How do I convert a CGImage to CMSampleBufferRef?

I’d like to convert a CGImage to CMSampleBufferRef and append it to a AVAssetWriterInput using the appendSampleBuffer: method. I’ve managed to get the CMSampleBufferRef using the following code, but the appendSampleBuffer: simply returns NO when I supply the resulting CMSampleBufferRef. What am I doing wrong?

- (void) appendCGImage: (CGImageRef) frame
{
    const int width = CGImageGetWidth(frame);
    const int height = CGImageGetHeight(frame);

    // Create a dummy pixel buffer to try the encoding
    // on something simple.
    CVPixelBufferRef pixelBuffer = NULL;
    CVReturn status = CVPixelBufferCreate(kCFAllocatorDefault, width, height,
        kCVPixelFormatType_32BGRA, NULL, &pixelBuffer);
    NSParameterAssert(status == kCVReturnSuccess && pixelBuffer != NULL);

    // Sample timing info.
    CMTime frameTime = CMTimeMake(1, 30);
    CMTime currentTime = CMTimeAdd(lastSampleTime, frameTime);
    CMSampleTimingInfo timing = {frameTime, currentTime, kCMTimeInvalid};

    OSStatus result = 0;

    // Sample format.
    CMVideoFormatDescriptionRef videoInfo = NULL;
    result = CMVideoFormatDescriptionCreateForImageBuffer(NULL,
         pixelBuffer, &videoInfo);
    NSParameterAssert(result == 0 && videoInfo != NULL);

    // Create sample buffer.
    CMSampleBufferRef sampleBuffer = NULL;
    result = CMSampleBufferCreateForImageBuffer(kCFAllocatorDefault,
        pixelBuffer, true, NULL, NULL, videoInfo, &timing, &sampleBuffer);
    NSParameterAssert(result == 0 && sampleBuffer != NULL);

    // Ship out the frame.
    NSParameterAssert(CMSampleBufferDataIsReady(sampleBuffer));
    NSParameterAssert([writerInput isReadyForMoreMediaData]);
    BOOL success = [writerInput appendSampleBuffer:frame];
    NSParameterAssert(success); // no go :(
}

P.S. I know there are memory leaks in this code, I’ve omitted some of the code for simplicity.

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    Aha, I’ve completely missed the AVAssetWriterInputPixelBufferAdaptor class that’s made especially for piping the pixel buffers into a writer input. Now the code works, even without the messy CMSampleBuffer stuff.