ios – 如何控制AVAssetWriter以正确的FPS写入

在目前最先进的硬件上,iOS允许我以下列fps进行录制：30,60,120和240.

但这些fps表现不同.如果我以30或60 fps拍摄,我希望通过这些fps拍摄创建的视频文件分别以30和60 fps播放.

但是如果我以120或240 fps的速度拍摄,我希望以这些fps拍摄的视频文件以30 fps的速度播放,否则我将看不到慢动作.

几个问题：

>我是对的吗？
>有没有办法以120或240 fps的速度分别以120和240 fps的速度进行拍摄？我的意思是在没有慢动作的情况下拍摄视频的fps？
>我在编写文件时如何控制该帧速率？

我正在创建这样的AVAssetWriter输入……

NSDictionary *videoCompressionSettings = @{AVVideoCodecKey                  : AVVideoCodecH264,AVVideoWidthKey                  : @(videoWidth),AVVideoHeightKey                 : @(videoHeight),AVVideoCompressionPropertiesKey  : @{ AVVideoAverageBitRateKey      : @(bitsPerSecond),AVVideoMaxKeyFrameIntervalKey : @(1)}
                                             };

    _assetWriterVideoInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeVideo outputSettings:videoCompressionSettings];

并没有明显的方法来控制它.

注意：我尝试了不同的数字,其中1是.我试过1.0 / fps,我试过fps,我已经删除了密钥.没有不同.

这就是我设置`AVAssetWriter的方法：

AVAssetWriter *newAssetWriter = [[AVAssetWriter alloc] initWithURL:_movieURL fileType:AVFileTypeQuickTimeMovie
                                          error:&error];

  _assetWriter = newAssetWriter;
  _assetWriter.shouldOptimizeforNetworkUse = NO;

  CGFloat videoWidth = size.width;
  CGFloat videoHeight  = size.height;

  NSUInteger numPixels = videoWidth * videoHeight;
  NSUInteger bitsPerSecond;

  // Assume that lower-than-SD resolutions are intended for streaming,and use a lower bitrate
  //  if ( numPixels < (640 * 480) )
  //    bitsPerPixel = 4.05; // This bitrate matches the quality produced by AVCaptureSessionPresetMedium or Low.
  //  else
  NSUInteger bitsPerPixel = 11.4; // This bitrate matches the quality produced by AVCaptureSessionPresetHigh.

  bitsPerSecond = numPixels * bitsPerPixel;

  NSDictionary *videoCompressionSettings = @{AVVideoCodecKey                  : AVVideoCodecH264,AVVideoCompressionPropertiesKey  : @{ AVVideoAverageBitRateKey      : @(bitsPerSecond)}
                                             };

  if (![_assetWriter canApplyOutputSettings:videoCompressionSettings forMediaType:AVMediaTypeVideo]) {
    NSLog(@"Couldn't add asset writer video input.");
    return;
  }

 _assetWriterVideoInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeVideo
                                                              outputSettings:videoCompressionSettings
                                                            sourceFormatHint:formatDescription];
  _assetWriterVideoInput.expectsMediaDataInRealTime = YES;      

  NSDictionary *adaptorDict = @{
                                (id)kCVPixelBufferPixelFormatTypeKey : @(kCVPixelFormatType_32BGRA),(id)kCVPixelBufferWidthKey : @(videoWidth),(id)kCVPixelBufferHeightKey : @(videoHeight)
                                };

  _pixelBufferAdaptor = [[AVAssetWriterInputPixelBufferAdaptor alloc]
                         initWithAssetWriterInput:_assetWriterVideoInput
                         sourcePixelBufferAttributes:adaptorDict];


  // Add asset writer input to asset writer
  if (![_assetWriter canAddInput:_assetWriterVideoInput]) {
    return;
  }

  [_assetWriter addInput:_assetWriterVideoInput];

captureOutput方法非常简单.我从过滤器获取图像并使用以下方法将其写入文件：

if (videoJustStartWriting)
    [_assetWriter startSessionAtSourceTime:presentationTime];

  CVPixelBufferRef renderedOutputPixelBuffer = NULL;
  Osstatus err = CVPixelBufferPoolCreatePixelBuffer(nil,_pixelBufferAdaptor.pixelBufferPool,&renderedOutputPixelBuffer);

  if (err) return; //          NSLog(@"Cannot obtain a pixel buffer from the buffer pool");

  //_ciContext is a Metal context
  [_ciContext render:finalImage
     toCVPixelBuffer:renderedOutputPixelBuffer
              bounds:[finalImage extent]
          colorSpace:_sDeviceRgbColorSpace];

   [self writeVideoPixelBuffer:renderedOutputPixelBuffer
                  withInitialTime:presentationTime];


- (void)writeVideoPixelBuffer:(CVPixelBufferRef)pixelBuffer withInitialTime:(CMTime)presentationTime
{

  if ( _assetWriter.status == AVAssetWriterStatusUnkNown ) {
    // If the asset writer status is unkNown,implies writing hasn't started yet,hence start writing with start time as the buffer's presentation timestamp
    if ([_assetWriter startWriting]) {
      [_assetWriter startSessionAtSourceTime:presentationTime];
    }
  }

  if ( _assetWriter.status == AVAssetWriterStatusWriting ) {
    // If the asset writer status is writing,append sample buffer to its corresponding asset writer input

      if (_assetWriterVideoInput.readyForMoreMediaData) {
        if (![_pixelBufferAdaptor appendPixelBuffer:pixelBuffer withPresentationTime:presentationTime]) {
          NSLog(@"error",[_assetWriter.error localizedFailureReason]);
        }
      }
  }

  if ( _assetWriter.status == AVAssetWriterStatusFailed ) {
    NSLog(@"Failed");
  }

}

我把整个东西以240 fps的速度拍摄.这些是附加帧的呈现时间.

time ======= 113594.311510508
time ======= 113594.324011508
time ======= 113594.328178716
time ======= 113594.340679424
time ======= 113594.344846383

如果你在它们之间进行一些计算,你会发现帧速率约为240 fps.因此帧以正确的时间存储.

但是,当我观看视频时,动作不是慢动作,快速时间表示视频是30 fps.

注意：此应用程序从相机抓取帧,帧进入CIF过滤器,这些过滤器的结果将转换回存储到文件并显示在屏幕上的样本缓冲区.