captureOutput:didOutputSampleBuffer:fromConnection Performance Issues

[iSkythe]

I use AVCaptureSessionPhoto to allow the user to take high-resolution photos. Upon taking a photo, I use the captureOutput:didOutputSampleBuffer:fromConnection : method to retrieve a thumbnail at the time of capture. However, although I try to do minimal work in the delegate method, the app becomes sort of laggy (I say sort of because it is still useable). Also, the iPhone tends to run hot.

Is there some way of reducing the amount of work the iPhone has to do?

I set up the AVCaptureVideoDataOutput by doing the following:

self.videoDataOutput = [[AVCaptureVideoDataOutput alloc] init]; 
self.videoDataOutput.alwaysDiscardsLateVideoFrames = YES;

// Specify the pixel format
dispatch_queue_t queue = dispatch_queue_create("com.myapp.videoDataOutput", NULL);
[self.videoDataOutput setSampleBufferDelegate:self queue:queue];
dispatch_release(queue);
self.videoDataOutput.videoSettings = [NSDictionary dictionaryWithObject: [NSNumber numberWithInt:kCVPixelFormatType_32BGRA] 
                                                   forKey:(id)kCVPixelBufferPixelFormatTypeKey];

Here's my captureOutput:didOutputSampleBuffer:fromConnection (and assisting imageRefFromSampleBuffer method):

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

NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
if (videoDataOutputConnection == nil) {
    videoDataOutputConnection = connection;
}
if (getThumbnail > 0) {
    getThumbnail--;
    CGImageRef tempThumbnail = [self imageRefFromSampleBuffer:sampleBuffer];
    UIImage *image;
    if (self.prevLayer.mirrored) {
        image = [[UIImage alloc] initWithCGImage:tempThumbnail scale:1.0 orientation:UIImageOrientationLeftMirrored];
    }
    else {
        image = [[UIImage alloc] initWithCGImage:tempThumbnail scale:1.0 orientation:UIImageOrientationRight];
    }
    [self.cameraThumbnailArray insertObject:image atIndex:0];
    dispatch_async(dispatch_get_main_queue(), ^{
        self.freezeCameraView.image = image;
    });
    CFRelease(tempThumbnail);
}
sampleBuffer = nil;
[pool release];

}

-(CGImageRef)imageRefFromSampleBuffer:(CMSampleBufferRef)sampleBuffer {

CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer); 
CVPixelBufferLockBaseAddress(imageBuffer,0); 
uint8_t *baseAddress = (uint8_t *)CVPixelBufferGetBaseAddress(imageBuffer); 
size_t bytesPerRow = CVPixelBufferGetBytesPerRow(imageBuffer); 
size_t width = CVPixelBufferGetWidth(imageBuffer); 
size_t height = CVPixelBufferGetHeight(imageBuffer); 

CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB(); 
CGContextRef context = CGBitmapContextCreate(baseAddress, width, height, 8, bytesPerRow, colorSpace, kCGBitmapByteOrder32Little | kCGImageAlphaPremultipliedFirst); 
CGImageRef newImage = CGBitmapContextCreateImage(context); 
CVPixelBufferUnlockBaseAddress(imageBuffer,0);
CGContextRelease(context); 
CGColorSpaceRelease(colorSpace);
return newImage;

}