AVAssetWriter速度减慢

Question

我正在使用AVAssetWriter保存来自摄像机的实时提要。使用下面的代码可以很好地工作

- (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer  fromConnection:(AVCaptureConnection *)connection{ 

 CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
 CMTime lastSampleTime = CMSampleBufferGetPresentationTimeStamp(sampleBuffer);

 if(videoWriter.status != AVAssetWriterStatusWriting){
    [videoWriter startWriting];
    [videoWriter startSessionAtSourceTime:lastSampleTime];
 }

 if(adaptor.assetWriterInput.readyForMoreMediaData) [adaptor appendPixelBuffer:imageBuffer withPresentationTime:lastSampleTime];
 else NSLog(@"adaptor not ready",);
}

我通常接近30fps(然而，在iPhone 4s上不是别人提到的60fps)，当计时适配器appendPixelBuffer时，只需要几毫秒。

然而，我不需要完整的帧，但我需要高质量(低压缩，每一帧的关键帧)，我将在几次之后重新读取它。因此，我想在写作之前裁剪图像。幸运的是，我只需要中间的一个条带，这样我就可以对缓冲区做一个简单的memcpy。为此，我创建了一个要复制到其中并使用适配器编写的CVPixelBufferRef：

- (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer  fromConnection:(AVCaptureConnection *)connection{ 

 CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
 CMTime lastSampleTime = CMSampleBufferGetPresentationTimeStamp(sampleBuffer);

 if(videoWriter.status != AVAssetWriterStatusWriting){
    [videoWriter startWriting];
    [videoWriter startSessionAtSourceTime:lastSampleTime];
 }

 CVPixelBufferLockBaseAddress(imageBuffer,0);
 size_t bytesPerRow = CVPixelBufferGetBytesPerRow(imageBuffer);
 size_t width = CVPixelBufferGetWidth(imageBuffer);
 size_t height = CVPixelBufferGetHeight(imageBuffer);
 void * buffIn = CVPixelBufferGetBaseAddress(imageBuffer);

 CVPixelBufferRef pxbuffer = NULL;
 CVReturn status = CVPixelBufferCreate(kCFAllocatorDefault, width, height, kCVPixelFormatType_32BGRA, nil, &pxbuffer);

 NSParameterAssert(status == kCVReturnSuccess && pxbuffer != NULL);
 CVPixelBufferLockBaseAddress(pxbuffer, 0);

  void *buffOut = CVPixelBufferGetBaseAddress(pxbuffer);
  NSParameterAssert(buffOut != NULL);

  //Copy the whole buffer while testing
  memcpy(buffOut, buffIn, width * height * 4); 
  //memcpy(buffOut, buffIn+sidecrop, width * 100 * 4); 

  if (adaptor.assetWriterInput.readyForMoreMediaData) [adaptor appendPixelBuffer:pxbuffer withPresentationTime:lastSampleTime];
  else NSLog(@"adaptor not ready");

   CVPixelBufferUnlockBaseAddress(pxbuffer, 0);
   CVPixelBufferUnlockBaseAddress(imageBuffer, 0);
}

这也行得通，视频看起来没问题。然而，它非常慢，并且帧速率变得不可接受。奇怪的是，最大的减速不是复制，而是适配器appendPixelBuffer步骤现在比以前多花了10-100倍的时间。所以我猜它不喜欢我创建的pxbuffer，但我知道为什么。我在设置视频输出和适配器时使用的是kCVPixelFormatType_32BGRA。

有没有人能推荐一种更好的复制/裁剪方法？你能直接在ImageBuffer上做吗？

Answer 1

我找到了一个解决方案。在iOS5中(我错过了更新)，你可以设置AVAssetWriter来裁剪你的视频(史蒂夫也注意到了)。将AVVideoScalingModeKey设置为AVVideoScalingModeResizeAspectFill

videoWriter = [[AVAssetWriter alloc] initWithURL:filmurl 
                                        fileType:AVFileTypeQuickTimeMovie 
                                           error:&error];  
NSDictionary *videoSettings = [NSDictionary dictionaryWithObjectsAndKeys: 
     AVVideoCodecH264, AVVideoCodecKey, 
     [NSNumber numberWithInt:1280], AVVideoWidthKey,  
     [NSNumber numberWithInt:200], AVVideoHeightKey,
     AVVideoScalingModeResizeAspectFill, AVVideoScalingModeKey,// This turns the
                                                               // scale into a crop
     nil]; 
videoWriterInput = [[AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeVideo 
                                                       outputSettings:videoSettings] retain];