51127979: Bitrates set by DataRateLimits are not respected

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Description

Summary:

When used in the form @[(targetBitrate / 8.f), @1], where targetBitrate is the desired average bitrate in bits per second, the kVTCompressionPropertyKey_DataRateLimits setting can produce anything from an average bitrate of 80% to 150% of the target, depending on content and target bitrate. According to these docs, an average is expected to be maintained: https://developer.apple.com/library/archive/qa/qa1958/_index.html

AVVideoAverageBitRateKey, in comparison, seems to do a much better job adhering, especially at higher bitrates.

Steps to Reproduce: See attached code.

Bitrates are derived by the following command: ffprobe -v error $FILE -select_streams v:0 -show_entries stream=bit_rate -of default=noprint_wrappers=1

Expected Results: Target bitrates are roughly respected

Actual Results: Target bitrates are often missed, in either direction.

Version/Build: iOS 12.3

Configuration: N/A

---

• (void)test { NSArray *const bitRates = @[ @(2e6), @(3e6), @(5e6), @(7e6), @(9e6), @(12e6), @(15e6) ];

  NSArray *const urls = @[ [[NSBundle bundleForClass:[self class]] URLForResource:@"file1" withExtension:@"mp4"], [[NSBundle bundleForClass:[self class]] URLForResource:@"file2" withExtension:@"mp4"], [[NSBundle bundleForClass:[self class]] URLForResource:@"file3" withExtension:@"mp4"], ];

  for (NSURL url in urls) { for (NSNumber bitRate in bitRates) { [self testUrl:url bitRate:[bitRate floatValue]]; } } }

• (void)testUrl:(NSURL )url bitRate:(CGFloat)bitRate { NSString filename = [NSString stringWithFormat:@"test-%@-%ld-bps-drl", [[url URLByDeletingPathExtension] lastPathComponent], (long)bitRate];

  NSURL *const outputUrl = [[[NSURL fileURLWithPath:NSTemporaryDirectory()] URLByAppendingPathComponent:filename] URLByAppendingPathExtension:@"mp4"];

  if ([[NSFileManager defaultManager]fileExistsAtPath:outputUrl.path]) { [[NSFileManager defaultManager] removeItemAtURL:outputUrl error:nil]; }

  AVAsset const asset = [AVAsset assetWithURL:url]; AVAssetTrack const videoTrack = [[asset tracksWithMediaType:AVMediaTypeVideo] firstObject]; NSError error = nil; AVAssetReader const assetReader = [AVAssetReader assetReaderWithAsset:asset error:&error]; XCTAssertNil(error); AVAssetWriter *const assetWriter = [AVAssetWriter assetWriterWithURL:outputUrl fileType:AVFileTypeMPEG4 error:&error]; XCTAssertNil(error);

  AVAssetWriterInput const input = [[AVAssetWriterInput alloc] initWithMediaType:AVMediaTypeVideo outputSettings:@{ AVVideoCodecKey: AVVideoCodecTypeH264, AVVideoWidthKey: @(videoTrack.naturalSize.width), AVVideoHeightKey: @(videoTrack.naturalSize.height), AVVideoCompressionPropertiesKey: @{ // AVVideoAverageBitRateKey: @(bitRate), (NSString )kVTCompressionPropertyKey_DataRateLimits: @[ @((bitRate) / 8.f), @1, ], AVVideoProfileLevelKey: AVVideoProfileLevelH264High41, } }];

  AVAssetReaderOutput *const output = [[AVAssetReaderTrackOutput alloc] initWithTrack:videoTrack outputSettings:@{ (id)kCVPixelBufferPixelFormatTypeKey: @(kCVPixelFormatType_32BGRA), }];

  [assetReader addOutput:output]; [assetWriter addInput:input];

  XCTAssertNil(assetReader.error); XCTAssertNil(assetWriter.error);

  [assetReader startReading]; [assetWriter startWriting];

  [assetWriter startSessionAtSourceTime:kCMTimeZero];

  const dispatch_queue_t queue = dispatch_queue_create("transcode", DISPATCH_QUEUE_SERIAL); const dispatch_semaphore_t transcode = dispatch_semaphore_create(0); [input requestMediaDataWhenReadyOnQueue:queue usingBlock:^{ while ([input isReadyForMoreMediaData]) { const CMSampleBufferRef sampleBuffer = [output copyNextSampleBuffer];

      XCTAssertNil(assetReader.error);
      XCTAssertNil(assetWriter.error);
  
      if (sampleBuffer) {
          [input appendSampleBuffer:sampleBuffer];
          CFRelease(sampleBuffer);
      } else {
          [input markAsFinished];
          dispatch_semaphore_signal(transcode);
      }
  }

  }];

  dispatch_semaphore_wait(transcode, DISPATCH_TIME_FOREVER);

  const dispatch_semaphore_t write = dispatch_semaphore_create(0); [assetWriter finishWritingWithCompletionHandler:^{ dispatch_semaphore_signal(write); }];

  XCTAssertNil(assetWriter.error);

  dispatch_semaphore_wait(write, DISPATCH_TIME_FOREVER);

  NSLog(@"Wrote to %@", outputUrl); }

• Product Version: Created: 2019-05-25T00:47:18.075283 Originated: 2019-05-24T00:00:00 Open Radar Link: http://www.openradar.me/51127979