webrtc/sdk/objc/Framework/Classes/avfoundationvideocapturer.mm

/*
 *  Copyright 2015 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 "avfoundationvideocapturer.h"

#import <AVFoundation/AVFoundation.h>
#import <Foundation/Foundation.h>
#if TARGET_OS_IPHONE
#import <UIKit/UIKit.h>
#endif

#import "RTCDispatcher+Private.h"
#import "WebRTC/RTCLogging.h"

#include "webrtc/base/bind.h"
#include "webrtc/base/checks.h"
#include "webrtc/base/thread.h"
#include "webrtc/common_video/include/corevideo_frame_buffer.h"

// TODO(tkchin): support other formats.
static NSString *const kDefaultPreset = AVCaptureSessionPreset640x480;
static cricket::VideoFormat const kDefaultFormat =
    cricket::VideoFormat(640,
                         480,
                         cricket::VideoFormat::FpsToInterval(30),
                         cricket::FOURCC_NV12);

// This class used to capture frames using AVFoundation APIs on iOS. It is meant
// to be owned by an instance of AVFoundationVideoCapturer. The reason for this
// because other webrtc objects own cricket::VideoCapturer, which is not
// ref counted. To prevent bad behavior we do not expose this class directly.
@interface RTCAVFoundationVideoCapturerInternal : NSObject
    <AVCaptureVideoDataOutputSampleBufferDelegate>

@property(nonatomic, readonly) AVCaptureSession *captureSession;
@property(nonatomic, readonly) dispatch_queue_t frameQueue;
@property(nonatomic, readonly) BOOL canUseBackCamera;
@property(nonatomic, assign) BOOL useBackCamera;  // Defaults to NO.
@property(nonatomic, assign) BOOL isRunning;  // Whether the capture session is running.
@property(atomic, assign) BOOL hasStarted;  // Whether we have an unmatched start.

// We keep a pointer back to AVFoundationVideoCapturer to make callbacks on it
// when we receive frames. This is safe because this object should be owned by
// it.
- (instancetype)initWithCapturer:(webrtc::AVFoundationVideoCapturer *)capturer;

// Starts and stops the capture session asynchronously. We cannot do this
// synchronously without blocking a WebRTC thread.
- (void)start;
- (void)stop;

@end

@implementation RTCAVFoundationVideoCapturerInternal {
  // Keep pointers to inputs for convenience.
  AVCaptureDeviceInput *_frontCameraInput;
  AVCaptureDeviceInput *_backCameraInput;
  AVCaptureVideoDataOutput *_videoDataOutput;
  // The cricket::VideoCapturer that owns this class. Should never be NULL.
  webrtc::AVFoundationVideoCapturer *_capturer;
  BOOL _orientationHasChanged;
  BOOL _hasRetriedOnFatalError;
  BOOL _isRunning;
  BOOL _hasStarted;
  rtc::CriticalSection _crit;
}

@synthesize captureSession = _captureSession;
@synthesize frameQueue = _frameQueue;
@synthesize useBackCamera = _useBackCamera;
@synthesize hasStarted = _hasStarted;

// This is called from the thread that creates the video source, which is likely
// the main thread.
- (instancetype)initWithCapturer:(webrtc::AVFoundationVideoCapturer *)capturer {
  RTC_DCHECK(capturer);
  if (self = [super init]) {
    _capturer = capturer;
    // Create the capture session and all relevant inputs and outputs. We need
    // to do this in init because the application may want the capture session
    // before we start the capturer for e.g. AVCapturePreviewLayer. All objects
    // created here are retained until dealloc and never recreated.
    if (![self setupCaptureSession]) {
      return nil;
    }
    NSNotificationCenter *center = [NSNotificationCenter defaultCenter];
#if TARGET_OS_IPHONE
    [center addObserver:self
               selector:@selector(deviceOrientationDidChange:)
                   name:UIDeviceOrientationDidChangeNotification
                 object:nil];
    [center addObserver:self
               selector:@selector(handleCaptureSessionInterruption:)
                   name:AVCaptureSessionWasInterruptedNotification
                 object:_captureSession];
    [center addObserver:self
               selector:@selector(handleCaptureSessionInterruptionEnded:)
                   name:AVCaptureSessionInterruptionEndedNotification
                 object:_captureSession];
#endif
    [center addObserver:self
               selector:@selector(handleCaptureSessionRuntimeError:)
                   name:AVCaptureSessionRuntimeErrorNotification
                 object:_captureSession];
    [center addObserver:self
               selector:@selector(handleCaptureSessionDidStartRunning:)
                   name:AVCaptureSessionDidStartRunningNotification
                 object:_captureSession];
    [center addObserver:self
               selector:@selector(handleCaptureSessionDidStopRunning:)
                   name:AVCaptureSessionDidStopRunningNotification
                 object:_captureSession];
  }
  return self;
}

- (void)dealloc {
  RTC_DCHECK(!self.hasStarted);
  [[NSNotificationCenter defaultCenter] removeObserver:self];
  _capturer = nullptr;
}

- (AVCaptureSession *)captureSession {
  return _captureSession;
}

- (dispatch_queue_t)frameQueue {
  if (!_frameQueue) {
    _frameQueue =
        dispatch_queue_create("org.webrtc.avfoundationvideocapturer.video",
                              DISPATCH_QUEUE_SERIAL);
    dispatch_set_target_queue(
        _frameQueue,
        dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0));
  }
  return _frameQueue;
}

// Called from any thread (likely main thread).
- (BOOL)canUseBackCamera {
  return _backCameraInput != nil;
}

// Called from any thread (likely main thread).
- (BOOL)useBackCamera {
  @synchronized(self) {
    return _useBackCamera;
  }
}

// Called from any thread (likely main thread).
- (void)setUseBackCamera:(BOOL)useBackCamera {
  if (!self.canUseBackCamera) {
    if (useBackCamera) {
      RTCLogWarning(@"No rear-facing camera exists or it cannot be used;"
                    "not switching.");
    }
    return;
  }
  @synchronized(self) {
    if (_useBackCamera == useBackCamera) {
      return;
    }
    _useBackCamera = useBackCamera;
    [self updateSessionInputForUseBackCamera:useBackCamera];
  }
}

- (BOOL)isRunning {
  rtc::CritScope cs(&_crit);
  return _isRunning;
}

- (void)setIsRunning:(BOOL)isRunning {
  rtc::CritScope cs(&_crit);
  _isRunning = isRunning;
}

// Called from WebRTC thread.
- (void)start {
  if (self.hasStarted) {
    return;
  }
  self.hasStarted = YES;
  [RTCDispatcher dispatchAsyncOnType:RTCDispatcherTypeCaptureSession
                               block:^{
    _orientationHasChanged = NO;
    [self updateOrientation];
#if TARGET_OS_IPHONE
    [[UIDevice currentDevice] beginGeneratingDeviceOrientationNotifications];
#endif
    AVCaptureSession *captureSession = self.captureSession;
    [captureSession startRunning];
  }];
}

// Called from same thread as start.
- (void)stop {
  if (!self.hasStarted) {
    return;
  }
  self.hasStarted = NO;
  // Due to this async block, it's possible that the ObjC object outlives the
  // C++ one. In order to not invoke functions on the C++ object, we set
  // hasStarted immediately instead of dispatching it async.
  [RTCDispatcher dispatchAsyncOnType:RTCDispatcherTypeCaptureSession
                               block:^{
    [_videoDataOutput setSampleBufferDelegate:nil queue:nullptr];
    [_captureSession stopRunning];
#if TARGET_OS_IPHONE
    [[UIDevice currentDevice] endGeneratingDeviceOrientationNotifications];
#endif
  }];
}

#pragma mark iOS notifications

#if TARGET_OS_IPHONE
- (void)deviceOrientationDidChange:(NSNotification *)notification {
  [RTCDispatcher dispatchAsyncOnType:RTCDispatcherTypeCaptureSession
                               block:^{
    _orientationHasChanged = YES;
    [self updateOrientation];
  }];
}
#endif

#pragma mark AVCaptureVideoDataOutputSampleBufferDelegate

- (void)captureOutput:(AVCaptureOutput *)captureOutput
    didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer
           fromConnection:(AVCaptureConnection *)connection {
  NSParameterAssert(captureOutput == _videoDataOutput);
  if (!self.hasStarted) {
    return;
  }
  _capturer->CaptureSampleBuffer(sampleBuffer);
}

- (void)captureOutput:(AVCaptureOutput *)captureOutput
    didDropSampleBuffer:(CMSampleBufferRef)sampleBuffer
         fromConnection:(AVCaptureConnection *)connection {
  RTCLogError(@"Dropped sample buffer.");
}

#pragma mark - AVCaptureSession notifications

- (void)handleCaptureSessionInterruption:(NSNotification *)notification {
  NSString *reasonString = nil;
#if defined(__IPHONE_9_0) && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_9_0
  NSNumber *reason =
      notification.userInfo[AVCaptureSessionInterruptionReasonKey];
  if (reason) {
    switch (reason.intValue) {
      case AVCaptureSessionInterruptionReasonVideoDeviceNotAvailableInBackground:
        reasonString = @"VideoDeviceNotAvailableInBackground";
        break;
      case AVCaptureSessionInterruptionReasonAudioDeviceInUseByAnotherClient:
        reasonString = @"AudioDeviceInUseByAnotherClient";
        break;
      case AVCaptureSessionInterruptionReasonVideoDeviceInUseByAnotherClient:
        reasonString = @"VideoDeviceInUseByAnotherClient";
        break;
      case AVCaptureSessionInterruptionReasonVideoDeviceNotAvailableWithMultipleForegroundApps:
        reasonString = @"VideoDeviceNotAvailableWithMultipleForegroundApps";
        break;
    }
  }
#endif
  RTCLog(@"Capture session interrupted: %@", reasonString);
  // TODO(tkchin): Handle this case.
}

- (void)handleCaptureSessionInterruptionEnded:(NSNotification *)notification {
  RTCLog(@"Capture session interruption ended.");
  // TODO(tkchin): Handle this case.
}

- (void)handleCaptureSessionRuntimeError:(NSNotification *)notification {
  NSError *error =
      [notification.userInfo objectForKey:AVCaptureSessionErrorKey];
  RTCLogError(@"Capture session runtime error: %@", error.localizedDescription);

  [RTCDispatcher dispatchAsyncOnType:RTCDispatcherTypeCaptureSession
                               block:^{
#if TARGET_OS_IPHONE
    if (error.code == AVErrorMediaServicesWereReset) {
      [self handleNonFatalError];
    } else {
      [self handleFatalError];
    }
#else
    [self handleFatalError];
#endif
  }];
}

- (void)handleCaptureSessionDidStartRunning:(NSNotification *)notification {
  RTCLog(@"Capture session started.");
  self.isRunning = YES;
  [RTCDispatcher dispatchAsyncOnType:RTCDispatcherTypeCaptureSession
                               block:^{
    // If we successfully restarted after an unknown error, allow future
    // retries on fatal errors.
    _hasRetriedOnFatalError = NO;
  }];
}

- (void)handleCaptureSessionDidStopRunning:(NSNotification *)notification {
  RTCLog(@"Capture session stopped.");
  self.isRunning = NO;
}

- (void)handleFatalError {
  [RTCDispatcher dispatchAsyncOnType:RTCDispatcherTypeCaptureSession
                               block:^{
    if (!_hasRetriedOnFatalError) {
      RTCLogWarning(@"Attempting to recover from fatal capture error.");
      [self handleNonFatalError];
      _hasRetriedOnFatalError = YES;
    } else {
      RTCLogError(@"Previous fatal error recovery failed.");
    }
  }];
}

- (void)handleNonFatalError {
  [RTCDispatcher dispatchAsyncOnType:RTCDispatcherTypeCaptureSession
                               block:^{
    if (self.hasStarted) {
      RTCLog(@"Restarting capture session after error.");
      [self.captureSession startRunning];
    }
  }];
}

#pragma mark - Private

- (BOOL)setupCaptureSession {
  AVCaptureSession *captureSession = [[AVCaptureSession alloc] init];
#if defined(__IPHONE_7_0) && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_7_0
  NSString *version = [[UIDevice currentDevice] systemVersion];
  if ([version integerValue] >= 7) {
    captureSession.usesApplicationAudioSession = NO;
  }
#endif
  if (![captureSession canSetSessionPreset:kDefaultPreset]) {
    RTCLogError(@"Session preset unsupported.");
    return NO;
  }
  captureSession.sessionPreset = kDefaultPreset;

  // Add the output.
  AVCaptureVideoDataOutput *videoDataOutput = [self videoDataOutput];
  if (![captureSession canAddOutput:videoDataOutput]) {
    RTCLogError(@"Video data output unsupported.");
    return NO;
  }
  [captureSession addOutput:videoDataOutput];

  // Get the front and back cameras. If there isn't a front camera
  // give up.
  AVCaptureDeviceInput *frontCameraInput = [self frontCameraInput];
  AVCaptureDeviceInput *backCameraInput = [self backCameraInput];
  if (!frontCameraInput) {
    RTCLogError(@"No front camera for capture session.");
    return NO;
  }

  // Add the inputs.
  if (![captureSession canAddInput:frontCameraInput] ||
      (backCameraInput && ![captureSession canAddInput:backCameraInput])) {
    RTCLogError(@"Session does not support capture inputs.");
    return NO;
  }
  AVCaptureDeviceInput *input = self.useBackCamera ?
      backCameraInput : frontCameraInput;
  [captureSession addInput:input];
  _captureSession = captureSession;
  return YES;
}

- (AVCaptureVideoDataOutput *)videoDataOutput {
  if (!_videoDataOutput) {
    // Make the capturer output NV12. Ideally we want I420 but that's not
    // currently supported on iPhone / iPad.
    AVCaptureVideoDataOutput *videoDataOutput =
        [[AVCaptureVideoDataOutput alloc] init];
    videoDataOutput = [[AVCaptureVideoDataOutput alloc] init];
    videoDataOutput.videoSettings = @{
      (NSString *)kCVPixelBufferPixelFormatTypeKey :
        @(kCVPixelFormatType_420YpCbCr8BiPlanarFullRange)
    };
    videoDataOutput.alwaysDiscardsLateVideoFrames = NO;
    [videoDataOutput setSampleBufferDelegate:self queue:self.frameQueue];
    _videoDataOutput = videoDataOutput;
  }
  return _videoDataOutput;
}

- (AVCaptureDevice *)videoCaptureDeviceForPosition:
    (AVCaptureDevicePosition)position {
  for (AVCaptureDevice *captureDevice in
       [AVCaptureDevice devicesWithMediaType:AVMediaTypeVideo]) {
    if (captureDevice.position == position) {
      return captureDevice;
    }
  }
  return nil;
}

- (AVCaptureDeviceInput *)frontCameraInput {
  if (!_frontCameraInput) {
#if TARGET_OS_IPHONE
    AVCaptureDevice *frontCameraDevice =
        [self videoCaptureDeviceForPosition:AVCaptureDevicePositionFront];
#else
    AVCaptureDevice *frontCameraDevice =
        [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
#endif
    if (!frontCameraDevice) {
      RTCLogWarning(@"Failed to find front capture device.");
      return nil;
    }
    NSError *error = nil;
    AVCaptureDeviceInput *frontCameraInput =
        [AVCaptureDeviceInput deviceInputWithDevice:frontCameraDevice
                                              error:&error];
    if (!frontCameraInput) {
      RTCLogError(@"Failed to create front camera input: %@",
                  error.localizedDescription);
      return nil;
    }
    _frontCameraInput = frontCameraInput;
  }
  return _frontCameraInput;
}

- (AVCaptureDeviceInput *)backCameraInput {
  if (!_backCameraInput) {
    AVCaptureDevice *backCameraDevice =
        [self videoCaptureDeviceForPosition:AVCaptureDevicePositionBack];
    if (!backCameraDevice) {
      RTCLogWarning(@"Failed to find front capture device.");
      return nil;
    }
    NSError *error = nil;
    AVCaptureDeviceInput *backCameraInput =
        [AVCaptureDeviceInput deviceInputWithDevice:backCameraDevice
                                              error:&error];
    if (!backCameraInput) {
      RTCLogError(@"Failed to create front camera input: %@",
                  error.localizedDescription);
      return nil;
    }
    _backCameraInput = backCameraInput;
  }
  return _backCameraInput;
}

// Called from capture session queue.
- (void)updateOrientation {
  AVCaptureConnection *connection =
      [_videoDataOutput connectionWithMediaType:AVMediaTypeVideo];
  if (!connection.supportsVideoOrientation) {
    // TODO(tkchin): set rotation bit on frames.
    return;
  }
#if TARGET_OS_IPHONE
  AVCaptureVideoOrientation orientation = AVCaptureVideoOrientationPortrait;
  switch ([UIDevice currentDevice].orientation) {
    case UIDeviceOrientationPortrait:
      orientation = AVCaptureVideoOrientationPortrait;
      break;
    case UIDeviceOrientationPortraitUpsideDown:
      orientation = AVCaptureVideoOrientationPortraitUpsideDown;
      break;
    case UIDeviceOrientationLandscapeLeft:
      orientation = AVCaptureVideoOrientationLandscapeRight;
      break;
    case UIDeviceOrientationLandscapeRight:
      orientation = AVCaptureVideoOrientationLandscapeLeft;
      break;
    case UIDeviceOrientationFaceUp:
    case UIDeviceOrientationFaceDown:
    case UIDeviceOrientationUnknown:
      if (!_orientationHasChanged) {
        connection.videoOrientation = orientation;
      }
      return;
  }
  connection.videoOrientation = orientation;
#endif
}

// Update the current session input to match what's stored in _useBackCamera.
- (void)updateSessionInputForUseBackCamera:(BOOL)useBackCamera {
  [RTCDispatcher dispatchAsyncOnType:RTCDispatcherTypeCaptureSession
                               block:^{
    [_captureSession beginConfiguration];
    AVCaptureDeviceInput *oldInput = _backCameraInput;
    AVCaptureDeviceInput *newInput = _frontCameraInput;
    if (useBackCamera) {
      oldInput = _frontCameraInput;
      newInput = _backCameraInput;
    }
    if (oldInput) {
      // Ok to remove this even if it's not attached. Will be no-op.
      [_captureSession removeInput:oldInput];
    }
    if (newInput) {
      [_captureSession addInput:newInput];
    }
    [self updateOrientation];
    [_captureSession commitConfiguration];
  }];
}

@end

namespace webrtc {

enum AVFoundationVideoCapturerMessageType : uint32_t {
  kMessageTypeFrame,
};

struct AVFoundationFrame {
  AVFoundationFrame(CVImageBufferRef buffer, int64_t time)
    : image_buffer(buffer), capture_time(time) {}
  CVImageBufferRef image_buffer;
  int64_t capture_time;
};

AVFoundationVideoCapturer::AVFoundationVideoCapturer()
    : _capturer(nil), _startThread(nullptr) {
  // Set our supported formats. This matches kDefaultPreset.
  std::vector<cricket::VideoFormat> supportedFormats;
  supportedFormats.push_back(cricket::VideoFormat(kDefaultFormat));
  SetSupportedFormats(supportedFormats);
  _capturer =
      [[RTCAVFoundationVideoCapturerInternal alloc] initWithCapturer:this];
}

AVFoundationVideoCapturer::~AVFoundationVideoCapturer() {
  _capturer = nil;
}

cricket::CaptureState AVFoundationVideoCapturer::Start(
    const cricket::VideoFormat& format) {
  if (!_capturer) {
    LOG(LS_ERROR) << "Failed to create AVFoundation capturer.";
    return cricket::CaptureState::CS_FAILED;
  }
  if (_capturer.isRunning) {
    LOG(LS_ERROR) << "The capturer is already running.";
    return cricket::CaptureState::CS_FAILED;
  }
  if (format != kDefaultFormat) {
    LOG(LS_ERROR) << "Unsupported format provided.";
    return cricket::CaptureState::CS_FAILED;
  }

  // Keep track of which thread capture started on. This is the thread that
  // frames need to be sent to.
  RTC_DCHECK(!_startThread);
  _startThread = rtc::Thread::Current();

  SetCaptureFormat(&format);
  // This isn't super accurate because it takes a while for the AVCaptureSession
  // to spin up, and this call returns async.
  // TODO(tkchin): make this better.
  [_capturer start];
  SetCaptureState(cricket::CaptureState::CS_RUNNING);

  return cricket::CaptureState::CS_STARTING;
}

void AVFoundationVideoCapturer::Stop() {
  [_capturer stop];
  SetCaptureFormat(NULL);
  _startThread = nullptr;
}

bool AVFoundationVideoCapturer::IsRunning() {
  return _capturer.isRunning;
}

AVCaptureSession* AVFoundationVideoCapturer::GetCaptureSession() {
  return _capturer.captureSession;
}

bool AVFoundationVideoCapturer::CanUseBackCamera() const {
  return _capturer.canUseBackCamera;
}

void AVFoundationVideoCapturer::SetUseBackCamera(bool useBackCamera) {
  _capturer.useBackCamera = useBackCamera;
}

bool AVFoundationVideoCapturer::GetUseBackCamera() const {
  return _capturer.useBackCamera;
}

void AVFoundationVideoCapturer::CaptureSampleBuffer(
    CMSampleBufferRef sampleBuffer) {
  if (CMSampleBufferGetNumSamples(sampleBuffer) != 1 ||
      !CMSampleBufferIsValid(sampleBuffer) ||
      !CMSampleBufferDataIsReady(sampleBuffer)) {
    return;
  }

  CVImageBufferRef image_buffer = CMSampleBufferGetImageBuffer(sampleBuffer);
  if (image_buffer == NULL) {
    return;
  }

  // Retain the buffer and post it to the webrtc thread. It will be released
  // after it has successfully been signaled.
  CVBufferRetain(image_buffer);
  AVFoundationFrame frame(image_buffer, rtc::TimeNanos());
  _startThread->Post(RTC_FROM_HERE, this, kMessageTypeFrame,
                     new rtc::TypedMessageData<AVFoundationFrame>(frame));
}

void AVFoundationVideoCapturer::OnMessage(rtc::Message *msg) {
  switch (msg->message_id) {
    case kMessageTypeFrame: {
      rtc::TypedMessageData<AVFoundationFrame>* data =
        static_cast<rtc::TypedMessageData<AVFoundationFrame>*>(msg->pdata);
      const AVFoundationFrame& frame = data->data();
      OnFrameMessage(frame.image_buffer, frame.capture_time);
      delete data;
      break;
    }
  }
}

void AVFoundationVideoCapturer::OnFrameMessage(CVImageBufferRef image_buffer,
                                               int64_t capture_time_ns) {
  RTC_DCHECK(_startThread->IsCurrent());

  rtc::scoped_refptr<webrtc::VideoFrameBuffer> buffer =
      new rtc::RefCountedObject<webrtc::CoreVideoFrameBuffer>(image_buffer);

  const int captured_width = buffer->width();
  const int captured_height = buffer->height();

  int adapted_width;
  int adapted_height;
  int crop_width;
  int crop_height;
  int crop_x;
  int crop_y;
  int64_t translated_camera_time_us;

  if (!AdaptFrame(captured_width, captured_height,
                  capture_time_ns / rtc::kNumNanosecsPerMicrosec,
                  rtc::TimeMicros(), &adapted_width, &adapted_height,
                  &crop_width, &crop_height, &crop_x, &crop_y,
                  &translated_camera_time_us)) {
    CVBufferRelease(image_buffer);
    return;
  }

  if (adapted_width != captured_width || crop_width != captured_width ||
      adapted_height != captured_height || crop_height != captured_height) {
    // TODO(magjed): Avoid converting to I420.
    rtc::scoped_refptr<webrtc::I420Buffer> scaled_buffer(
        _buffer_pool.CreateBuffer(adapted_width, adapted_height));
    scaled_buffer->CropAndScaleFrom(buffer->NativeToI420Buffer(), crop_x,
                                    crop_y, crop_width, crop_height);
    buffer = scaled_buffer;
  }

  OnFrame(cricket::WebRtcVideoFrame(buffer, webrtc::kVideoRotation_0,
                                    translated_camera_time_us),
          captured_width, captured_height);

  CVBufferRelease(image_buffer);
}

}  // namespace webrtc