text: /*
   *  Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
   *
   *  Use of this source code is governed by a BSD-style license
   *  that can be found in the LICENSE file in the root of the source
   *  tree. An additional intellectual property rights grant can be found
   *  in the file PATENTS.  All contributing project authors may
   *  be found in the AUTHORS file in the root of the source tree.
   */
  
  #include "media/engine/simulcast.h"
  
  #include <stdint.h>
  #include <stdio.h>
  
  #include <algorithm>
  #include <string>
  #include <vector>
  
  #include "absl/strings/match.h"
  #include "absl/types/optional.h"
  #include "api/video/video_codec_constants.h"
  #include "media/base/media_constants.h"
  #include "modules/video_coding/utility/simulcast_rate_allocator.h"
  #include "rtc_base/checks.h"
  #include "rtc_base/experiments/field_trial_parser.h"
  #include "rtc_base/experiments/min_video_bitrate_experiment.h"
  #include "rtc_base/experiments/normalize_simulcast_size_experiment.h"
  #include "rtc_base/experiments/rate_control_settings.h"
  #include "rtc_base/logging.h"
  
  namespace cricket {
  
  namespace {
  
  constexpr webrtc::DataRate Interpolate(const webrtc::DataRate& a,
                                         const webrtc::DataRate& b,
                                         float rate) {
    return a * (1.0 - rate) + b * rate;
  }
  
  constexpr char kUseLegacySimulcastLayerLimitFieldTrial[] =
      "WebRTC-LegacySimulcastLayerLimit";
  
  constexpr double kDefaultMaxRoundupRate = 0.1;
  
  // TODO(webrtc:12415): Flip this to a kill switch when this feature launches.
  bool EnableLowresBitrateInterpolation(const webrtc::FieldTrialsView& trials) {
    return absl::StartsWith(
        trials.Lookup("WebRTC-LowresSimulcastBitrateInterpolation"), "Enabled");
  }
  
  // Limits for legacy conference screensharing mode. Currently used for the
  // lower of the two simulcast streams.
  constexpr webrtc::DataRate kScreenshareDefaultTl0Bitrate =
      webrtc::DataRate::KilobitsPerSec(200);
  constexpr webrtc::DataRate kScreenshareDefaultTl1Bitrate =
      webrtc::DataRate::KilobitsPerSec(1000);
  
  // Min/max bitrate for the higher one of the two simulcast stream used for
  // screen content.
  constexpr webrtc::DataRate kScreenshareHighStreamMinBitrate =
      webrtc::DataRate::KilobitsPerSec(600);
  constexpr webrtc::DataRate kScreenshareHighStreamMaxBitrate =
      webrtc::DataRate::KilobitsPerSec(1250);
  
  }  // namespace
  
  struct SimulcastFormat {
    int width;
    int height;
    // The maximum number of simulcast layers can be used for
    // resolutions at `widthxheight` for legacy applications.
    size_t max_layers;
    // The maximum bitrate for encoding stream at `widthxheight`, when we are
    // not sending the next higher spatial stream.
    webrtc::DataRate max_bitrate;
    // The target bitrate for encoding stream at `widthxheight`, when this layer
    // is not the highest layer (i.e., when we are sending another higher spatial
    // stream).
    webrtc::DataRate target_bitrate;
    // The minimum bitrate needed for encoding stream at `widthxheight`.
    webrtc::DataRate min_bitrate;
  };
  
  // These tables describe from which resolution we can use how many
  // simulcast layers at what bitrates (maximum, target, and minimum).
  // Important!! Keep this table from high resolution to low resolution.
  constexpr const SimulcastFormat kSimulcastFormats[] = {
      {1920, 1080, 3, webrtc::DataRate::KilobitsPerSec(5000),
       webrtc::DataRate::KilobitsPerSec(4000),
       webrtc::DataRate::KilobitsPerSec(800)},
      {1280, 720, 3, webrtc::DataRate::KilobitsPerSec(2500),
       webrtc::DataRate::KilobitsPerSec(2500),
       webrtc::DataRate::KilobitsPerSec(600)},
      {960, 540, 3, webrtc::DataRate::KilobitsPerSec(1200),
       webrtc::DataRate::KilobitsPerSec(1200),
       webrtc::DataRate::KilobitsPerSec(350)},
      {640, 360, 2, webrtc::DataRate::KilobitsPerSec(700),
       webrtc::DataRate::KilobitsPerSec(500),
       webrtc::DataRate::KilobitsPerSec(150)},
      {480, 270, 2, webrtc::DataRate::KilobitsPerSec(450),
       webrtc::DataRate::KilobitsPerSec(350),
       webrtc::DataRate::KilobitsPerSec(150)},
      {320, 180, 1, webrtc::DataRate::KilobitsPerSec(200),
       webrtc::DataRate::KilobitsPerSec(150),
       webrtc::DataRate::KilobitsPerSec(30)},
      // As the resolution goes down, interpolate the target and max bitrates down
      // towards zero. The min bitrate is still limited at 30 kbps and the target
      // and the max will be capped from below accordingly.
      {0, 0, 1, webrtc::DataRate::KilobitsPerSec(0),
       webrtc::DataRate::KilobitsPerSec(0),
       webrtc::DataRate::KilobitsPerSec(30)}};
  
  std::vector<SimulcastFormat> GetSimulcastFormats(
      bool enable_lowres_bitrate_interpolation) {
    std::vector<SimulcastFormat> formats;
    formats.insert(formats.begin(), std::begin(kSimulcastFormats),
                   std::end(kSimulcastFormats));
    if (!enable_lowres_bitrate_interpolation) {
      RTC_CHECK_GE(formats.size(), 2u);
      SimulcastFormat& format0x0 = formats[formats.size() - 1];
      const SimulcastFormat& format_prev = formats[formats.size() - 2];
      format0x0.max_bitrate = format_prev.max_bitrate;
      format0x0.target_bitrate = format_prev.target_bitrate;
      format0x0.min_bitrate = format_prev.min_bitrate;
    }
    return formats;
  }
  
  const int kMaxScreenshareSimulcastLayers = 2;
  
  // Multiway: Number of temporal layers for each simulcast stream.
  int DefaultNumberOfTemporalLayers(int simulcast_id,
                                    bool screenshare,
                                    const webrtc::FieldTrialsView& trials) {
    RTC_CHECK_GE(simulcast_id, 0);
    RTC_CHECK_LT(simulcast_id, webrtc::kMaxSimulcastStreams);
  
    const int kDefaultNumTemporalLayers = 3;
    const int kDefaultNumScreenshareTemporalLayers = 2;
    int default_num_temporal_layers = screenshare
                                          ? kDefaultNumScreenshareTemporalLayers
                                          : kDefaultNumTemporalLayers;
  
    const std::string group_name =
        screenshare ? trials.Lookup("WebRTC-VP8ScreenshareTemporalLayers")
                    : trials.Lookup("WebRTC-VP8ConferenceTemporalLayers");
    if (group_name.empty())
      return default_num_temporal_layers;
  
    int num_temporal_layers = default_num_temporal_layers;
    if (sscanf(group_name.c_str(), "%d", &num_temporal_layers) == 1 &&
        num_temporal_layers > 0 &&
        num_temporal_layers <= webrtc::kMaxTemporalStreams) {
      return num_temporal_layers;
    }
  
    RTC_LOG(LS_WARNING) << "Attempt to set number of temporal layers to "
                           "incorrect value: "
                        << group_name;
  
    return default_num_temporal_layers;
  }
  
  int FindSimulcastFormatIndex(int width,
                               int height,
                               bool enable_lowres_bitrate_interpolation) {
    RTC_DCHECK_GE(width, 0);
    RTC_DCHECK_GE(height, 0);
    const auto formats = GetSimulcastFormats(enable_lowres_bitrate_interpolation);
    for (uint32_t i = 0; i < formats.size(); ++i) {
      if (width * height >= formats[i].width * formats[i].height) {
        return i;
      }
    }
    RTC_DCHECK_NOTREACHED();
    return -1;
  }
  
  // Round size to nearest simulcast-friendly size.
  // Simulcast stream width and height must both be dividable by
  // |2 ^ (simulcast_layers - 1)|.
  int NormalizeSimulcastSize(int size, size_t simulcast_layers) {
    int base2_exponent = static_cast<int>(simulcast_layers) - 1;
    const absl::optional<int> experimental_base2_exponent =
        webrtc::NormalizeSimulcastSizeExperiment::GetBase2Exponent();
    if (experimental_base2_exponent &&
        (size > (1 << *experimental_base2_exponent))) {
      base2_exponent = *experimental_base2_exponent;
    }
    return ((size >> base2_exponent) << base2_exponent);
  }
  
  SimulcastFormat InterpolateSimulcastFormat(
      int width,
      int height,
      absl::optional<double> max_roundup_rate,
      bool enable_lowres_bitrate_interpolation) {
    const auto formats = GetSimulcastFormats(enable_lowres_bitrate_interpolation);
    const int index = FindSimulcastFormatIndex(
        width, height, enable_lowres_bitrate_interpolation);
    if (index == 0)
      return formats[index];
    const int total_pixels_up =
        formats[index - 1].width * formats[index - 1].height;
    const int total_pixels_down = formats[index].width * formats[index].height;
    const int total_pixels = width * height;
    const float rate = (total_pixels_up - total_pixels) /
                       static_cast<float>(total_pixels_up - total_pixels_down);
  
    // Use upper resolution if `rate` is below the configured threshold.
    size_t max_layers = (rate < max_roundup_rate.value_or(kDefaultMaxRoundupRate))
                            ? formats[index - 1].max_layers
                            : formats[index].max_layers;
    webrtc::DataRate max_bitrate = Interpolate(formats[index - 1].max_bitrate,
                                               formats[index].max_bitrate, rate);
    webrtc::DataRate target_bitrate = Interpolate(
        formats[index - 1].target_bitrate, formats[index].target_bitrate, rate);
    webrtc::DataRate min_bitrate = Interpolate(formats[index - 1].min_bitrate,
                                               formats[index].min_bitrate, rate);
  
    return {width, height, max_layers, max_bitrate, target_bitrate, min_bitrate};
  }
  
  SimulcastFormat InterpolateSimulcastFormat(
      int width,
      int height,
      bool enable_lowres_bitrate_interpolation) {
    return InterpolateSimulcastFormat(width, height, absl::nullopt,
                                      enable_lowres_bitrate_interpolation);
  }
  
  webrtc::DataRate FindSimulcastMaxBitrate(
      int width,
      int height,
      bool enable_lowres_bitrate_interpolation) {
    return InterpolateSimulcastFormat(width, height,
                                      enable_lowres_bitrate_interpolation)
        .max_bitrate;
  }
  
  webrtc::DataRate FindSimulcastTargetBitrate(
      int width,
      int height,
      bool enable_lowres_bitrate_interpolation) {
    return InterpolateSimulcastFormat(width, height,
                                      enable_lowres_bitrate_interpolation)
        .target_bitrate;
  }
  
  webrtc::DataRate FindSimulcastMinBitrate(
      int width,
      int height,
      bool enable_lowres_bitrate_interpolation) {
    return InterpolateSimulcastFormat(width, height,
                                      enable_lowres_bitrate_interpolation)
        .min_bitrate;
  }
  
  void BoostMaxSimulcastLayer(webrtc::DataRate max_bitrate,
                              std::vector<webrtc::VideoStream>* layers) {
    if (layers->empty())
      return;
  
    const webrtc::DataRate total_bitrate = GetTotalMaxBitrate(*layers);
  
    // We're still not using all available bits.
    if (total_bitrate < max_bitrate) {
      // Spend additional bits to boost the max layer.
      const webrtc::DataRate bitrate_left = max_bitrate - total_bitrate;
      layers->back().max_bitrate_bps += bitrate_left.bps();
    }
  }
  
  webrtc::DataRate GetTotalMaxBitrate(
      const std::vector<webrtc::VideoStream>& layers) {
    if (layers.empty())
      return webrtc::DataRate::Zero();
  
    int total_max_bitrate_bps = 0;
    for (size_t s = 0; s < layers.size() - 1; ++s) {
      total_max_bitrate_bps += layers[s].target_bitrate_bps;
    }
    total_max_bitrate_bps += layers.back().max_bitrate_bps;
    return webrtc::DataRate::BitsPerSec(total_max_bitrate_bps);
  }
  
  size_t LimitSimulcastLayerCount(int width,
                                  int height,
                                  size_t need_layers,
                                  size_t layer_count,
                                  const webrtc::FieldTrialsView& trials) {
    if (!absl::StartsWith(trials.Lookup(kUseLegacySimulcastLayerLimitFieldTrial),
                          "Disabled")) {
      // Max layers from one higher resolution in kSimulcastFormats will be used
      // if the ratio (pixels_up - pixels) / (pixels_up - pixels_down) is less
      // than configured `max_ratio`. pixels_down is the selected index in
      // kSimulcastFormats based on pixels.
      webrtc::FieldTrialOptional<double> max_ratio("max_ratio");
      webrtc::ParseFieldTrial({&max_ratio},
                              trials.Lookup("WebRTC-SimulcastLayerLimitRoundUp"));
  
      const bool enable_lowres_bitrate_interpolation =
          EnableLowresBitrateInterpolation(trials);
      size_t adaptive_layer_count = std::max(
          need_layers,
          InterpolateSimulcastFormat(width, height, max_ratio.GetOptional(),
                                     enable_lowres_bitrate_interpolation)
              .max_layers);
      if (layer_count > adaptive_layer_count) {
        RTC_LOG(LS_WARNING) << "Reducing simulcast layer count from "
                            << layer_count << " to " << adaptive_layer_count;
        layer_count = adaptive_layer_count;
      }
    }
    return layer_count;
  }
  
  std::vector<webrtc::VideoStream> GetSimulcastConfig(
      size_t min_layers,
      size_t max_layers,
      int width,
      int height,
      double bitrate_priority,
      int max_qp,
      bool is_screenshare_with_conference_mode,
      bool temporal_layers_supported,
      const webrtc::FieldTrialsView& trials) {
    RTC_DCHECK_LE(min_layers, max_layers);
    RTC_DCHECK(max_layers > 1 || is_screenshare_with_conference_mode);
  
    const bool base_heavy_tl3_rate_alloc =
        webrtc::RateControlSettings::ParseFromKeyValueConfig(&trials)
            .Vp8BaseHeavyTl3RateAllocation();
    if (is_screenshare_with_conference_mode) {
      return GetScreenshareLayers(max_layers, width, height, bitrate_priority,
                                  max_qp, temporal_layers_supported,
                                  base_heavy_tl3_rate_alloc, trials);
    } else {
      // Some applications rely on the old behavior limiting the simulcast layer
      // count based on the resolution automatically, which they can get through
      // the WebRTC-LegacySimulcastLayerLimit field trial until they update.
      max_layers =
          LimitSimulcastLayerCount(width, height, min_layers, max_layers, trials);
  
      return GetNormalSimulcastLayers(max_layers, width, height, bitrate_priority,
                                      max_qp, temporal_layers_supported,
                                      base_heavy_tl3_rate_alloc, trials);
    }
  }
  
  std::vector<webrtc::VideoStream> GetNormalSimulcastLayers(
      size_t layer_count,
      int width,
      int height,
      double bitrate_priority,
      int max_qp,
      bool temporal_layers_supported,
      bool base_heavy_tl3_rate_alloc,
      const webrtc::FieldTrialsView& trials) {
    std::vector<webrtc::VideoStream> layers(layer_count);
  
    const bool enable_lowres_bitrate_interpolation =
        EnableLowresBitrateInterpolation(trials);
  
    // Format width and height has to be divisible by |2 ^ num_simulcast_layers -
    // 1|.
    width = NormalizeSimulcastSize(width, layer_count);
    height = NormalizeSimulcastSize(height, layer_count);
    // Add simulcast streams, from highest resolution (`s` = num_simulcast_layers
    // -1) to lowest resolution at `s` = 0.
    for (size_t s = layer_count - 1;; --s) {
      layers[s].width = width;
      layers[s].height = height;
      // TODO(pbos): Fill actual temporal-layer bitrate thresholds.
      layers[s].max_qp = max_qp;
      layers[s].num_temporal_layers =
          temporal_layers_supported
              ? DefaultNumberOfTemporalLayers(s, false, trials)
              : 1;
      layers[s].max_bitrate_bps =
          FindSimulcastMaxBitrate(width, height,
                                  enable_lowres_bitrate_interpolation)
              .bps();
      layers[s].target_bitrate_bps =
          FindSimulcastTargetBitrate(width, height,
                                     enable_lowres_bitrate_interpolation)
              .bps();
      int num_temporal_layers = DefaultNumberOfTemporalLayers(s, false, trials);
      if (s == 0) {
        // If alternative temporal rate allocation is selected, adjust the
        // bitrate of the lowest simulcast stream so that absolute bitrate for
        // the base temporal layer matches the bitrate for the base temporal
        // layer with the default 3 simulcast streams. Otherwise we risk a
        // higher threshold for receiving a feed at all.
        float rate_factor = 1.0;
        if (num_temporal_layers == 3) {
          if (base_heavy_tl3_rate_alloc) {
            // Base heavy allocation increases TL0 bitrate from 40% to 60%.
            rate_factor = 0.4 / 0.6;
          }
        } else {
          rate_factor =
              webrtc::SimulcastRateAllocator::GetTemporalRateAllocation(
                  3, 0, /*base_heavy_tl3_rate_alloc=*/false) /
              webrtc::SimulcastRateAllocator::GetTemporalRateAllocation(
                  num_temporal_layers, 0, /*base_heavy_tl3_rate_alloc=*/false);
        }
  
        layers[s].max_bitrate_bps =
            static_cast<int>(layers[s].max_bitrate_bps * rate_factor);
        layers[s].target_bitrate_bps =
            static_cast<int>(layers[s].target_bitrate_bps * rate_factor);
      }
      layers[s].min_bitrate_bps =
          FindSimulcastMinBitrate(width, height,
                                  enable_lowres_bitrate_interpolation)
              .bps();
  
      // Ensure consistency.
      layers[s].max_bitrate_bps =
          std::max(layers[s].min_bitrate_bps, layers[s].max_bitrate_bps);
      layers[s].target_bitrate_bps =
          std::max(layers[s].min_bitrate_bps, layers[s].target_bitrate_bps);
  
      layers[s].max_framerate = kDefaultVideoMaxFramerate;
  
      width /= 2;
      height /= 2;
  
      if (s == 0) {
        break;
      }
    }
    // Currently the relative bitrate priority of the sender is controlled by
    // the value of the lowest VideoStream.
    // TODO(bugs.webrtc.org/8630): The web specification describes being able to
    // control relative bitrate for each individual simulcast layer, but this
    // is currently just implemented per rtp sender.
    layers[0].bitrate_priority = bitrate_priority;
    return layers;
  }
  
  std::vector<webrtc::VideoStream> GetScreenshareLayers(
      size_t max_layers,
      int width,
      int height,
      double bitrate_priority,
      int max_qp,
      bool temporal_layers_supported,
      bool base_heavy_tl3_rate_alloc,
      const webrtc::FieldTrialsView& trials) {
    auto max_screenshare_layers = kMaxScreenshareSimulcastLayers;
    size_t num_simulcast_layers =
        std::min<int>(max_layers, max_screenshare_layers);
  
    std::vector<webrtc::VideoStream> layers(num_simulcast_layers);
    // For legacy screenshare in conference mode, tl0 and tl1 bitrates are
    // piggybacked on the VideoCodec struct as target and max bitrates,
    // respectively. See eg. webrtc::LibvpxVp8Encoder::SetRates().
    layers[0].width = width;
    layers[0].height = height;
    layers[0].max_qp = max_qp;
    layers[0].max_framerate = 5;
    layers[0].min_bitrate_bps = webrtc::kDefaultMinVideoBitrateBps;
    layers[0].target_bitrate_bps = kScreenshareDefaultTl0Bitrate.bps();
    layers[0].max_bitrate_bps = kScreenshareDefaultTl1Bitrate.bps();
    layers[0].num_temporal_layers = temporal_layers_supported ? 2 : 1;
  
    // With simulcast enabled, add another spatial layer. This one will have a
    // more normal layout, with the regular 3 temporal layer pattern and no fps
    // restrictions. The base simulcast layer will still use legacy setup.
    if (num_simulcast_layers == kMaxScreenshareSimulcastLayers) {
      // Add optional upper simulcast layer.
      const int num_temporal_layers =
          DefaultNumberOfTemporalLayers(1, true, trials);
      int max_bitrate_bps;
      bool using_boosted_bitrate = false;
      if (!temporal_layers_supported) {
        // Set the max bitrate to where the base layer would have been if temporal
        // layers were enabled.
        max_bitrate_bps = static_cast<int>(
            kScreenshareHighStreamMaxBitrate.bps() *
            webrtc::SimulcastRateAllocator::GetTemporalRateAllocation(
                num_temporal_layers, 0, base_heavy_tl3_rate_alloc));
      } else if (DefaultNumberOfTemporalLayers(1, true, trials) != 3 ||
                 base_heavy_tl3_rate_alloc) {
        // Experimental temporal layer mode used, use increased max bitrate.
        max_bitrate_bps = kScreenshareHighStreamMaxBitrate.bps();
        using_boosted_bitrate = true;
      } else {
        // Keep current bitrates with default 3tl/8 frame settings.
        // Lowest temporal layers of a 3 layer setup will have 40% of the total
        // bitrate allocation for that simulcast layer. Make sure the gap between
        // the target of the lower simulcast layer and first temporal layer of the
        // higher one is at most 2x the bitrate, so that upswitching is not
        // hampered by stalled bitrate estimates.
        max_bitrate_bps = 2 * ((layers[0].target_bitrate_bps * 10) / 4);
      }
  
      layers[1].width = width;
      layers[1].height = height;
      layers[1].max_qp = max_qp;
      layers[1].max_framerate = kDefaultVideoMaxFramerate;
      layers[1].num_temporal_layers =
          temporal_layers_supported
              ? DefaultNumberOfTemporalLayers(1, true, trials)
              : 1;
      layers[1].min_bitrate_bps = using_boosted_bitrate
                                      ? kScreenshareHighStreamMinBitrate.bps()
                                      : layers[0].target_bitrate_bps * 2;
      layers[1].target_bitrate_bps = max_bitrate_bps;
      layers[1].max_bitrate_bps = max_bitrate_bps;
    }
  
    // The bitrate priority currently implemented on a per-sender level, so we
    // just set it for the first simulcast layer.
    layers[0].bitrate_priority = bitrate_priority;
    return layers;
  }
  
  }  // namespace cricket
  