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[chromium-blink-merge.git] / media / cast / sender / external_video_encoder.cc

// Copyright 2014 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "media/cast/sender/external_video_encoder.h"

#include <cmath>

#include "base/bind.h"
#include "base/logging.h"
#include "base/memory/scoped_vector.h"
#include "base/memory/shared_memory.h"
#include "base/message_loop/message_loop.h"
#include "base/metrics/histogram.h"
#include "media/base/video_frame.h"
#include "media/base/video_types.h"
#include "media/base/video_util.h"
#include "media/cast/cast_defines.h"
#include "media/cast/logging/logging_defines.h"
#include "media/cast/net/cast_transport_config.h"

namespace {

static const size_t kOutputBufferCount = 3;

void LogFrameEncodedEvent(
    const scoped_refptr<media::cast::CastEnvironment>& cast_environment,
    base::TimeTicks event_time,
    media::cast::RtpTimestamp rtp_timestamp,
    uint32 frame_id) {
  cast_environment->Logging()->InsertFrameEvent(
      event_time, media::cast::FRAME_ENCODED, media::cast::VIDEO_EVENT,
      rtp_timestamp, frame_id);
}

}  // namespace

namespace media {
namespace cast {

// Container for the associated data of a video frame being processed.
struct InProgressFrameEncode {
  // The source content to encode.
  const scoped_refptr<VideoFrame> video_frame;

  // The reference time for this frame.
  const base::TimeTicks reference_time;

  // The callback to run when the result is ready.
  const VideoEncoder::FrameEncodedCallback frame_encoded_callback;

  // The target encode bit rate.
  const int target_bit_rate;

  // The real-world encode start time.  This is used to compute the encoded
  // frame's |deadline_utilization| and so it uses the real-world clock instead
  // of the CastEnvironment clock, the latter of which might be simulated.
  const base::TimeTicks start_time;

  InProgressFrameEncode(const scoped_refptr<VideoFrame>& v_frame,
                        base::TimeTicks r_time,
                        VideoEncoder::FrameEncodedCallback callback,
                        int bit_rate)
      : video_frame(v_frame),
        reference_time(r_time),
        frame_encoded_callback(callback),
        target_bit_rate(bit_rate),
        start_time(base::TimeTicks::Now()) {}
};

// Owns a VideoEncoderAccelerator instance and provides the necessary adapters
// to encode media::VideoFrames and emit media::cast::EncodedFrames.  All
// methods must be called on the thread associated with the given
// SingleThreadTaskRunner, except for the task_runner() accessor.
class ExternalVideoEncoder::VEAClientImpl
    : public VideoEncodeAccelerator::Client,
      public base::RefCountedThreadSafe<VEAClientImpl> {
 public:
  VEAClientImpl(
      const scoped_refptr<CastEnvironment>& cast_environment,
      const scoped_refptr<base::SingleThreadTaskRunner>& encoder_task_runner,
      scoped_ptr<media::VideoEncodeAccelerator> vea,
      int max_frame_rate,
      const StatusChangeCallback& status_change_cb,
      const CreateVideoEncodeMemoryCallback& create_video_encode_memory_cb)
      : cast_environment_(cast_environment),
        task_runner_(encoder_task_runner),
        max_frame_rate_(max_frame_rate),
        status_change_cb_(status_change_cb),
        create_video_encode_memory_cb_(create_video_encode_memory_cb),
        video_encode_accelerator_(vea.Pass()),
        encoder_active_(false),
        next_frame_id_(0u),
        key_frame_encountered_(false),
        requested_bit_rate_(-1) {
  }

  base::SingleThreadTaskRunner* task_runner() const {
    return task_runner_.get();
  }

  void Initialize(const gfx::Size& frame_size,
                  VideoCodecProfile codec_profile,
                  int start_bit_rate,
                  uint32 first_frame_id) {
    DCHECK(task_runner_->RunsTasksOnCurrentThread());

    requested_bit_rate_ = start_bit_rate;
    encoder_active_ = video_encode_accelerator_->Initialize(
        media::PIXEL_FORMAT_I420, frame_size, codec_profile, start_bit_rate,
        this);
    next_frame_id_ = first_frame_id;

    UMA_HISTOGRAM_BOOLEAN("Cast.Sender.VideoEncodeAcceleratorInitializeSuccess",
                          encoder_active_);

    cast_environment_->PostTask(
        CastEnvironment::MAIN,
        FROM_HERE,
        base::Bind(status_change_cb_,
                   encoder_active_ ? STATUS_INITIALIZED :
                       STATUS_CODEC_INIT_FAILED));
  }

  void SetBitRate(int bit_rate) {
    DCHECK(task_runner_->RunsTasksOnCurrentThread());

    requested_bit_rate_ = bit_rate;
    video_encode_accelerator_->RequestEncodingParametersChange(bit_rate,
                                                               max_frame_rate_);
  }

  void EncodeVideoFrame(
      const scoped_refptr<media::VideoFrame>& video_frame,
      const base::TimeTicks& reference_time,
      bool key_frame_requested,
      const VideoEncoder::FrameEncodedCallback& frame_encoded_callback) {
    DCHECK(task_runner_->RunsTasksOnCurrentThread());

    if (!encoder_active_)
      return;

    in_progress_frame_encodes_.push_back(InProgressFrameEncode(
        video_frame, reference_time, frame_encoded_callback,
        requested_bit_rate_));

    // BitstreamBufferReady will be called once the encoder is done.
    video_encode_accelerator_->Encode(video_frame, key_frame_requested);
  }

 protected:
  void NotifyError(VideoEncodeAccelerator::Error error) final {
    DCHECK(task_runner_->RunsTasksOnCurrentThread());

    DCHECK(error != VideoEncodeAccelerator::kInvalidArgumentError &&
           error != VideoEncodeAccelerator::kIllegalStateError);

    encoder_active_ = false;

    cast_environment_->PostTask(
        CastEnvironment::MAIN,
        FROM_HERE,
        base::Bind(status_change_cb_, STATUS_CODEC_RUNTIME_ERROR));

    // TODO(miu): Force-flush all |in_progress_frame_encodes_| immediately so
    // pending frames do not become stuck, freezing VideoSender.
  }

  // Called to allocate the input and output buffers.
  void RequireBitstreamBuffers(unsigned int input_count,
                               const gfx::Size& input_coded_size,
                               size_t output_buffer_size) final {
    DCHECK(task_runner_->RunsTasksOnCurrentThread());

    // TODO(miu): Investigate why we are ignoring |input_count| (4) and instead
    // using |kOutputBufferCount| (3) here.
    for (size_t j = 0; j < kOutputBufferCount; ++j) {
      create_video_encode_memory_cb_.Run(
          output_buffer_size,
          base::Bind(&VEAClientImpl::OnCreateSharedMemory, this));
    }
  }

  // Encoder has encoded a frame and it's available in one of the output
  // buffers.  Package the result in a media::cast::EncodedFrame and post it
  // to the Cast MAIN thread via the supplied callback.
  void BitstreamBufferReady(int32 bitstream_buffer_id,
                            size_t payload_size,
                            bool key_frame) final {
    DCHECK(task_runner_->RunsTasksOnCurrentThread());
    if (bitstream_buffer_id < 0 ||
        bitstream_buffer_id >= static_cast<int32>(output_buffers_.size())) {
      NOTREACHED();
      VLOG(1) << "BitstreamBufferReady(): invalid bitstream_buffer_id="
              << bitstream_buffer_id;
      NotifyError(media::VideoEncodeAccelerator::kPlatformFailureError);
      return;
    }
    base::SharedMemory* output_buffer = output_buffers_[bitstream_buffer_id];
    if (payload_size > output_buffer->mapped_size()) {
      NOTREACHED();
      VLOG(1) << "BitstreamBufferReady(): invalid payload_size = "
              << payload_size;
      NotifyError(media::VideoEncodeAccelerator::kPlatformFailureError);
      return;
    }
    if (key_frame)
      key_frame_encountered_ = true;
    if (!key_frame_encountered_) {
      // Do not send video until we have encountered the first key frame.
      // Save the bitstream buffer in |stream_header_| to be sent later along
      // with the first key frame.
      //
      // TODO(miu): Should |stream_header_| be an std::ostringstream for
      // performance reasons?
      stream_header_.append(static_cast<const char*>(output_buffer->memory()),
                            payload_size);
    } else if (!in_progress_frame_encodes_.empty()) {
      const InProgressFrameEncode& request = in_progress_frame_encodes_.front();

      scoped_ptr<SenderEncodedFrame> encoded_frame(new SenderEncodedFrame());
      encoded_frame->dependency = key_frame ? EncodedFrame::KEY :
          EncodedFrame::DEPENDENT;
      encoded_frame->frame_id = next_frame_id_++;
      if (key_frame)
        encoded_frame->referenced_frame_id = encoded_frame->frame_id;
      else
        encoded_frame->referenced_frame_id = encoded_frame->frame_id - 1;
      encoded_frame->rtp_timestamp = TimeDeltaToRtpDelta(
          request.video_frame->timestamp(), kVideoFrequency);
      encoded_frame->reference_time = request.reference_time;
      if (!stream_header_.empty()) {
        encoded_frame->data = stream_header_;
        stream_header_.clear();
      }
      encoded_frame->data.append(
          static_cast<const char*>(output_buffer->memory()), payload_size);

      // If FRAME_DURATION metadata was provided in the source VideoFrame,
      // compute the utilization metrics.
      base::TimeDelta frame_duration;
      if (request.video_frame->metadata()->GetTimeDelta(
              media::VideoFrameMetadata::FRAME_DURATION, &frame_duration) &&
          frame_duration > base::TimeDelta()) {
        // Compute deadline utilization as the real-world time elapsed divided
        // by the frame duration.
        const base::TimeDelta processing_time =
            base::TimeTicks::Now() - request.start_time;
        encoded_frame->deadline_utilization =
            processing_time.InSecondsF() / frame_duration.InSecondsF();

        // See vp8_encoder.cc for an explanation of this math.  Here, we are
        // computing a substitute value for |quantizer| using the
        // QuantizerEstimator.
        const double actual_bit_rate =
            encoded_frame->data.size() * 8.0 / frame_duration.InSecondsF();
        DCHECK_GT(request.target_bit_rate, 0);
        const double bitrate_utilization =
            actual_bit_rate / request.target_bit_rate;
        const double quantizer =
            (encoded_frame->dependency == EncodedFrame::KEY) ?
            quantizer_estimator_.EstimateForKeyFrame(*request.video_frame) :
            quantizer_estimator_.EstimateForDeltaFrame(*request.video_frame);
        if (quantizer != QuantizerEstimator::NO_RESULT) {
          encoded_frame->lossy_utilization = bitrate_utilization *
              (quantizer / QuantizerEstimator::MAX_VP8_QUANTIZER);
        }
      } else {
        quantizer_estimator_.Reset();
      }

      cast_environment_->PostTask(
          CastEnvironment::MAIN,
          FROM_HERE,
          base::Bind(&LogFrameEncodedEvent,
                     cast_environment_,
                     cast_environment_->Clock()->NowTicks(),
                     encoded_frame->rtp_timestamp,
                     encoded_frame->frame_id));

      cast_environment_->PostTask(
          CastEnvironment::MAIN,
          FROM_HERE,
          base::Bind(request.frame_encoded_callback,
                     base::Passed(&encoded_frame)));

      in_progress_frame_encodes_.pop_front();
    } else {
      VLOG(1) << "BitstreamBufferReady(): no encoded frame data available";
    }

    // We need to re-add the output buffer to the encoder after we are done
    // with it.
    video_encode_accelerator_->UseOutputBitstreamBuffer(media::BitstreamBuffer(
        bitstream_buffer_id,
        output_buffers_[bitstream_buffer_id]->handle(),
        output_buffers_[bitstream_buffer_id]->mapped_size()));
  }

 private:
  friend class base::RefCountedThreadSafe<VEAClientImpl>;

  ~VEAClientImpl() final {
    // According to the media::VideoEncodeAccelerator interface, Destroy()
    // should be called instead of invoking its private destructor.
    task_runner_->PostTask(
        FROM_HERE,
        base::Bind(&media::VideoEncodeAccelerator::Destroy,
                   base::Unretained(video_encode_accelerator_.release())));
  }

  // Note: This method can be called on any thread.
  void OnCreateSharedMemory(scoped_ptr<base::SharedMemory> memory) {
    task_runner_->PostTask(FROM_HERE,
                           base::Bind(&VEAClientImpl::OnReceivedSharedMemory,
                                      this,
                                      base::Passed(&memory)));
  }

  void OnReceivedSharedMemory(scoped_ptr<base::SharedMemory> memory) {
    DCHECK(task_runner_->RunsTasksOnCurrentThread());

    output_buffers_.push_back(memory.Pass());

    // Wait until all requested buffers are received.
    if (output_buffers_.size() < kOutputBufferCount)
      return;

    // Immediately provide all output buffers to the VEA.
    for (size_t i = 0; i < output_buffers_.size(); ++i) {
      video_encode_accelerator_->UseOutputBitstreamBuffer(
          media::BitstreamBuffer(static_cast<int32>(i),
                                 output_buffers_[i]->handle(),
                                 output_buffers_[i]->mapped_size()));
    }
  }

  const scoped_refptr<CastEnvironment> cast_environment_;
  const scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
  const int max_frame_rate_;
  const StatusChangeCallback status_change_cb_;  // Must be run on MAIN thread.
  const CreateVideoEncodeMemoryCallback create_video_encode_memory_cb_;
  scoped_ptr<media::VideoEncodeAccelerator> video_encode_accelerator_;
  bool encoder_active_;
  uint32 next_frame_id_;
  bool key_frame_encountered_;
  std::string stream_header_;

  // Shared memory buffers for output with the VideoAccelerator.
  ScopedVector<base::SharedMemory> output_buffers_;

  // FIFO list.
  std::list<InProgressFrameEncode> in_progress_frame_encodes_;

  // The requested encode bit rate for the next frame.
  int requested_bit_rate_;

  // Used to compute utilization metrics for each frame.
  QuantizerEstimator quantizer_estimator_;

  DISALLOW_COPY_AND_ASSIGN(VEAClientImpl);
};

// static
bool ExternalVideoEncoder::IsSupported(const VideoSenderConfig& video_config) {
  if (video_config.codec != CODEC_VIDEO_VP8 &&
      video_config.codec != CODEC_VIDEO_H264)
    return false;

  // TODO(miu): "Layering hooks" are needed to be able to query outside of
  // libmedia, to determine whether the system provides a hardware encoder.  For
  // now, assume that this was already checked by this point.
  // http://crbug.com/454029
  return video_config.use_external_encoder;
}

ExternalVideoEncoder::ExternalVideoEncoder(
    const scoped_refptr<CastEnvironment>& cast_environment,
    const VideoSenderConfig& video_config,
    const gfx::Size& frame_size,
    uint32 first_frame_id,
    const StatusChangeCallback& status_change_cb,
    const CreateVideoEncodeAcceleratorCallback& create_vea_cb,
    const CreateVideoEncodeMemoryCallback& create_video_encode_memory_cb)
    : cast_environment_(cast_environment),
      create_video_encode_memory_cb_(create_video_encode_memory_cb),
      frame_size_(frame_size),
      bit_rate_(video_config.start_bitrate),
      key_frame_requested_(false),
      weak_factory_(this) {
  DCHECK(cast_environment_->CurrentlyOn(CastEnvironment::MAIN));
  DCHECK_GT(video_config.max_frame_rate, 0);
  DCHECK(!frame_size_.IsEmpty());
  DCHECK(!status_change_cb.is_null());
  DCHECK(!create_vea_cb.is_null());
  DCHECK(!create_video_encode_memory_cb_.is_null());
  DCHECK_GT(bit_rate_, 0);

  create_vea_cb.Run(
      base::Bind(&ExternalVideoEncoder::OnCreateVideoEncodeAccelerator,
                 weak_factory_.GetWeakPtr(),
                 video_config,
                 first_frame_id,
                 status_change_cb));
}

ExternalVideoEncoder::~ExternalVideoEncoder() {
}

bool ExternalVideoEncoder::EncodeVideoFrame(
    const scoped_refptr<media::VideoFrame>& video_frame,
    const base::TimeTicks& reference_time,
    const FrameEncodedCallback& frame_encoded_callback) {
  DCHECK(cast_environment_->CurrentlyOn(CastEnvironment::MAIN));
  DCHECK(!frame_encoded_callback.is_null());

  if (!client_ || video_frame->visible_rect().size() != frame_size_)
    return false;

  client_->task_runner()->PostTask(FROM_HERE,
                                   base::Bind(&VEAClientImpl::EncodeVideoFrame,
                                              client_,
                                              video_frame,
                                              reference_time,
                                              key_frame_requested_,
                                              frame_encoded_callback));
  key_frame_requested_ = false;
  return true;
}

void ExternalVideoEncoder::SetBitRate(int new_bit_rate) {
  DCHECK(cast_environment_->CurrentlyOn(CastEnvironment::MAIN));
  DCHECK_GT(new_bit_rate, 0);

  bit_rate_ = new_bit_rate;
  if (!client_)
    return;
  client_->task_runner()->PostTask(
      FROM_HERE, base::Bind(&VEAClientImpl::SetBitRate, client_, bit_rate_));
}

void ExternalVideoEncoder::GenerateKeyFrame() {
  DCHECK(cast_environment_->CurrentlyOn(CastEnvironment::MAIN));
  key_frame_requested_ = true;
}

void ExternalVideoEncoder::LatestFrameIdToReference(uint32 /*frame_id*/) {
  // Do nothing.  Not supported.
}

void ExternalVideoEncoder::OnCreateVideoEncodeAccelerator(
    const VideoSenderConfig& video_config,
    uint32 first_frame_id,
    const StatusChangeCallback& status_change_cb,
    scoped_refptr<base::SingleThreadTaskRunner> encoder_task_runner,
    scoped_ptr<media::VideoEncodeAccelerator> vea) {
  DCHECK(cast_environment_->CurrentlyOn(CastEnvironment::MAIN));

  // The callback will be invoked with null pointers in the case where the
  // system does not support or lacks the resources to provide GPU-accelerated
  // video encoding.
  if (!encoder_task_runner || !vea) {
    cast_environment_->PostTask(
        CastEnvironment::MAIN,
        FROM_HERE,
        base::Bind(status_change_cb, STATUS_CODEC_INIT_FAILED));
    return;
  }

  VideoCodecProfile codec_profile;
  switch (video_config.codec) {
    case CODEC_VIDEO_VP8:
      codec_profile = media::VP8PROFILE_ANY;
      break;
    case CODEC_VIDEO_H264:
      codec_profile = media::H264PROFILE_MAIN;
      break;
    case CODEC_VIDEO_FAKE:
      NOTREACHED() << "Fake software video encoder cannot be external";
      // ...flow through to next case...
    default:
      cast_environment_->PostTask(
        CastEnvironment::MAIN,
        FROM_HERE,
        base::Bind(status_change_cb, STATUS_UNSUPPORTED_CODEC));
      return;
  }

  DCHECK(!client_);
  client_ = new VEAClientImpl(cast_environment_,
                              encoder_task_runner,
                              vea.Pass(),
                              video_config.max_frame_rate,
                              status_change_cb,
                              create_video_encode_memory_cb_);
  client_->task_runner()->PostTask(FROM_HERE,
                                   base::Bind(&VEAClientImpl::Initialize,
                                              client_,
                                              frame_size_,
                                              codec_profile,
                                              bit_rate_,
                                              first_frame_id));
}

SizeAdaptableExternalVideoEncoder::SizeAdaptableExternalVideoEncoder(
    const scoped_refptr<CastEnvironment>& cast_environment,
    const VideoSenderConfig& video_config,
    const StatusChangeCallback& status_change_cb,
    const CreateVideoEncodeAcceleratorCallback& create_vea_cb,
    const CreateVideoEncodeMemoryCallback& create_video_encode_memory_cb)
    : SizeAdaptableVideoEncoderBase(cast_environment,
                                    video_config,
                                    status_change_cb),
      create_vea_cb_(create_vea_cb),
      create_video_encode_memory_cb_(create_video_encode_memory_cb) {}

SizeAdaptableExternalVideoEncoder::~SizeAdaptableExternalVideoEncoder() {}

scoped_ptr<VideoEncoder> SizeAdaptableExternalVideoEncoder::CreateEncoder() {
  return scoped_ptr<VideoEncoder>(new ExternalVideoEncoder(
      cast_environment(),
      video_config(),
      frame_size(),
      last_frame_id() + 1,
      CreateEncoderStatusChangeCallback(),
      create_vea_cb_,
      create_video_encode_memory_cb_));
}

QuantizerEstimator::QuantizerEstimator() {}

QuantizerEstimator::~QuantizerEstimator() {}

void QuantizerEstimator::Reset() {
  last_frame_pixel_buffer_.reset();
}

double QuantizerEstimator::EstimateForKeyFrame(const VideoFrame& frame) {
  if (!CanExamineFrame(frame))
    return NO_RESULT;

  // If the size of the frame is different from the last frame, allocate a new
  // buffer.  The buffer only needs to be a fraction of the size of the entire
  // frame, since the entropy analysis only examines a subset of each frame.
  const gfx::Size size = frame.visible_rect().size();
  const int rows_in_subset =
      std::max(1, size.height() * FRAME_SAMPLING_PERCENT / 100);
  if (last_frame_size_ != size || !last_frame_pixel_buffer_) {
    last_frame_pixel_buffer_.reset(new uint8[size.width() * rows_in_subset]);
    last_frame_size_ = size;
  }

  // Compute a histogram where each bucket represents the number of times two
  // neighboring pixels were different by a specific amount.  511 buckets are
  // needed, one for each integer in the range [-255,255].
  int histogram[511];
  memset(histogram, 0, sizeof(histogram));
  const int row_skip = size.height() / rows_in_subset;
  int y = 0;
  for (int i = 0; i < rows_in_subset; ++i, y += row_skip) {
    const uint8* const row_begin = frame.visible_data(VideoFrame::kYPlane) +
        y * frame.stride(VideoFrame::kYPlane);
    const uint8* const row_end = row_begin + size.width();
    int left_hand_pixel_value = static_cast<int>(*row_begin);
    for (const uint8* p = row_begin + 1; p < row_end; ++p) {
      const int right_hand_pixel_value = static_cast<int>(*p);
      const int difference = right_hand_pixel_value - left_hand_pixel_value;
      const int histogram_index = difference + 255;
      ++histogram[histogram_index];
      left_hand_pixel_value = right_hand_pixel_value;  // For next iteration.
    }

    // Copy the row of pixels into the buffer.  This will be used when
    // generating histograms for future delta frames.
    memcpy(last_frame_pixel_buffer_.get() + i * size.width(),
           row_begin,
           size.width());
  }

  // Estimate a quantizer value depending on the difference data in the
  // histogram and return it.
  const int num_samples = (size.width() - 1) * rows_in_subset;
  return ToQuantizerEstimate(ComputeEntropyFromHistogram(
      histogram, arraysize(histogram), num_samples));
}

double QuantizerEstimator::EstimateForDeltaFrame(const VideoFrame& frame) {
  if (!CanExamineFrame(frame))
    return NO_RESULT;

  // If the size of the |frame| has changed, no difference can be examined.
  // In this case, process this frame as if it were a key frame.
  const gfx::Size size = frame.visible_rect().size();
  if (last_frame_size_ != size || !last_frame_pixel_buffer_)
    return EstimateForKeyFrame(frame);
  const int rows_in_subset =
      std::max(1, size.height() * FRAME_SAMPLING_PERCENT / 100);

  // Compute a histogram where each bucket represents the number of times the
  // same pixel in this frame versus the last frame was different by a specific
  // amount.  511 buckets are needed, one for each integer in the range
  // [-255,255].
  int histogram[511];
  memset(histogram, 0, sizeof(histogram));
  const int row_skip = size.height() / rows_in_subset;
  int y = 0;
  for (int i = 0; i < rows_in_subset; ++i, y += row_skip) {
    const uint8* const row_begin = frame.visible_data(VideoFrame::kYPlane) +
        y * frame.stride(VideoFrame::kYPlane);
    const uint8* const row_end = row_begin + size.width();
    uint8* const last_frame_row_begin =
        last_frame_pixel_buffer_.get() + i * size.width();
    for (const uint8* p = row_begin, *q = last_frame_row_begin; p < row_end;
         ++p, ++q) {
      const int difference = static_cast<int>(*p) - static_cast<int>(*q);
      const int histogram_index = difference + 255;
      ++histogram[histogram_index];
    }

    // Copy the row of pixels into the buffer.  This will be used when
    // generating histograms for future delta frames.
    memcpy(last_frame_row_begin, row_begin, size.width());
  }

  // Estimate a quantizer value depending on the difference data in the
  // histogram and return it.
  const int num_samples = size.width() * rows_in_subset;
  return ToQuantizerEstimate(ComputeEntropyFromHistogram(
      histogram, arraysize(histogram), num_samples));
}

// static
bool QuantizerEstimator::CanExamineFrame(const VideoFrame& frame) {
  DCHECK_EQ(8, VideoFrame::PlaneHorizontalBitsPerPixel(frame.format(),
                                                       VideoFrame::kYPlane));
  return media::IsYuvPlanar(frame.format()) &&
      !frame.visible_rect().IsEmpty();
}

// static
double QuantizerEstimator::ComputeEntropyFromHistogram(const int* histogram,
                                                       size_t num_buckets,
                                                       int num_samples) {
  DCHECK_LT(0, num_samples);
  double entropy = 0.0;
  for (size_t i = 0; i < num_buckets; ++i) {
    const double probability = static_cast<double>(histogram[i]) / num_samples;
    if (probability > 0.0)
      entropy = entropy - probability * log2(probability);
  }
  return entropy;
}

// static
double QuantizerEstimator::ToQuantizerEstimate(double shannon_entropy) {
  DCHECK_GE(shannon_entropy, 0.0);

  // This math is based on an analysis of data produced by running a wide range
  // of mirroring content in a Cast streaming session on a Chromebook Pixel
  // (2013 edition).  The output from the Pixel's built-in hardware encoder was
  // compared to an identically-configured software implementation (libvpx)
  // running alongside.  Based on an analysis of the data, the following linear
  // mapping seems to produce reasonable VP8 quantizer values from the
  // |shannon_entropy| values.
  //
  // TODO(miu): Confirm whether this model and value work well on other
  // platforms.
  const double kEntropyAtMaxQuantizer = 7.5;
  const double slope =
      (MAX_VP8_QUANTIZER - MIN_VP8_QUANTIZER) / kEntropyAtMaxQuantizer;
  const double quantizer = std::min<double>(
      MAX_VP8_QUANTIZER, MIN_VP8_QUANTIZER + slope * shannon_entropy);
  return quantizer;
}

}  //  namespace cast
}  //  namespace media