cocoa capture frame from webcam

Question

Hi to all

I'm implementing a software to caputre video from webcam. I've seen MyRecorder sample in Apple Dev and it works fine.

I've tried to add a button to take a snapshot from video with this code:

- (IBAction)addFrame:(id)sender
{
    CVImageBufferRef imageBuffer;
    @synchronized (self) {
        imageBuffer = CVBufferRetain(mCurrentImageBuffer);
    }
    if (imageBuffer) { 
    [ bla bla bla ]  
    }
}

but mCurrentImageBuffer is always empty. How can I take current frame from my webcam and put on mCurrentImageBuffer?

I've tried to use

(void)captureOutput:(QTCaptureOutput *)captureOutput 
        didOutputVideoFrame:(CVImageBufferRef)videoFrame 
        withSampleBuffer:(QTSampleBuffer *)sampleBuffer 
        fromConnection:(QTCaptureConnection *)connection
{
    CVImageBufferRef imageBufferToRelease;

    CVBufferRetain(videoFrame);

    @synchronized (self) {
        imageBufferToRelease = mCurrentImageBuffer;
        mCurrentImageBuffer = videoFrame;
    }
    CVBufferRelease(imageBufferToRelease);  
}

but it's never called. How can I decide when call captureOutput delegate method? Any idea?

thanks, Andrea

Answer 1

I've tried to use

- (void)captureOutput:(QTCaptureOutput *)captureOutput 
                                didOutputVideoFrame:(CVImageBufferRef)videoFrame 
                                withSampleBuffer:(QTSampleBuffer *)sampleBuffer 
                                fromConnection:(QTCaptureConnection *)connection

but it's never called.

Is the object implementing this method the capture output object's delegate?

Answer 2

It looks like you're trying to use the QTKit Capture API for capturing video from your webcam. The MyRecorder sample application is pretty much the simplest functioning video capture program you can make using this API. It wasn't clear from your description, but you need to make sure that you follow their example, and initialize your video session in the same manner as they do in the -awakeFromNib method within MyRecorderController. If you don't, you won't get any video being captured.

As far as the method you're trying to use, -captureOutput:didOutputVideoFrame:withSampleBuffer:fromConnection: is a delegate method for QTCaptureDecompressedVideoOutput. An instance of this class is not present in the MyRecorder sample, because that sample only records compressed video to disk. To use this, you'll need to create an instance of QTCaptureDecompressedVideoOutput, attach it to your QTCaptureSession using -addOutput:error:, and set the delegate for the QTCaptureDecompressedVideoOutput instance to be your class.

For more information on how QTKit handles this sort of thing, you can consult the QTKit Capture section of the QTKit Application Programming Guide.