AVFoundation - Retiming CMSampleBufferRef Video Output

Question

First time asking a question here. I\'m hoping the post is clear and sample code is formatted correctly.

I\'m experimenting with AVFoundation and time lapse photography.

Answer 1

With a little more searching and reading I have a working solution. Don't know that it is best method, but so far, so good.

In my setup area I've setup an AVAssetWriterInputPixelBufferAdaptor. The code addition looks like this.

InputWriterBufferAdaptor = [AVAssetWriterInputPixelBufferAdaptor
            assetWriterInputPixelBufferAdaptorWithAssetWriterInput: inputWriterBuffer
            sourcePixelBufferAttributes: nil];
[inputWriterBufferAdaptor retain];

For completeness to understand the code below, I also have these three lines in the setup method.

fpsOutput = 30; //Some possible values: 30, 10, 15 24, 25, 30/1.001 or 29.97;
cmTimeSecondsDenominatorTimescale = 600 * 100000; //To more precisely handle 29.97.
cmTimeNumeratorValue = cmTimeSecondsDenominatorTimescale / fpsOutput;

Instead of applying a retiming to a copy of the sample buffer. I now have the following three lines of code that effectively does the same thing. Notice the withPresentationTime parameter for the adapter. By passing my custom value to that, I gain the correct timing I'm seeking.

CVPixelBufferRef myImage = CMSampleBufferGetImageBuffer( sampleBuffer );
imageSourceTime = CMTimeMake( writtenFrames * cmTimeNumeratorValue, cmTimeSecondsDenominatorTimescale);
appendSuccessFlag = [inputWriterBufferAdaptor appendPixelBuffer: myImage withPresentationTime: imageSourceTime];

Use of the AVAssetWriterInputPixelBufferAdaptor.pixelBufferPool property may have some gains, but I haven't figured that out.

Answer 2

OK, I found the bug in my first post.

When using

myStatus = CMSampleBufferCreateCopyWithNewTiming(kCFAllocatorDefault,
                                                 sampleBuffer,
                                                 1,
                                                 &sampleTimingInfo, 
                                                 &newSampleBuffer);

you need to balance that with a CFRelease(newSampleBuffer);

The same idea holds true when using a CVPixelBufferRef with a piexBufferPool of an AVAssetWriterInputPixelBufferAdaptor instance. You would use CVPixelBufferRelease(yourCVPixelBufferRef); after calling the appendPixelBuffer: withPresentationTime: method.

Hope this is helpful to someone else.