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• h264 lossless coding

  19 juillet 2022, par cloudraven

  Is it possible to do completely lossless encoding in h264 ? By lossless, I mean that if I feed it a series of frames and encode them, and then if I extract all the frames from the encoded video, I will get the exact same frames as in the input, pixel by pixel, frame by frame. Is that actually possible ?
Take this example :

  



  I generate a bunch of frames, then I encode the image sequence to an uncompressed AVI (with something like virtualdub), I then apply lossless h264 (the help files claim that setting —qp 0 makes lossless compression, but I am not sure if that means that there is no loss at any point of the process or that just the quantization is lossless). I can then extract the frames from the resulting h264 video with something like mplayer.

  



  I tried with Handbrake first, but it turns out it doesn't support lossless encoding. I tried x264 but it crashes. It may be because my source AVI file is in RGB colorspace instead of YV12. I don't know how to feed a series of YV12 bitmaps and in what format to x264 anyway, so I cannot even try.

  



  In summary what I want to know if that is there a way to go from

  



  Series of lossless bitmaps (in any colorspace) -> some transformation -> h264 encode -> h264 decode -> some transformation -> the original series of lossless bitmaps

  



  If there a way to achieve this ?

  



  EDIT : There is a VERY valid point about lossless H264 not making too much sense. I am well aware that there is no way I could tell (with just my eyes) the difference between and uncompressed clip and another compressed at a high rate in H264, but I don't think it is not without uses. For example, it may be useful for storing video for editing without taking huge amounts of space and not losing quality and spending too much encoding time every time the file is saved.

  



  UPDATE 2 : Now x264 doesn't crash. I can use as sources either avisynth or lossless yv12 lagarith (to avoid the colorspace compression warning). Howerver, even with —qp 0 and a rgb or yv12 source I still get some differences, minimal but present. This is troubling, because all the information I have found on lossless predictive coding (—qp 0) claims that the whole encoding should be lossless, but I am unable to verifiy this.

  


• h264 lossless coding

  29 septembre 2014, par cloudraven

  Is it possible to do completely lossless encoding in h264 ? By lossless, I mean that if I feed it a series of frames and encode them, and then if I extract all the frames from the encoded video, I will get the exact same frames as in the input, pixel by pixel, frame by frame. Is that actually possible ?
  Take this example :

  I generate a bunch of frames, then I encode the image sequence to an uncompressed AVI (with something like virtualdub), I then apply lossless h264 (the help files claim that setting —qp 0 makes lossless compression, but I am not sure if that means that there is no loss at any point of the process or that just the quantization is lossless). I can then extract the frames from the resulting h264 video with something like mplayer.

  I tried with Handbrake first, but it turns out it doesn’t support lossless encoding. I tried x264 but it crashes. It may be because my source AVI file is in RGB colorspace instead of YV12. I don’t know how to feed a series of YV12 bitmaps and in what format to x264 anyway, so I cannot even try.

  In summary what I want to know if that is there a way to go from

  Series of lossless bitmaps (in any colorspace) -> some transformation -> h264 encode -> h264 decode -> some transformation -> the original series of lossless bitmaps

  If there a way to achieve this ?

  EDIT : There is a VERY valid point about lossless H264 not making too much sense. I am well aware that there is no way I could tell (with just my eyes) the difference between and uncompressed clip and another compressed at a high rate in H264, but I don’t think it is not without uses. For example, it may be useful for storing video for editing without taking huge amounts of space and not losing quality and spending too much encoding time every time the file is saved.

  UPDATE 2 : Now x264 doesn’t crash. I can use as sources either avisynth or lossless yv12 lagarith (to avoid the colorspace compression warning). Howerver, even with —qp 0 and a rgb or yv12 source I still get some differences, minimal but present. This is troubling, because all the information I have found on lossless predictive coding (—qp 0) claims that the whole encoding should be lossless, but I am unable to verifiy this.

• iOS Radio App : Need to extract and stream audio-only from HLS streams with video content

  20 décembre 2024, par Bader Alghamdi

  I'm developing an iOS radio app that plays various HLS streams. The challenge is that some stations broadcast HLS streams containing both audio and video (example : https://svs.itworkscdn.net/smcwatarlive/smcwatar/chunks.m3u8), but I want to :

  


  Extract and play only the audio track
Support AirPlay for audio-only streaming
Minimize data usage by not downloading video content
Technical Details :

  


  iOS 17+
Swift 6
Using AVFoundation for playback
Current implementation uses AVPlayer with AVPlayerItem
Current Code Structure :

  


  class StreamPlayer: ObservableObject { @Published var isPlaying = false private var player: AVPlayer? private var playerItem: AVPlayerItem?

func playStream(url: URL) {
    let asset = AVURLAsset(url: url)
    playerItem = AVPlayerItem(asset: asset)
    player = AVPlayer(playerItem: playerItem)
    player?.play()
}



  


  Stream Analysis : When analyzing the video stream using FFmpeg :

  


  CopyInput #0, hls, from 'https://svs.itworkscdn.net/smcwatarlive/smcwatar/chunks.m3u8':
  Stream #0:0: Video: h264, yuv420p(tv, bt709), 1920x1080 [SAR 1:1 DAR 16:9], 25 fps
  Stream #0:1: Audio: aac, 44100 Hz, stereo, fltp



  


  Attempted Solutions :

  


  Using MobileFFmpeg :

  


  let command = [
    "-i", streamUrl,
    "-vn",
    "-acodec", "aac",
    "-ac", "2",
    "-ar", "44100",
    "-b:a", "128k",
    "-f", "mpegts",
    "udp://127.0.0.1:12345"
].joined(separator: " ")

ffmpegProcess = MobileFFmpeg.execute(command)

I


  


  ssue : While FFmpeg successfully extracts audio, playback through AVPlayer doesn't work reliably.

  


  Tried using HLS output :

  


  let command = [
    "-i", streamUrl,
    "-vn",
    "-acodec", "aac",
    "-ac", "2",
    "-ar", "44100",
    "-b:a", "128k",
    "-f", "hls",
    "-hls_time", "2",
    "-hls_list_size", "3",
    outputUrl.path
]



  


  Issue : Creates temporary files but faces synchronization issues with live streams.

  


  ---

  


  Testing URLs :

  


  Audio+Video : https://svs.itworkscdn.net/smcwatarlive/smcwatar/chunks.m3u8
Audio Only : https://mbcfm-radio.mbc.net/mbcfm-radio.m3u8

  


  ---

  


  Requirements :

  


  

  • Real-time audio extraction from HLS stream

  


  • Maintain live streaming capabilities

  


  • Full AirPlay support

  


  • Minimal data usage (avoid downloading video content)

  


  • Handle network interruptions gracefully

  


  


  Questions :

  


  

  • What's the most efficient way to extract only audio from an HLS stream in real-time ?

  


  • Is there a way to tell AVPlayer to ignore video tracks completely ?

  


  • Are there better alternatives to FFmpeg for this specific use case ?

  


  • What's the recommended approach for handling AirPlay with modified streams ?

  


  


  Any guidance or alternative approaches would be greatly appreciated. Thank you !

  


  What I Tried :

  


  

  1. Direct AVPlayer Implementation :

  


  


  

  • Used standard AVPlayer to play HLS stream

  


  • Expected it to allow selecting audio-only tracks

  


  • Result : Always downloads both video and audio, consuming unnecessary bandwidth

  


  


  ---

  


  

  2. FFmpeg Audio Extraction :

  


  


  let command = [
    "-i", "https://svs.itworkscdn.net/smcwatarlive/smcwatar/chunks.m3u8",
    "-vn",                    // Remove video
    "-acodec", "aac",        // Audio codec
    "-ac", "2",              // 2 channels
    "-ar", "44100",          // Sample rate
    "-b:a", "128k",          // Bitrate
    "-f", "mpegts",          // Output format
    "udp://127.0.0.1:12345"  // Local stream
]
ffmpegProcess = MobileFFmpeg.execute(command)



  


  Expected : Clean audio stream that AVPlayer could play
Result : FFmpeg extracts audio but AVPlayer can't play the UDP stream

  


  ---

  


  

  3. HLS Segmented Approach :

  


  


  swiftCopylet command = [
    "-i", streamUrl,
    "-vn",
    "-acodec", "aac",
    "-f", "hls",
    "-hls_time", "2",
    "-hls_list_size", "3",
    outputUrl.path
]



  


  Expected : Create local HLS playlist with audio-only segments
Result : Creates files but faces sync issues with live stream

  


  ---

  


  Expected Behavior :

  


  

  • Stream plays audio only

  


  • Minimal data usage (no video download)

  


  • Working AirPlay support

  


  • Real-time playback without delays

  


  


  Actual Results :

  


  

  • Either downloads full video stream (wasteful)

  


  • Or fails to play extracted audio

  


  • AirPlay issues with modified streams

  


  • Sync problems with live content