Incorrect Frames by AVAssetImageGenerator.generateCGImagesAsynchronously

Question

As per the doc, generateCGImagesAsynchronously takes in an array of NSValue and generates the frames from the video and given time and return it as a callback.

I generate a list of values using following code

    var values : [NSValue] = []
    let frameDuration = CMTimeMake(1, timeScale)
    for i in 0..<duration{
        let lastFrameTime = CMTimeMake(Int64(i), timeScale)
        let presentationTime = (i == 0) ? lastFrameTime : CMTimeAdd(lastFrameTime, frameDuration)

        values.append(NSValue(time: presentationTime))

        //Next two lines of codes are just to cross check the output
        let image = try! imageGenerator.copyCGImage(at: presentationTime, actualTime: nil)
        let imageUrl = FileManagerUtil.getTempFileName(parentFolder: FrameExtractor.EXTRACTED_IMAGE, fileNameWithExtension: "\(Constants.FRAME_SUFFIX)\(i)\(".jpeg")")
    }

As you can see in the code above, I cross checked the result by using synchronous method and I can confirm that the values array hold the correct reference of time.

But when same array is passed to generateImageAsynchronously method, I get duplicate of same frame 10 times for different time stamps. That is if my video is of 10 seconds, then I get 300 frames (with 30 fps) but frames of 1st second repeated 10 times each. It something like returning frame for time 0.1 seconds when requested for 1 second.

P.S: Though synchronous method is working fine, it is taking twice the time taken by the asynchronous method. May be because it is returning same frames. But I need it working to check the actual time usages.

Answer 1

You need to set requestedTimeToleranceBefore and requestedTimeToleranceAfter to your desired accuracy, for example .zero but this might incur on extra delay and cost to generate the images.

For example:

imageGenerator.requestedTimeToleranceBefore = .zero
imageGenerator.requestedTimeToleranceAfter = .zero

This way you would get the exact frame at that time.