Hold multiple Frames in Memory before sending them to AVAssetWriter

Question

I need to hold some video frames from a captureSession in memory and write them to a file when 'something' happens.

Similar to this solution, i use this code to put a frame into a NSMutableArray:

- (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection*)connection
{       
    //...
    CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
    uint8 *baseAddress = (uint8*)CVPixelBufferGetBaseAddress(imageBuffer);
    NSData *rawFrame = [[NSData alloc] initWithBytes:(void*)baseAddress length:(height * bytesPerRow)];
    [m_frameDataArray addObject:rawFrame];
    [rawFrame release];
    //...
}

And this to write the video file:

-(void)writeFramesToFile
{
    //...
    NSDictionary *outputSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                                    [NSNumber numberWithInt:640], AVVideoWidthKey,
                                    [NSNumber numberWithInt:480], AVVideoHeightKey,
                                    AVVideoCodecH264, AVVideoCodecKey,
                                    nil ];
    AVAssetWriterInput *bufferAssetWriterInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeVideo outputSettings:outputSettings];
    AVAssetWriter *bufferAssetWriter = [[AVAssetWriter alloc]initWithURL:pathURL fileType:AVFileTypeQuickTimeMovie error:&error];
    [bufferAssetWriter addInput:bufferAssetWriterInput];

    [bufferAssetWriter startWriting];
    [bufferAssetWriter startSessionAtSourceTime:startTime];
    for (NSInteger i = 1; i < m_frameDataArray.count; i++){
        NSData *rawFrame = [m_frameDataArray objectAtIndex:i];
        CVImageBufferRef imgBuf = [rawFrame bytes];
        [pixelBufferAdaptor appendPixelBuffer:imgBuf withPresentationTime:CMTimeMake(1,10)]; //<-- EXC_BAD_ACCESS
        [rawFrame release];
    }
    //... (finishing video file)
}

But something is wrong with the imgBuf reference. Any suggestions? Thanks in advance.

Answer 1

You're supposed to lock base address before accessing imageBuffer's properties.

CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
CVPixelBufferLockBaseAddress(imageBuffer, 0);
uint8 *baseAddress = (uint8*)CVPixelBufferGetBaseAddress(imageBuffer);
NSData *rawFrame = [[NSData alloc] initWithBytes:(void*)baseAddress length:(height * bytesPerRow)];
[m_frameDataArray addObject:rawFrame];
CVPixelBufferUnlockBaseAddress(imageBuffer, 0);