Compressing an AVAsset (mainly AVMutableComposition) - ios

Question

I have a video with these specs
Format : H.264 , 1280x544
FPS : 25
Data Size : 26MB
Duration : 3:00
Data Rate : 1.17 Mbit/s
While experimenting ,I performed a removeTimeRange(range : CMTimeRange) on every other frame (total frames = 4225). This results in the video becoming 2x faster , so the duration becomes 1:30.
However when I export the video, the video becomes 12x larger in size i.e. 325MB.This makes sense since this technique is decomposing the video into about 2112 pieces and stitching it back together. Apparently, in doing so the compression among individual frames is lost, thus causing the enormous size.
This causes stuttering in the video when played with an AVPlayer and therefore poor performance.
Question : How can I apply some kind of compression while stitching back the frames so that the video can play smoothly and also be less in size?
I only want a point in the right direction. Thanks!
CODE
1) Creating an AVMutableComposition from Asset & Configuring it
func configureAssets(){
let options = [AVURLAssetPreferPreciseDurationAndTimingKey : "true"]
let videoAsset = AVURLAsset(url: Bundle.main.url(forResource: "Push", withExtension: "mp4")! , options : options)
let videoAssetSourceTrack = videoAsset.tracks(withMediaType: AVMediaTypeVideo).first! as AVAssetTrack
let comp = AVMutableComposition()
let videoCompositionTrack = comp.addMutableTrack(withMediaType: AVMediaTypeVideo, preferredTrackID: kCMPersistentTrackID_Invalid)
do {
try videoCompositionTrack.insertTimeRange(
CMTimeRangeMake(kCMTimeZero, videoAsset.duration),
of: videoAssetSourceTrack,
at: kCMTimeZero)
deleteSomeFrames(from: comp)
videoCompositionTrack.preferredTransform = videoAssetSourceTrack.preferredTransform
}catch { print(error) }
asset = comp }
2) Deleting every other frame.
func deleteSomeFrames(from asset : AVMutableComposition){
let fps = Int32(asset.tracks(withMediaType: AVMediaTypeVideo).first!.nominalFrameRate)
let sumTime = Int32(asset.duration.value) / asset.duration.timescale;
let totalFrames = sumTime * fps
let totalTime = Float(CMTimeGetSeconds(asset.duration))
let frameDuration = Double(totalTime / Float(totalFrames))
let frameTime = CMTime(seconds: frameDuration, preferredTimescale: 1000)
for frame in Swift.stride(from: 0, to: totalFrames, by: 2){
let timing = CMTimeMultiplyByFloat64(frameTime, Float64(frame))
print("Asset Duration = \(CMTimeGetSeconds(asset.duration))")
print("")
let timeRange = CMTimeRange(start: timing, duration : frameTime)
asset.removeTimeRange(timeRange)
}
print("duration after time removed = \(CMTimeGetSeconds(asset.duration))")
}
3) Saving the file
func createFileFromAsset(_ asset: AVAsset){
let documentsDirectory = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0] as URL
let filePath = documentsDirectory.appendingPathComponent("rendered-vid.mp4")
if let exportSession = AVAssetExportSession(asset: asset, presetName: AVAssetExportPresetHighestQuality){
exportSession.outputURL = filePath
exportSession.shouldOptimizeForNetworkUse = true
exportSession.timeRange = CMTimeRangeMake(kCMTimeZero, asset.duration)
exportSession.outputFileType = AVFileTypeQuickTimeMovie
exportSession.exportAsynchronously {
print("finished: \(filePath) : \(exportSession.status.rawValue) ")
if exportSession.status.rawValue == 4{
print("Export failed -> Reason: \(exportSession.error!.localizedDescription))")
print(exportSession.error!)
}
}}}
4) Finally update the ViewController to play the new Composition!
override func viewDidLoad() {
super.viewDidLoad()
// Create the AVPlayer and play the composition
assetConfig.configureAssets()
let snapshot : AVComposition = assetConfig.asset as! AVComposition
let playerItem = AVPlayerItem(asset : snapshot)
player = AVPlayer(playerItem: playerItem)
let playerLayer = AVPlayerLayer(player: player)
playerLayer.frame = CGRect(x : 0, y : 0, width : self.view.frame.width , height : self.view.frame.height)
self.view.layer.addSublayer(playerLayer)
player?.play()
}

Answer 1

If you are using AVMutableComposition,you will notice that each composition may contain one or more AVCompositionTrack(or AVMutableCompositionTrack),and the best way to edit your composition was to operate each track, but not the whole composition.
but if your purpose is to faster your video's rate, editing tracks will not be necessary.
so i will try my best to tell you what i know about your question
About video Stuttering while playing
Possible reason of stuttering
notice that you are using method removeTimeRange(range: CMTimeRange),this method will remove the timeRange on composition yes, but will NOT auto fill the empty of each time range
Visualize Example
[F stand for Frame,E stand for Empty]
org_video --> F-F-F-F-F-F-F-F...
after remove time range, the composition will be like this
after_video --> F-E-F-E-F-E-F-E...
and you might think that the video will be like this
target_video --> F-F-F-F...
this is the most possible reason about stuttering during playback.
Suggested solution
So if you want to shorten your video, make it rate more faster/slower,you possible need to use the method scaleTimeRange:(CMTimeRange) toDuration:(CMTime)
Example
AVMutableComposition * project;//if this video is 200s
//Scale
project.scaleTimeRange:CMTimeRangeMake(kCMTimeZero, project.duration) toDuration:CMTimeMakeWithSeconds(100,kCMTimeMaxTimescale)
this method is to make the video faster/slower.
About the file size
a video file's size might effected by bit rate and format type ,if your using H.264,the most possible reason causing size enlarge will be bit rate.
in your code,you are using AVAssetExportSession
AVAssetExportSession(asset: asset, presetName: AVAssetExportPresetHighestQuality
you gave the preset which is AVAssetExportPresetHighestQuality
in my own application project, after i was using this preset, the video's bit rate will be 20~30Mbps,no matter your source video's bit rate. and, well using apple's preset will not allowed you to set the bit rate manually, so.
Possible Solution
There is a third part tool called SDAVAssetExportSession,this session will allowed you to fully config your export session, you might want to try to study this code about custom export session's preset.
here is what i can tell you right now. wish could help :>