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On-demand and seamless transcoding of individual HLS segments
5 janvier 2024, par Omid AriyanBackground


I've been meaning to implement on-demand transcoding of certain video formats such as ".mkv", ".wmv", ".mov", etc. in order to serve them on a media management server using ASP.NET Core 6.0, C# and ffmpeg.


My Approach


The approach I've decided to use is to serve a dynamically generated .m3u8 file which is simply generated using a segment duration of choice e.g. 10s and the known video duration. Here's how I've done it. Note that the resolution is currently not implemented and discarded :


public string GenerateVideoOnDemandPlaylist(double duration, int segment)
{
 double interval = (double)segment;
 var content = new StringBuilder();

 content.AppendLine("#EXTM3U");
 content.AppendLine("#EXT-X-VERSION:6");
 content.AppendLine(String.Format("#EXT-X-TARGETDURATION:{0}", segment));
 content.AppendLine("#EXT-X-MEDIA-SEQUENCE:0");
 content.AppendLine("#EXT-X-PLAYLIST-TYPE:VOD");
 content.AppendLine("#EXT-X-INDEPENDENT-SEGMENTS");

 for (double index = 0; (index * interval) < duration; index++)
 {
 content.AppendLine(String.Format("#EXTINF:{0:#.000000},", ((duration - (index * interval)) > interval) ? interval : ((duration - (index * interval)))));
 content.AppendLine(String.Format("{0:00000}.ts", index));
 }

 content.AppendLine("#EXT-X-ENDLIST");

 return content.ToString();
}

[HttpGet]
[Route("stream/{id}/{resolution}.m3u8")]
public IActionResult Stream(string id, string resolution)
{
 double duration = RetrieveVideoLengthInSeconds();
 return Content(GenerateVideoOnDemandPlaylist(duration, 10), "application/x-mpegURL", Encoding.UTF8);
}


Here's an example of how the .m3u8 file looks like :


#EXTM3U
#EXT-X-VERSION:6
#EXT-X-TARGETDURATION:10
#EXT-X-MEDIA-SEQUENCE:0
#EXT-X-PLAYLIST-TYPE:VOD
#EXT-X-INDEPENDENT-SEGMENTS
#EXTINF:10.000000,
00000.ts
#EXTINF:3.386667,
00001.ts
#EXT-X-ENDLIST


So the player would ask for 00000.ts, 00001.ts, etc. and the next step is to have them generated on demand :


public byte[] GenerateVideoOnDemandSegment(int index, int duration, string path)
{
 int timeout = 30000;
 int totalWaitTime = 0;
 int waitInterval = 100;
 byte[] output = Array.Empty<byte>();
 string executable = "/opt/homebrew/bin/ffmpeg";
 DirectoryInfo temp = Directory.CreateDirectory(System.IO.Path.Combine(System.IO.Path.GetTempPath(), System.IO.Path.GetRandomFileName()));
 string format = System.IO.Path.Combine(temp.FullName, "output-%05d.ts");

 using (Process ffmpeg = new())
 {
 ffmpeg.StartInfo.FileName = executable;

 ffmpeg.StartInfo.Arguments = String.Format("-ss {0} ", index * duration);
 ffmpeg.StartInfo.Arguments += String.Format("-y -t {0} ", duration);
 ffmpeg.StartInfo.Arguments += String.Format("-i \"{0}\" ", path);
 ffmpeg.StartInfo.Arguments += String.Format("-c:v libx264 -c:a aac ");
 ffmpeg.StartInfo.Arguments += String.Format("-segment_time {0} -reset_timestamps 1 -break_non_keyframes 1 -map 0 ", duration);
 ffmpeg.StartInfo.Arguments += String.Format("-initial_offset {0} ", index * duration);
 ffmpeg.StartInfo.Arguments += String.Format("-f segment -segment_format mpegts {0}", format);

 ffmpeg.StartInfo.CreateNoWindow = true;
 ffmpeg.StartInfo.UseShellExecute = false;
 ffmpeg.StartInfo.RedirectStandardError = false;
 ffmpeg.StartInfo.RedirectStandardOutput = false;

 ffmpeg.Start();

 do
 {
 Thread.Sleep(waitInterval);
 totalWaitTime += waitInterval;
 }
 while ((!ffmpeg.HasExited) && (totalWaitTime < timeout));

 if (ffmpeg.HasExited)
 {
 string filename = System.IO.Path.Combine(temp.FullName, "output-00000.ts");

 if (!File.Exists(filename))
 {
 throw new FileNotFoundException("Unable to find the generated segment: " + filename);
 }

 output = File.ReadAllBytes(filename);
 }
 else
 {
 // It's been too long. Kill it!
 ffmpeg.Kill();
 }
 }

 // Remove the temporary directory and all its contents.
 temp.Delete(true);

 return output;
}

[HttpGet]
[Route("stream/{id}/{index}.ts")]
public IActionResult Segment(string id, int index)
{
 string path = RetrieveVideoPath(id);
 return File(GenerateVideoOnDemandSegment(index, 10, path), "application/x-mpegURL", true);
}
</byte>

So as you can see, here's the command I use to generate each segment incrementing -ss and -initial_offset by 10 for each segment :


ffmpeg -ss 0 -y -t 10 -i "video.mov" -c:v libx264 -c:a aac -segment_time 10 -reset_timestamps 1 -break_non_keyframes 1 -map 0 -initial_offset 0 -f segment -segment_format mpegts /var/folders/8h/3xdhhky96b5bk2w2br6bt8n00000gn/T/4ynrwu0q.z24/output-%05d.ts


The Problem


Things work on a functional level, however the transition between segments is slightly glitchy and especially the audio has very short interruptions at each 10 second mark. How can I ensure the segments are seamless ? What can I improve in this process ?


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Revision efcdf946ed : Fix a decoding mismatch in sub-pixel filters This did the same correction as th
23 mai 2014, par Yunqing WangChanged Paths :
Modify /vp9/common/x86/vp9_subpixel_8t_ssse3.asm
Fix a decoding mismatch in sub-pixel filtersThis did the same correction as the one in commit "Correct ssse3
8/16-pixel wide sub-pixel filter calculation" to avoid saturation
during filtering.Change-Id : Ife9aa3f62daf9114eb24fe38f7baa3c3f361b2d6
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avcodec/nvenc : fix signedness of timing fields
3 août 2024, par Timo Rothenpieler
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