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• Creating an Init file from existing non-fragmented, segmented MP4 files

  20 novembre 2018, par slhck

  I am performing chunked encodes of longer video files, where I’ve split the original file into individual sequences that I have encoded separately. These sequences are files of different length, depending on where the scene cuts appear—they may be between 2 and 5 seconds long. They all start with an I-frame and are standalone.

  My encoded sequences are all MP4s, e.g. :

  test_0000.mp4
  
  test_0001.mp4
  
  test_0002.mp4
  
  test_0003.mp4
  
  test_0004.mp4

  They all have common properties :

  $ mp4info test_0000.mp4
  
  
  
  File:
  
    major brand:      isom
  
    minor version:    200
  
    compatible brand: isom
  
    compatible brand: iso2
  
    compatible brand: mp41
  
    fast start:       no
  
  
  
  Movie:
  
    duration:   2016 ms
  
    time scale: 1000
  
    fragments:  no
  
  
  
  ...

  Now, in order to play those with a DASH player, I have to create an initialization segment and individual fragmented MP4s.

  I could generate the fragmented MP4s via mp4fragment which I run on each standalone MP4 file :

  $ mp4info test_0000.m4s
  
  File:
  
    major brand:      isom
  
    minor version:    200
  
    compatible brand: isom
  
    compatible brand: iso2
  
    compatible brand: mp41
  
    compatible brand: iso5
  
    fast start:       yes
  
  
  
  Movie:
  
    duration:   2016 ms
  
    time scale: 1000
  
    fragments:  yes
  
  
  
  ...

  But obviously, these are now not according to spec, and all contain a moov atom :

  

  What I’d need is individual media segments with only one moof and mdat box, which then require an initialization segment with only a moov box.

  How can I generate that from the existing, already encoded segments ?

  I know this appears like an XY problem. In principle, I could already segment my original file directly after encoding, and run those encodes at the same time, e.g. using ffmpeg’s dash muxer, or MP4Box, however :

  • There is almost no control over the resulting segment sizes, with respect to minimum and maximum duration
  • This approach does not parallelize

  I have also checked Bento4 ; it does not seem to offer this functionality. Neither does FFmpeg. MP4Box behaves similarly. They all assume you have one long file to start with.

  ---

  I see I could splice off the ftyp and moov boxes from these “fake fragments” in order to create an initialization segment. But I would end up with segments containing multiple moof and mdat boxes, which is not according to the specification – it only allows one fragment and media data box :

  4. Media Segments

  […] one optional Segment Type Box (styp) followed by a single Movie Fragment Box (moof) followed by one or more Media Data Boxes (mdat).

  I guess I can live with this the styp not being present.

• AV1 extract keyframes

  18 avril 2021, par Steven Penny

  Using this file [1], I can run this command with no problem :

  


  ffmpeg -i crushed-between-two-portals.mp4 %d.jpg


  


  but if I run either of these commands [2] :

  


  ffmpeg -i crushed-between-two-portals.mp4 -vf "select='eq(pict_type,I)'" %d.jpg
ffmpeg -i crushed-between-two-portals.mp4 -vf "select='eq(pict_type,PICT_TYPE_I)'" %d.jpg


  


  I get this result :

  


  Output file is empty, nothing was encoded (check -ss / -t / -frames parameters
if used)


  


  I think it is because of the video stream :

  


  Stream #0:0(und): Video: av1 (Main) (av01 / 0x31307661), yuv420p(tv, bt709),
1280x720 [SAR 1:1 DAR 16:9], 19 kb/s, 59.94 fps, 59.94 tbr, 60k tbn, 60k tbc
(default)


  


  I have used this command before with other codecs. How can I extract keyframes
or similar from AV1 codec ?

  


  

  1. https://f002.backblazeb2.com/file/ql8mlh/crushed-between-two-portals.mp4

  


  2. Create a thumbnail image every X seconds of the video

  


  


• x264 Downsides of a high CRF (22) intermediary codec between conversions instead of lossless

  18 décembre 2019, par bobtheencoder

  I have a huge collection of video files that are in the range of CRF 16-20 taking up TB’s of space. The only need I have for these originals is that I have to encode them from time to time but the CRF of these final encodes is very low (CRF 26-28).

  I understand that a lossy to lossy converstion ALWAYS results in some quality loss but my question is what if the intermediate file is almost visually lossless compared to the final output.

  So to sum up, what quality difference should I expect from the following routes ?

  CRF 18 (original) -----> CRF 28 (final)
  
  CRF 18 (original) -----> CRF 22 (long-term storage) -----> Lossy  CRF 28 (final)