AVAssetWriter slowdown - ios

Question

I am using AVAssetWriter to save the live feed from the camera. This works well using this code
- (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",);
}
I am usually getting close to 30 fps (however not 60 fps on iPhone 4s as noted by others) and when timing [adaptor appendPixelBuffer] it only takes a few ms.
However, I don't need the full frame, but I need high quality (low compression, key frame every frame) and I am going to read it back a process several times later. I therefore would like to crop the image before writing. Fortunately I only need a strip in the middle so I can do a simple memcpy of the buffer. To do this I am creating a CVPixelBufferRef that I am copying into and writing with the adaptor:
- (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);
}
This also works and the video looks OK. However it is very slow and the frame rate becomes unacceptable. And strangely, the big slowdown isn't the copying but that the [adaptor appendPixelBuffer] step now takes 10-100 times longer than before. So I guess that it doesn't like the pxbuffer I create, but I can see why. I am using kCVPixelFormatType_32BGRA when setting up both the video out and the adaptor.
Can anyone suggest a better way to do the copying/cropping? Can you do that directly on the ImageBuffer?

Answer 1

I found a solution. In iOS5 (I had missed the updates) you can set AVAssetWriter to crop your video (as also noted by Steve). Set AVVideoScalingModeKey to 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];