iOS AVFoundation how to convert camera frames per second to create a timelapse using CMTime?

Question

I have an iPhone camera attachment that captures video at 9FPS and makes it available as individual UIImages. I'm trying to stitch these images together to create a timelapse video of what the camera sees using AVFoundation.

I'm not sure how to properly convert frames and timing to achieve the time compression I want.

For example - I want 1 hour of real life footage to be converted into 1 minute of time lapse. This tells me that I need to capture every 60th frame and append it to timelapse.

Does the code below accomplish 60 seconds to 1 second time lapse conversion? Or do I need to add some more multiplication/division by kRecordingFPS?

#define kRecordingFPS 9
#define kTimelapseCaptureFrameWithMod 60
//frameCount is the number of frames that the camera has output so far
if(frameCount % kTimelapseCaptureFrameWithMod == 0)
{
    //...
    //convert image and prepare it for recording
    [self appendImage:image 
               atTime:CMTimeMake(currentRecordingFrameNumber++, kRecordingFPS)];
}

Answer 1

Your code will make 1 frame out of 60 go in your film every 1/9s, and increase the frameIndex by one each time.

The result should be a film of [max value for frameCount] / (60 * 9). If you have 32400 frames (1h of film at 9fps), the film will be of {540:9 = 60s}.

Try printing the CMTime using CMTimeShow() at every call to appendImage:atTime: to check.

//at 5 fps, 300 frames recorded per 60 seconds
//to compress 300 frames into 1 second at 5 fps, we need to take every 300/5 = 60th frame
//to compress 300 frames into 1 second at 15 fps, take every 300/15 = 20th frame
//to compress 300 frames into 1 sec at 30 fps, take every 300/30 = 10th frame

#define kReceivedFPS 9
#define kStoreFPS 9
#define kSecondsPerSecondCompression 60

#define kTimelapseCaptureFrameWithMod (kSecondsPerSecondCompression * kReceivedFPS) / kStoreFPS