modules/video_coding/frame_buffer2.cc

/*
 *  Copyright (c) 2016 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 "modules/video_coding/frame_buffer2.h"

#include <algorithm>
#include <cstdlib>
#include <iterator>
#include <memory>
#include <queue>
#include <utility>
#include <vector>

#include "absl/container/inlined_vector.h"
#include "api/video/encoded_image.h"
#include "api/video/video_timing.h"
#include "modules/video_coding/frame_helpers.h"
#include "modules/video_coding/include/video_coding_defines.h"
#include "modules/video_coding/jitter_estimator.h"
#include "modules/video_coding/timing.h"
#include "rtc_base/checks.h"
#include "rtc_base/experiments/rtt_mult_experiment.h"
#include "rtc_base/logging.h"
#include "rtc_base/numerics/sequence_number_util.h"
#include "rtc_base/trace_event.h"
#include "system_wrappers/include/clock.h"
#include "system_wrappers/include/field_trial.h"

namespace webrtc {
namespace video_coding {

namespace {
// Max number of frames the buffer will hold.
constexpr size_t kMaxFramesBuffered = 800;

// Default value for the maximum decode queue size that is used when the
// low-latency renderer is used.
constexpr size_t kZeroPlayoutDelayDefaultMaxDecodeQueueSize = 8;

// Max number of decoded frame info that will be saved.
constexpr int kMaxFramesHistory = 1 << 13;

// The time it's allowed for a frame to be late to its rendering prediction and
// still be rendered.
constexpr int kMaxAllowedFrameDelayMs = 5;

constexpr int64_t kLogNonDecodedIntervalMs = 5000;
}  // namespace

FrameBuffer::FrameBuffer(Clock* clock,
                         VCMTiming* timing,
                         VCMReceiveStatisticsCallback* stats_callback)
    : decoded_frames_history_(kMaxFramesHistory),
      clock_(clock),
      callback_queue_(nullptr),
      jitter_estimator_(clock),
      timing_(timing),
      inter_frame_delay_(clock_->TimeInMilliseconds()),
      stopped_(false),
      protection_mode_(kProtectionNack),
      stats_callback_(stats_callback),
      last_log_non_decoded_ms_(-kLogNonDecodedIntervalMs),
      rtt_mult_settings_(RttMultExperiment::GetRttMultValue()),
      zero_playout_delay_max_decode_queue_size_(
          "max_decode_queue_size",
          kZeroPlayoutDelayDefaultMaxDecodeQueueSize) {
  ParseFieldTrial({&zero_playout_delay_max_decode_queue_size_},
                  field_trial::FindFullName("WebRTC-ZeroPlayoutDelay"));
  callback_checker_.Detach();
}

FrameBuffer::~FrameBuffer() {
  RTC_DCHECK_RUN_ON(&construction_checker_);
}

void FrameBuffer::NextFrame(int64_t max_wait_time_ms,
                            bool keyframe_required,
                            rtc::TaskQueue* callback_queue,
                            NextFrameCallback handler) {
  RTC_DCHECK_RUN_ON(&callback_checker_);
  RTC_DCHECK(callback_queue->IsCurrent());
  TRACE_EVENT0("webrtc", "FrameBuffer::NextFrame");
  int64_t latest_return_time_ms =
      clock_->TimeInMilliseconds() + max_wait_time_ms;

  MutexLock lock(&mutex_);
  if (stopped_) {
    return;
  }
  latest_return_time_ms_ = latest_return_time_ms;
  keyframe_required_ = keyframe_required;
  frame_handler_ = handler;
  callback_queue_ = callback_queue;
  StartWaitForNextFrameOnQueue();
}

void FrameBuffer::StartWaitForNextFrameOnQueue() {
  RTC_DCHECK(callback_queue_);
  RTC_DCHECK(!callback_task_.Running());
  int64_t wait_ms = FindNextFrame(clock_->TimeInMilliseconds());
  callback_task_ = RepeatingTaskHandle::DelayedStart(
      callback_queue_->Get(), TimeDelta::Millis(wait_ms),
      [this] {
        RTC_DCHECK_RUN_ON(&callback_checker_);
        // If this task has not been cancelled, we did not get any new frames
        // while waiting. Continue with frame delivery.
        std::unique_ptr<EncodedFrame> frame;
        NextFrameCallback frame_handler;
        {
          MutexLock lock(&mutex_);
          if (!frames_to_decode_.empty()) {
            // We have frames, deliver!
            frame = GetNextFrame();
            timing_->SetLastDecodeScheduledTimestamp(
                clock_->TimeInMilliseconds());
          } else if (clock_->TimeInMilliseconds() < latest_return_time_ms_) {
            // If there's no frames to decode and there is still time left, it
            // means that the frame buffer was cleared between creation and
            // execution of this task. Continue waiting for the remaining time.
            int64_t wait_ms = FindNextFrame(clock_->TimeInMilliseconds());
            return TimeDelta::Millis(wait_ms);
          }
          frame_handler = std::move(frame_handler_);
          CancelCallback();
        }
        // Deliver frame, if any. Otherwise signal timeout.
        frame_handler(std::move(frame));
        return TimeDelta::Zero();  // Ignored.
      },
      TaskQueueBase::DelayPrecision::kHigh);
}

int64_t FrameBuffer::FindNextFrame(int64_t now_ms) {
  int64_t wait_ms = latest_return_time_ms_ - now_ms;
  frames_to_decode_.clear();

  // `last_continuous_frame_` may be empty below, but nullopt is smaller
  // than everything else and loop will immediately terminate as expected.
  for (auto frame_it = frames_.begin();
       frame_it != frames_.end() && frame_it->first <= last_continuous_frame_;
       ++frame_it) {
    if (!frame_it->second.continuous ||
        frame_it->second.num_missing_decodable > 0) {
      continue;
    }

    EncodedFrame* frame = frame_it->second.frame.get();

    if (keyframe_required_ && !frame->is_keyframe())
      continue;

    auto last_decoded_frame_timestamp =
        decoded_frames_history_.GetLastDecodedFrameTimestamp();

    // TODO(https://bugs.webrtc.org/9974): consider removing this check
    // as it may make a stream undecodable after a very long delay between
    // frames.
    if (last_decoded_frame_timestamp &&
        AheadOf(*last_decoded_frame_timestamp, frame->Timestamp())) {
      continue;
    }

    // Gather all remaining frames for the same superframe.
    std::vector<FrameMap::iterator> current_superframe;
    current_superframe.push_back(frame_it);
    bool last_layer_completed = frame_it->second.frame->is_last_spatial_layer;
    FrameMap::iterator next_frame_it = frame_it;
    while (!last_layer_completed) {
      ++next_frame_it;

      if (next_frame_it == frames_.end() || !next_frame_it->second.frame) {
        break;
      }

      if (next_frame_it->second.frame->Timestamp() != frame->Timestamp() ||
          !next_frame_it->second.continuous) {
        break;
      }

      if (next_frame_it->second.num_missing_decodable > 0) {
        bool has_inter_layer_dependency = false;
        for (size_t i = 0; i < EncodedFrame::kMaxFrameReferences &&
                           i < next_frame_it->second.frame->num_references;
             ++i) {
          if (next_frame_it->second.frame->references[i] >= frame_it->first) {
            has_inter_layer_dependency = true;
            break;
          }
        }

        // If the frame has an undecoded dependency that is not within the same
        // temporal unit then this frame is not yet ready to be decoded. If it
        // is within the same temporal unit then the not yet decoded dependency
        // is just a lower spatial frame, which is ok.
        if (!has_inter_layer_dependency ||
            next_frame_it->second.num_missing_decodable > 1) {
          break;
        }
      }

      current_superframe.push_back(next_frame_it);
      last_layer_completed = next_frame_it->second.frame->is_last_spatial_layer;
    }
    // Check if the current superframe is complete.
    // TODO(bugs.webrtc.org/10064): consider returning all available to
    // decode frames even if the superframe is not complete yet.
    if (!last_layer_completed) {
      continue;
    }

    frames_to_decode_ = std::move(current_superframe);

    if (frame->RenderTime() == -1) {
      frame->SetRenderTime(timing_->RenderTimeMs(frame->Timestamp(), now_ms));
    }
    bool too_many_frames_queued =
        frames_.size() > zero_playout_delay_max_decode_queue_size_ ? true
                                                                   : false;
    wait_ms = timing_->MaxWaitingTime(frame->RenderTime(), now_ms,
                                      too_many_frames_queued);

    // This will cause the frame buffer to prefer high framerate rather
    // than high resolution in the case of the decoder not decoding fast
    // enough and the stream has multiple spatial and temporal layers.
    // For multiple temporal layers it may cause non-base layer frames to be
    // skipped if they are late.
    if (wait_ms < -kMaxAllowedFrameDelayMs)
      continue;

    break;
  }
  wait_ms = std::min<int64_t>(wait_ms, latest_return_time_ms_ - now_ms);
  wait_ms = std::max<int64_t>(wait_ms, 0);
  return wait_ms;
}

std::unique_ptr<EncodedFrame> FrameBuffer::GetNextFrame() {
  RTC_DCHECK_RUN_ON(&callback_checker_);
  int64_t now_ms = clock_->TimeInMilliseconds();
  // TODO(ilnik): remove `frames_out` use frames_to_decode_ directly.
  std::vector<std::unique_ptr<EncodedFrame>> frames_out;

  RTC_DCHECK(!frames_to_decode_.empty());
  bool superframe_delayed_by_retransmission = false;
  size_t superframe_size = 0;
  const EncodedFrame& first_frame = *frames_to_decode_[0]->second.frame;
  int64_t render_time_ms = first_frame.RenderTime();
  int64_t receive_time_ms = first_frame.ReceivedTime();
  // Gracefully handle bad RTP timestamps and render time issues.
  if (FrameHasBadRenderTiming(first_frame.RenderTimeMs(), now_ms,
                              timing_->TargetVideoDelay())) {
    jitter_estimator_.Reset();
    timing_->Reset();
    render_time_ms = timing_->RenderTimeMs(first_frame.Timestamp(), now_ms);
  }

  for (FrameMap::iterator& frame_it : frames_to_decode_) {
    RTC_DCHECK(frame_it != frames_.end());
    std::unique_ptr<EncodedFrame> frame = std::move(frame_it->second.frame);

    frame->SetRenderTime(render_time_ms);

    superframe_delayed_by_retransmission |= frame->delayed_by_retransmission();
    receive_time_ms = std::max(receive_time_ms, frame->ReceivedTime());
    superframe_size += frame->size();

    PropagateDecodability(frame_it->second);
    decoded_frames_history_.InsertDecoded(frame_it->first, frame->Timestamp());

    // Remove decoded frame and all undecoded frames before it.
    if (stats_callback_) {
      unsigned int dropped_frames =
          std::count_if(frames_.begin(), frame_it,
                        [](const std::pair<const int64_t, FrameInfo>& frame) {
                          return frame.second.frame != nullptr;
                        });
      if (dropped_frames > 0) {
        stats_callback_->OnDroppedFrames(dropped_frames);
      }
    }

    frames_.erase(frames_.begin(), ++frame_it);

    frames_out.emplace_back(std::move(frame));
  }

  if (!superframe_delayed_by_retransmission) {
    int64_t frame_delay;

    if (inter_frame_delay_.CalculateDelay(first_frame.Timestamp(), &frame_delay,
                                          receive_time_ms)) {
      jitter_estimator_.UpdateEstimate(frame_delay, superframe_size);
    }

    float rtt_mult = protection_mode_ == kProtectionNackFEC ? 0.0 : 1.0;
    absl::optional<float> rtt_mult_add_cap_ms = absl::nullopt;
    if (rtt_mult_settings_.has_value()) {
      rtt_mult = rtt_mult_settings_->rtt_mult_setting;
      rtt_mult_add_cap_ms = rtt_mult_settings_->rtt_mult_add_cap_ms;
    }
    timing_->SetJitterDelay(
        jitter_estimator_.GetJitterEstimate(rtt_mult, rtt_mult_add_cap_ms));
    timing_->UpdateCurrentDelay(render_time_ms, now_ms);
  } else {
    if (RttMultExperiment::RttMultEnabled())
      jitter_estimator_.FrameNacked();
  }

  UpdateJitterDelay();
  UpdateTimingFrameInfo();

  if (frames_out.size() == 1) {
    return std::move(frames_out[0]);
  } else {
    return CombineAndDeleteFrames(std::move(frames_out));
  }
}

void FrameBuffer::SetProtectionMode(VCMVideoProtection mode) {
  TRACE_EVENT0("webrtc", "FrameBuffer::SetProtectionMode");
  MutexLock lock(&mutex_);
  protection_mode_ = mode;
}

void FrameBuffer::Stop() {
  TRACE_EVENT0("webrtc", "FrameBuffer::Stop");
  MutexLock lock(&mutex_);
  if (stopped_)
    return;
  stopped_ = true;

  CancelCallback();
}

void FrameBuffer::Clear() {
  MutexLock lock(&mutex_);
  ClearFramesAndHistory();
}

int FrameBuffer::Size() {
  MutexLock lock(&mutex_);
  return frames_.size();
}

void FrameBuffer::UpdateRtt(int64_t rtt_ms) {
  MutexLock lock(&mutex_);
  jitter_estimator_.UpdateRtt(rtt_ms);
}

bool FrameBuffer::ValidReferences(const EncodedFrame& frame) const {
  for (size_t i = 0; i < frame.num_references; ++i) {
    if (frame.references[i] >= frame.Id())
      return false;

    for (size_t j = i + 1; j < frame.num_references; ++j) {
      if (frame.references[i] == frame.references[j])
        return false;
    }
  }

  return true;
}

void FrameBuffer::CancelCallback() {
  // Called from the callback queue or from within Stop().
  frame_handler_ = {};
  callback_task_.Stop();
  callback_queue_ = nullptr;
  callback_checker_.Detach();
}

int64_t FrameBuffer::InsertFrame(std::unique_ptr<EncodedFrame> frame) {
  TRACE_EVENT0("webrtc", "FrameBuffer::InsertFrame");
  RTC_DCHECK(frame);

  MutexLock lock(&mutex_);

  int64_t last_continuous_frame_id = last_continuous_frame_.value_or(-1);

  if (!ValidReferences(*frame)) {
    RTC_LOG(LS_WARNING) << "Frame " << frame->Id()
                        << " has invalid frame references, dropping frame.";
    return last_continuous_frame_id;
  }

  if (frames_.size() >= kMaxFramesBuffered) {
    if (frame->is_keyframe()) {
      RTC_LOG(LS_WARNING) << "Inserting keyframe " << frame->Id()
                          << " but buffer is full, clearing"
                             " buffer and inserting the frame.";
      ClearFramesAndHistory();
    } else {
      RTC_LOG(LS_WARNING) << "Frame " << frame->Id()
                          << " could not be inserted due to the frame "
                             "buffer being full, dropping frame.";
      return last_continuous_frame_id;
    }
  }

  auto last_decoded_frame = decoded_frames_history_.GetLastDecodedFrameId();
  auto last_decoded_frame_timestamp =
      decoded_frames_history_.GetLastDecodedFrameTimestamp();
  if (last_decoded_frame && frame->Id() <= *last_decoded_frame) {
    if (AheadOf(frame->Timestamp(), *last_decoded_frame_timestamp) &&
        frame->is_keyframe()) {
      // If this frame has a newer timestamp but an earlier frame id then we
      // assume there has been a jump in the frame id due to some encoder
      // reconfiguration or some other reason. Even though this is not according
      // to spec we can still continue to decode from this frame if it is a
      // keyframe.
      RTC_LOG(LS_WARNING)
          << "A jump in frame id was detected, clearing buffer.";
      ClearFramesAndHistory();
      last_continuous_frame_id = -1;
    } else {
      RTC_LOG(LS_WARNING) << "Frame " << frame->Id() << " inserted after frame "
                          << *last_decoded_frame
                          << " was handed off for decoding, dropping frame.";
      return last_continuous_frame_id;
    }
  }

  // Test if inserting this frame would cause the order of the frames to become
  // ambiguous (covering more than half the interval of 2^16). This can happen
  // when the frame id make large jumps mid stream.
  if (!frames_.empty() && frame->Id() < frames_.begin()->first &&
      frames_.rbegin()->first < frame->Id()) {
    RTC_LOG(LS_WARNING) << "A jump in frame id was detected, clearing buffer.";
    ClearFramesAndHistory();
    last_continuous_frame_id = -1;
  }

  auto info = frames_.emplace(frame->Id(), FrameInfo()).first;

  if (info->second.frame) {
    return last_continuous_frame_id;
  }

  if (!UpdateFrameInfoWithIncomingFrame(*frame, info))
    return last_continuous_frame_id;

  if (!frame->delayed_by_retransmission())
    timing_->IncomingTimestamp(frame->Timestamp(), frame->ReceivedTime());

  // It can happen that a frame will be reported as fully received even if a
  // lower spatial layer frame is missing.
  if (stats_callback_ && frame->is_last_spatial_layer) {
    stats_callback_->OnCompleteFrame(frame->is_keyframe(), frame->size(),
                                     frame->contentType());
  }

  info->second.frame = std::move(frame);

  if (info->second.num_missing_continuous == 0) {
    info->second.continuous = true;
    PropagateContinuity(info);
    last_continuous_frame_id = *last_continuous_frame_;

    // Since we now have new continuous frames there might be a better frame
    // to return from NextFrame.
    if (callback_queue_) {
      callback_queue_->PostTask([this] {
        MutexLock lock(&mutex_);
        if (!callback_task_.Running())
          return;
        RTC_CHECK(frame_handler_);
        callback_task_.Stop();
        StartWaitForNextFrameOnQueue();
      });
    }
  }

  return last_continuous_frame_id;
}

void FrameBuffer::PropagateContinuity(FrameMap::iterator start) {
  TRACE_EVENT0("webrtc", "FrameBuffer::PropagateContinuity");
  RTC_DCHECK(start->second.continuous);

  std::queue<FrameMap::iterator> continuous_frames;
  continuous_frames.push(start);

  // A simple BFS to traverse continuous frames.
  while (!continuous_frames.empty()) {
    auto frame = continuous_frames.front();
    continuous_frames.pop();

    if (!last_continuous_frame_ || *last_continuous_frame_ < frame->first) {
      last_continuous_frame_ = frame->first;
    }

    // Loop through all dependent frames, and if that frame no longer has
    // any unfulfilled dependencies then that frame is continuous as well.
    for (size_t d = 0; d < frame->second.dependent_frames.size(); ++d) {
      auto frame_ref = frames_.find(frame->second.dependent_frames[d]);
      RTC_DCHECK(frame_ref != frames_.end());

      // TODO(philipel): Look into why we've seen this happen.
      if (frame_ref != frames_.end()) {
        --frame_ref->second.num_missing_continuous;
        if (frame_ref->second.num_missing_continuous == 0) {
          frame_ref->second.continuous = true;
          continuous_frames.push(frame_ref);
        }
      }
    }
  }
}

void FrameBuffer::PropagateDecodability(const FrameInfo& info) {
  TRACE_EVENT0("webrtc", "FrameBuffer::PropagateDecodability");
  for (size_t d = 0; d < info.dependent_frames.size(); ++d) {
    auto ref_info = frames_.find(info.dependent_frames[d]);
    RTC_DCHECK(ref_info != frames_.end());
    // TODO(philipel): Look into why we've seen this happen.
    if (ref_info != frames_.end()) {
      RTC_DCHECK_GT(ref_info->second.num_missing_decodable, 0U);
      --ref_info->second.num_missing_decodable;
    }
  }
}

bool FrameBuffer::UpdateFrameInfoWithIncomingFrame(const EncodedFrame& frame,
                                                   FrameMap::iterator info) {
  TRACE_EVENT0("webrtc", "FrameBuffer::UpdateFrameInfoWithIncomingFrame");
  auto last_decoded_frame = decoded_frames_history_.GetLastDecodedFrameId();
  RTC_DCHECK(!last_decoded_frame || *last_decoded_frame < info->first);

  // In this function we determine how many missing dependencies this `frame`
  // has to become continuous/decodable. If a frame that this `frame` depend
  // on has already been decoded then we can ignore that dependency since it has
  // already been fulfilled.
  //
  // For all other frames we will register a backwards reference to this `frame`
  // so that `num_missing_continuous` and `num_missing_decodable` can be
  // decremented as frames become continuous/are decoded.
  struct Dependency {
    int64_t frame_id;
    bool continuous;
  };
  std::vector<Dependency> not_yet_fulfilled_dependencies;

  // Find all dependencies that have not yet been fulfilled.
  for (size_t i = 0; i < frame.num_references; ++i) {
    // Does `frame` depend on a frame earlier than the last decoded one?
    if (last_decoded_frame && frame.references[i] <= *last_decoded_frame) {
      // Was that frame decoded? If not, this `frame` will never become
      // decodable.
      if (!decoded_frames_history_.WasDecoded(frame.references[i])) {
        int64_t now_ms = clock_->TimeInMilliseconds();
        if (last_log_non_decoded_ms_ + kLogNonDecodedIntervalMs < now_ms) {
          RTC_LOG(LS_WARNING)
              << "Frame " << frame.Id()
              << " depends on a non-decoded frame more previous than the last "
                 "decoded frame, dropping frame.";
          last_log_non_decoded_ms_ = now_ms;
        }
        return false;
      }
    } else {
      auto ref_info = frames_.find(frame.references[i]);
      bool ref_continuous =
          ref_info != frames_.end() && ref_info->second.continuous;
      not_yet_fulfilled_dependencies.push_back(
          {frame.references[i], ref_continuous});
    }
  }

  info->second.num_missing_continuous = not_yet_fulfilled_dependencies.size();
  info->second.num_missing_decodable = not_yet_fulfilled_dependencies.size();

  for (const Dependency& dep : not_yet_fulfilled_dependencies) {
    if (dep.continuous)
      --info->second.num_missing_continuous;

    frames_[dep.frame_id].dependent_frames.push_back(frame.Id());
  }

  return true;
}

void FrameBuffer::UpdateJitterDelay() {
  TRACE_EVENT0("webrtc", "FrameBuffer::UpdateJitterDelay");
  if (!stats_callback_)
    return;

  int max_decode_ms;
  int current_delay_ms;
  int target_delay_ms;
  int jitter_buffer_ms;
  int min_playout_delay_ms;
  int render_delay_ms;
  if (timing_->GetTimings(&max_decode_ms, &current_delay_ms, &target_delay_ms,
                          &jitter_buffer_ms, &min_playout_delay_ms,
                          &render_delay_ms)) {
    stats_callback_->OnFrameBufferTimingsUpdated(
        max_decode_ms, current_delay_ms, target_delay_ms, jitter_buffer_ms,
        min_playout_delay_ms, render_delay_ms);
  }
}

void FrameBuffer::UpdateTimingFrameInfo() {
  TRACE_EVENT0("webrtc", "FrameBuffer::UpdateTimingFrameInfo");
  absl::optional<TimingFrameInfo> info = timing_->GetTimingFrameInfo();
  if (info && stats_callback_)
    stats_callback_->OnTimingFrameInfoUpdated(*info);
}

void FrameBuffer::ClearFramesAndHistory() {
  TRACE_EVENT0("webrtc", "FrameBuffer::ClearFramesAndHistory");
  if (stats_callback_) {
    unsigned int dropped_frames =
        std::count_if(frames_.begin(), frames_.end(),
                      [](const std::pair<const int64_t, FrameInfo>& frame) {
                        return frame.second.frame != nullptr;
                      });
    if (dropped_frames > 0) {
      stats_callback_->OnDroppedFrames(dropped_frames);
    }
  }
  frames_.clear();
  last_continuous_frame_.reset();
  frames_to_decode_.clear();
  decoded_frames_history_.Clear();
}

// TODO(philipel): Avoid the concatenation of frames here, by replacing
// NextFrame and GetNextFrame with methods returning multiple frames.
std::unique_ptr<EncodedFrame> FrameBuffer::CombineAndDeleteFrames(
    std::vector<std::unique_ptr<EncodedFrame>> frames) const {
  RTC_DCHECK(!frames.empty());
  absl::InlinedVector<std::unique_ptr<EncodedFrame>, 4> inlined;
  for (auto& frame : frames) {
    inlined.push_back(std::move(frame));
  }
  return webrtc::CombineAndDeleteFrames(std::move(inlined));
}

FrameBuffer::FrameInfo::FrameInfo() = default;
FrameBuffer::FrameInfo::FrameInfo(FrameInfo&&) = default;
FrameBuffer::FrameInfo::~FrameInfo() = default;

}  // namespace video_coding
}  // namespace webrtc