Trouble applying scaleTimeRange on multiple videos in a AVMutableComposition video

After you scale the video, duration of the composition gets recalculated, so you need to append the next part according to this change. Replace

 insertTime = insertTime + duration

with

 insertTime = insertTime + newDuration

You also need to update setOpacity at value, I’d advise you to move that line after insertTime update and use new value, to remove duplicate work here

When you’re applying scale, it’s applied to new composition, so you need to use relative range:

 let currentRange = CMTimeRange(start: insertTime, duration: frameRange.duration)
 nthVideoTrack.scaleTimeRange(currentRange, toDuration: newDuration)
 nthAudioTrack.scaleTimeRange(currentRange, toDuration: newDuration)