访问iPhone视频输出图像缓冲区时的低FPS

我想在iPhone上做一些图像处理。我在用 http://developer.apple.com/library/ios/#qa/qa2010/qa1702.html 捕捉相机画面。

我的问题是,当我试图访问捕获的缓冲区时,相机的FPS从30下降到大约20。有人知道我该怎么修吗?

我使用kCVPixelFormatType_32BGRA格式的最低捕获质量(AVCaptureSessionPresetLow=192x144)。如果有人知道我可以用低质量的,我愿意试试。

当我在其他平台(如Symbian)上进行相同的图像访问时,它可以正常工作。

这是我的代码:

#pragma mark -
#pragma mark AVCaptureSession delegate
- (void)captureOutput:(AVCaptureOutput *)captureOutput 
didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer 
    fromConnection:(AVCaptureConnection *)connection 
{ 
 /*We create an autorelease pool because as we are not in the main_queue our code is
  not executed in the main thread. So we have to create an autorelease pool for the thread we are in*/
 NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
 //Lock the image buffer
    if (CVPixelBufferLockBaseAddress(imageBuffer, 0) == kCVReturnSuccess)
    {  

  // calculate FPS and display it using main thread
  [self performSelectorOnMainThread:@selector(updateFps:) withObject: (id) nil waitUntilDone:NO];


  UInt8 *base = (UInt8 *)CVPixelBufferGetBaseAddress(imageBuffer); //image buffer start address

  size_t width  = CVPixelBufferGetWidth(imageBuffer); 
  size_t height = CVPixelBufferGetHeight(imageBuffer);

  int size = (height*width);
  UInt8* pRGBtmp = m_pRGBimage;

        /*
        Here is the problem; m_pRGBimage is RGB image I want to process.
        In the 'for' loop I convert the image from BGRA to RGB. As a resault, the FPS drops to 20.
        */ 
  for (int i=0;i<size;i++)
  {   
    pRGBtmp[0] = base[2];
    pRGBtmp[1] = base[1];
    pRGBtmp[2] = base[0];
    base = base+4;
    pRGBtmp = pRGBtmp+3;     
  }


  // Display received action
  [self performSelectorOnMainThread:@selector(displayAction:) withObject: (id) nil waitUntilDone:NO];
  //[self displayAction:&eyePlayOutput];
  //saveFrame( imageBuffer );

  //unlock the  image buffer
  CVPixelBufferUnlockBaseAddress(imageBuffer,0);

 }


 [pool drain];
} 

作为答案的后续,我需要实时处理图像,它正在显示。

我注意到当我使用AVCaptureSessionPresetHigh时,我做的最简单的事情是:

 for (int i=0;i<size;i++)
    x = base[0];

使帧速率降至4-5 FPS。我想是因为这样大小的图像没有被缓存。

基本上我需要96x48图像。有没有一种简单的方法来缩小相机输出图像的比例,一种使用硬件加速的方法,这样我就可以使用小的一个?