Avfoundation create mini clips from recording - iphone

Question

i was wondering how would i be able to create mini videos every certain amount of time from my recording without stopping my recording? i tried to look for an equivalent of AvAssetImageGenerator for videos an example would be nice.

Answer 1

The easiest way is to use two AVAssetWriters and set up the next writer while the current one is recording, then stop after x time and swap the writers. You should be able to swap the writers without dropping any frames.
Edit:
How to do AVAssetWriter "juggling"
Step 1: Create instance objects for the writers and pixelbuffer adaptors (and you'll want file names for these files as well that you know)
AVAssetWriter* mWriter[2];
AVAssetWriterInputPixelBufferAdaptor* mPBAdaptor[2];
NSString* mOutFile[2];
int mCurrentWriter, mFrameCount, mTargetFrameCount;
Step 2: Create a method for setting up a writer (since you'll be doing this over and over again)
-(int) setupWriter: (int) writer
{
NSAutoreleasePool* p = [[NSAutoreleasePool alloc] init];
NSDictionary* writerSettings = [NSDictionary dictionaryWithObjectsAndKeys: AVVideoCodecH264, AVVideoCodecKey, [NSNumber numberWithInt: mVideoWidth], AVVideoWidthKey, [NSNumber numberWithInt: mVideoHeight], AVVideoHeightKey, nil];
NSDictionary* pbSettings = [NSDictionary dictionaryWithObjectsAndKeys: [NSNumber numberWithInt:mVivdeoWidth],kCVPixelBufferWidthKey,
[NSNumber numberWithInt:mVideoHeight], kCVPixelBufferHeightKey,
[NSNumber numberWithInt:0],kCVPixelBufferExtendedPixelsLeftKey,
[NSNumber numberWithInt:0],kCVPixelBufferExtendedPixelsRightKey,
[NSNumber numberWithInt:0],kCVPixelBufferExtendedPixelsTopKey,
[NSNumber numberWithInt:0],kCVPixelBufferExtendedPixelsBottomKey,
[NSNumber numberWithInt:mVideoWidth],kCVPixelBufferBytesPerRowAlignmentKey,
[NSNumber numberWithUnsignedInt:kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange], kCVPixelBufferPixelFormatTypeKey, nil];
AVAssetWriterInput* writerInput = [AVAssetWriterInput assetWriterWithMediaType: AVMediaTypeVideo outputSettings: writerSettings];
// Create an audio input here if you want...
mWriter[writer] = [[AVAssetWriter alloc] initWithURL: [NSURL fileURLWithPath:mOutfile[writer]] fileType: AVFileTypeMPEG4 error:nil];
mPBAdaptor[writer] = [[AVAssetWriterInputPixelBufferAdaptor alloc] initWithAssetWriterInput: writerInput sourcePixelBufferAttributes: pbSettings];
[mWriter[writer] addInput: writerInput];
// Add your audio input here if you want it
[p release];
}
Step 3: Gotta tear these things down!
- (void) tearDownWriter: (int) writer
{
if(mWriter[writer]) {
if(mWriter[writer].status == 1) [mWriter[writer] finishWriting]; // This will complete the movie.
[mWriter[writer] release]; mWriter[writer] = nil;
[mPBAdaptor[writer] release]; mPBAdaptor[writer] = nil;
}
}
Step 4: Swap! Tear down the current writer and recreate it asynchronously while the other writer is writing.
- (void) swapWriters
{
NSAutoreleasePool * p = [[NSAutoreleasePool alloc] init];
if(++mFrameCount > mCurrentTargetFrameCount)
{
mFrameCount = 0;
int c, n;
c = mCurrentWriter^1;
n = mCurrentWriter; // swap.
[self tearDownWriter:n];
__block VideoCaptureClass* bSelf = self;
dispatch_async(dispatch_get_global_queue(0,0), ^{
[bSelf setupWriter:n];
CMTime time;
time.value = 0;
time.timescale = 15; // or whatever the correct timescale for your movie is
time.epoch = 0;
time.flags = kCMTimeFlags_Valid;
[bSelf->mWriter[n] startWriting];
[bSelf->mWriter[n] startSessionAtSourceTime:time];
});
mCurrentWriter = c;
}
[p release];
}
Note: When starting up you will have to create and start both writers.
Step 5: Capturing output
- (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection {
// This method will only work with video; you'll have to check for audio if you're using that.
CMTime time = CMSampleBufferGetPresentationTimeStamp(sampleBuffer); // Note: you may have to create your own PTS.
CVImageBufferRef pixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
[mPBAdaptor[mCurrentWriter] appendPixelBuffer:pixelBuffer withPresentationTime: time];
[self swapBuffers];
}
You can probably skip the pixel buffer adaptor if you don't need it. This should give you an approximate idea of how to do what you want to do. mTargetFrameCount represents how many frames you want the current video to be in length. Audio will probably take additional consideration, you may want to base your length off your audio stream instead of the video stream if you are using audio.