleDirect = props.getBoolean("sampleDirect", true);
          m_config.showWeighted = props.getBoolean("showWeighted", false);
  
          #if BDPT_DEBUG == 1
          if (m_config.maxDepth == -1 || m_config.maxDepth > 6) {
              /* Limit the maximum depth when rendering image
                 matrices in BDPT debug mode (the number of
                 these images grows quadratically with depth) */
              Log(EWarn, "Limiting max. path depth to 6 to avoid an "
                  "extremely large number of debug output images");
              m_config.maxDepth = 6;
          }
          #endif
  
          if (m_config.rrDepth <= 0)
              Log(EError, "'rrDepth' must be set to a value greater than zero!");
  
          if (m_config.maxDepth <= 0 && m_config.maxDepth != -1)
              Log(EError, "'maxDepth' must be set to -1 (infinite) or a value greater than zero!");
      }
  
      /// Unserialize from a binary data stream
      BDPTIntegrator(Stream *stream, InstanceManager *manager)
       : Integrator(stream, manager) {
          m_config = BDPTConfiguration(stream);
      }
  
      void serialize(Stream *stream, InstanceManager *manager) const {
          Integrator::serialize(stream, manager);
          m_config.serialize(stream);
      }
  
      bool preprocess(const Scene *scene, RenderQueue *queue,
              const RenderJob *job, int sceneResID, int sensorResID,
              int samplerResID) {
          Integrator::preprocess(scene, queue, job, sceneResID,
                  sensorResID, samplerResID);
  
          if (scene->getSubsurfaceIntegrators().size() > 0)
              Log(EError, "Subsurface integrators are not supported "
                  "by the bidirectional path tracer!");
  
          return true;
      }
  
      void cancel() {
          Scheduler::getInstance()->cancel(m_process);
      }
  
      void configureSampler(const Scene *scene, Sampler *sampler) {
          /* Prepare the sampler for tile-based rendering */
          sampler->setFilmResolution(scene->getFilm()->getCropSize(), true);
      }
  
      bool render(Scene *scene, RenderQueue *queue, const RenderJob *job,
              int sceneResID, int sensorResID, int samplerResID) {
          ref<Scheduler> scheduler = Scheduler::getInstance();
          ref<Sensor> sensor = scene->getSensor();
          const Film *film = sensor->getFilm();
          size_t sampleCount = scene->getSampler()->getSampleCount();
          size_t nCores = scheduler->getCoreCount();
  
          Log(EDebug, "Size of data structures: PathVertex=%i bytes, PathEdge=%i bytes",
              (int) sizeof(PathVertex), (int) sizeof(PathEdge));
  
          Log(EInfo, "Starting render job (%ix%i, " SIZE_T_FMT " samples, " SIZE_T_FMT
              " %s, " SSE_STR ") ..", film->getCropSize().x, film->getCropSize().y,
              sampleCount, nCores, nCores == 1 ? "core" : "cores");
  
          m_config.blockSize = scene->getBlockSize();
          m_config.cropSize = film->getCropSize();
          m_config.sampleCount = sampleCount;
          m_config.dump();
  
          ref<BDPTProcess> process = new BDPTProcess(job, queue, m_config);
          m_process = process;
  
          process->bindResource("scene", sceneResID);
          process->bindResource("sensor", sensorResID);
          process->bindResource("sampler", samplerResID);
          scheduler->schedule(process);
  
          scheduler->wait(process);
          m_process = NULL;
          process->develop();
  
          #if BDPT_DEBUG == 1
              fs::path path = scene->getDestinationFile();
              if (m_config.lightImage)
                  process->getResult()->dump(m_config, path.parent_path(), path.stem());
          #endif
  
          return process->getReturnStatus() == ParallelProcess::ESuccess;
      }
  
      MTS_DECLARE_CLASS()
  private:
      ref<ParallelProcess> m_process;
      BDPTConfiguration m_config;
  };
  
  MTS_IMPLEMENT_CLASS_S(BDPTIntegrator, false, Integrator)
  MTS_EXPORT_PLUGIN(BDPTIntegrator, "Bidirectional path tracer");
  MTS_NAMESPACE_END
  