AVCaptureSession with multiple Outputs?

Question

I'm currently developing an iOS app that applies CoreImage to the camera feed in order to take photos and videos, and I've run into a bit of snag.

Up till now I've been using AVCaptureVideoDataOutput to obtain the sample buffers and manipulate them with CoreImage, and then displayed a simple preview, as well as using it to capture photos and saving them.

When I tried to implement Video Recording, by writing the SampleBuffers to a video as I received them from the AVCaptureVideoDataOutput, it had a very slow frame rate (probably because of the other image relating processing that was going on).

So I was wondering, is it possible to have an AVCaptureVideoDataOutput and a AVCaptureMoveFileOutput going on the same AVCaptureSession simultaneously?

I gave it a quick go, and found that when I added the extra output, my AVCaptureVideoDataOutput stopped receiving information.

If I can get it working, I'm hoping it means that I can simply use the 2nd output to record video at high frame rates, and do post-processing on the video after the user has stopped recording.

Any help will be greatly appreciated.

Answer 1

It's easier than you'd think.

See: AVCamDemo

1. Capture data using AVCaptureVideoDataOutput.
2. Create a new dispatch queue before recording, eg. recordingQueue: recordingQueue = dispatch_queue_create("Movie Recording Queue", DISPATCH_QUEUE_SERIAL);

3. In the captureOutput:didOutputSampleBuffer:fromConnection: delegate method, capture the samplebuffer, retain it, and in the recording queue, write it to the file:

   -(void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection {    
   
       CFRetain(sampleBuffer);
   
       dispatch_async(recordingQueue, ^{
   
           if (assetWriter) {
   
               if (connection == videoConnection) {
                   [self writeSampleBuffer:sampleBuffer ofType:AVMediaTypeVideo];
               } else if (connection == audioConnection) {
                   [self writeSampleBuffer:sampleBuffer ofType:AVMediaTypeAudio];
               }
   
           }
   
           CFRelease(sampleBuffer);        
       });
   }
   
       - (void) writeSampleBuffer:(CMSampleBufferRef)sampleBuffer ofType:(NSString *)mediaType
       {
           CMTime presentationTime = CMSampleBufferGetPresentationTimeStamp(sampleBuffer);
   
           if ( assetWriter.status == AVAssetWriterStatusUnknown ) {
   
               if ([assetWriter startWriting]) {
                   [assetWriter startSessionAtSourceTime:presentationTime];
               } else {
                   NSLog(@"Error writing initial buffer");
               }
           }
   
           if ( assetWriter.status == AVAssetWriterStatusWriting ) {
   
               if (mediaType == AVMediaTypeVideo) {
                   if (assetWriterVideoIn.readyForMoreMediaData) {
   
                       if (![assetWriterVideoIn appendSampleBuffer:sampleBuffer]) {
                           NSLog(@"Error writing video buffer");
                       }
                   }
               }
               else if (mediaType == AVMediaTypeAudio) {
                   if (assetWriterAudioIn.readyForMoreMediaData) {
   
                       if (![assetWriterAudioIn appendSampleBuffer:sampleBuffer]) {
                           NSLog(@"Error writing audio buffer");
                       }
                   }
               }
           }
       }