Re: [gentoo-user] FFmpeg and AVI1'd M-JPEGs

[Michael]

On Monday, 20 July 2020 19:10:17 BST Ashley Dixon wrote:
> Hi List,
>
> A while ago, I made the grave mistake of buying  a  Logitech  C270  webcam
> [1], which initially seemed to be well-supported under Linux as a U.V.C.\
> camera [2]. As shown by the following  (partial)  output  of  `v4l2-ctl
> --list-formats-ext` suggests, the full 720p@30fps is only supported when
> using MJPG.
>
>         ioctl: VIDIOC_ENUM_FMT
>                 Type: Video Capture
>
>                 [0]: 'YUYV' (YUYV 4:2:2)
>                         [...]
>                         Size: Discrete 1280x720
>                                 Interval: Discrete 0.133s (7.500 fps)
>                                 Interval: Discrete 0.200s (5.000 fps)
>                         [...]
>
>                 [1]: 'MJPG' (Motion-JPEG, compressed)
>                         Size: Discrete 1280x720
>                                 Interval: Discrete 0.033s (30.000 fps)
>                                 Interval: Discrete 0.040s (25.000 fps)
>                                 Interval: Discrete 0.050s (20.000 fps)
>                                 Interval: Discrete 0.067s (15.000 fps)
>                                 Interval: Discrete 0.100s (10.000 fps)
>                                 Interval: Discrete 0.200s (5.000 fps)
>                         [...]
>
> Unfortunately, as per [3], Logitech C-series cameras (among  others)
> utilise  a proprietary extension to the Motion-JPEG format, in which H.264
> data is attached to the various JPEG key and delta frames in the APP0 field
> [4],  as  shown  when inspecting the data of any frame ("AVI1" [5]):
>
>         $ ffmpeg -i video.mp4 -vcodec copy %02d.jpg
>         $ xxd 01.jpg | head -n 1
>
>         00000000: ffd8 ffe0 0021 4156 4931 0001 0101 0078  .....!AVI1.....x
>                        ^^^^^^^^^^^^^^^^^^^^^^
>
> Thus, FFmpeg (and programs using FFmpeg) seem to have trouble trouble
> correctly parsing these MJPG videos, and VLC  almost  crashes  when  trying
>  to  play  one (although the Firefox integrated mp4 player does fine):
>
>         $ vlc output.mp4
>
>         [00007f2eec008aa0] main video output error: video output creation
> failed [00007f2ee8c0ae50] main decoder error: failed to create video output
>
>         $ ffmpeg -i output.mp4 somethingelse.avi
>
>         [mjpeg @ 0x5611aeee7000] unable to decode APP fields: Invalid data
> found when processing input
>
> Some work was done on libav many years ago, in an effort to provide a
> bitstream filter to ffmpeg to properly convert these to JPEG/JFIF images,
> however I  doubt it's relevant any more [6],  especially  considering  that
>  Gentoo  has  removed libav, and the corresponding USE-flag in
> `virtual/ffmpeg` is forever-masked [7].
>
> To conclude, is there any method of making  these  Logitech'd  MJPG  files
> play nicely with FFmpeg (and VLC) ?  Someone on  the  aforementioned  Stack
>  Overflow question pointed out that Skype is able to read both the outer
> MJPG  and  inner H.264 streams, however I cannot confirm this myself. So
> far, Firefox is the only program which can play these video files, and I do
> not want to use  that  as  my primary video-player !
>
> I also need to provide these files to some non-technical (Windows)  users
> at  a local College, so I want to make them as portable  and
> standards-conformant  as possible.
>
>         Thanks for any guidance,
>         Ashley.
>
> P.S. Other than this, it's an extremely good camera for someone on a budget.
>
> [1] https://www.argos.co.uk/product/4034924
> [2] https://linux-hardware.org/index.php?id=usb:046d-0825
> [3] https://stackoverflow.com/a/56403628
> [4]
> https://en.wikipedia.org/wiki/JPEG_File_Interchange_Format#JFIF_APP0_marker
> _segment [5] https://exiftool.org/TagNames/JPEG.html#AVI1
> [6]
> https://code.videolan.org/libav/libav/commit/ba83c4bfb55d5c3fc6b3959edb0d3c
> da07a70241 [7]
> https://gitweb.gentoo.org/repo/gentoo.git/tree/profiles/base/use.mask#n15

I don't have this webcam so I don't know if it will or won't play nicely with 
Linux, but at least ffmpeg seems to contain the requisite mjpeg codec:

 $ ffmpeg -hide_banner -codecs | grep mjpeg
 DEVIL. mjpeg                Motion JPEG (encoders: mjpeg mjpeg_vaapi )
 D.VIL. mjpegb               Apple MJPEG-B
 D.A.L. adpcm_ima_smjpeg     ADPCM IMA Loki SDL MJPEG

Again, I don't know what VLC can do with this.  :-/