-dump_attachment option is poorly documented and/or doesn't work

[slhck]

The FFmpeg documentation gives the following example for dumping attachments from MKV files:

To extract all attachments to files determined by the filename tag:  
ffmpeg -dump_attachment:t "" INPUT

There are a few confusing points about this example:

• -dump_attachment seems to be a per-file option, and not a global one, so it should come _after_ the input.
• The input file would be missing the -i option altogether

Because of this, FFmpeg just overwrites and empties the input file when using the above command.

Here's an example with an attachment created with mkvmerge:

mkvmerge -o out.mkv -A in.mp4 \
--attachment-description "Test Attachment" \
--attachment-mime-type image/jpeg \
--attach-file out.jpg
mkvmerge v5.0.1 ('Es ist Sommer') built on Jul 30 2012 19:32:42
'in.mp4': Using the Quicktime/MP4 demultiplexer.
'in.mp4' track 1: Using the MPEG-4 part 10 (AVC) video output module.
The file 'out.mkv' has been opened for writing.
'in.mp4' track 1: Extracted the aspect ratio information from the MPEG-4 layer 10 (AVC) video data and set the display dimensions to 1280/720.
Progress: 100%
The cue entries (the index) are being written...
Muxing took 0 seconds.

Checking whether the file really contains an attachment:

mkvmerge -i out.mkv
File 'out.mkv': container: Matroska
Track ID 1: video (V_MPEG4/ISO/AVC)
Attachment ID 1: type 'image/jpeg', size 45690 bytes, description 'Test Attachment', file name 'out.jpg'

Then, with FFmpeg:

ffmpeg -dump_attachment:t "" out.mkv
ffmpeg version 1.1.2 Copyright (c) 2000-2013 the FFmpeg developers
  built on Feb  8 2013 22:55:29 with Apple LLVM version 4.2 (clang-425.0.24) (based on LLVM 3.2svn)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/1.1.2 --enable-shared --enable-pthreads --enable-gpl --enable-version3 --enable-nonfree --enable-hardcoded-tables --enable-avresample --cc=cc --host-cflags= --host-ldflags= --enable-libx264 --enable-libfaac --enable-libmp3lame --enable-libxvid --enable-libfreetype --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libvo-aacenc --enable-ffplay --enable-libfdk-aac --enable-libopus --enable-libopenjpeg --extra-cflags='-I/usr/local/Cellar/openjpeg/1.5.1/include/openjpeg-1.5 '
  libavutil      52. 13.100 / 52. 13.100
  libavcodec     54. 86.100 / 54. 86.100
  libavformat    54. 59.106 / 54. 59.106
  libavdevice    54.  3.102 / 54.  3.102
  libavfilter     3. 32.100 /  3. 32.100
  libswscale      2.  1.103 /  2.  1.103
  libswresample   0. 17.102 /  0. 17.102
  libpostproc    52.  2.100 / 52.  2.100
File 'out.mkv' already exists. Overwrite ? [y/N] 

Naturally, pressing y here would just overwrite the file and do nothing.
This also doesn't work, as a per-file option. It doesn't dump the attachment.

ffmpeg -i out.mkv -dump_attachment "" -c copy out2.mkv
ffmpeg version 1.1.2 Copyright (c) 2000-2013 the FFmpeg developers
  built on Feb  8 2013 22:55:29 with Apple LLVM version 4.2 (clang-425.0.24) (based on LLVM 3.2svn)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/1.1.2 --enable-shared --enable-pthreads --enable-gpl --enable-version3 --enable-nonfree --enable-hardcoded-tables --enable-avresample --cc=cc --host-cflags= --host-ldflags= --enable-libx264 --enable-libfaac --enable-libmp3lame --enable-libxvid --enable-libfreetype --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libvo-aacenc --enable-ffplay --enable-libfdk-aac --enable-libopus --enable-libopenjpeg --extra-cflags='-I/usr/local/Cellar/openjpeg/1.5.1/include/openjpeg-1.5 '
  libavutil      52. 13.100 / 52. 13.100
  libavcodec     54. 86.100 / 54. 86.100
  libavformat    54. 59.106 / 54. 59.106
  libavdevice    54.  3.102 / 54.  3.102
  libavfilter     3. 32.100 /  3. 32.100
  libswscale      2.  1.103 /  2.  1.103
  libswresample   0. 17.102 /  0. 17.102
  libpostproc    52.  2.100 / 52.  2.100
Input #0, matroska,webm, from 'out.mkv':
  Metadata:
    creation_time   : 2013-02-23 14:22:56
  Duration: 00:03:31.48, start: 0.000000, bitrate: 2232 kb/s
    Stream #0:0(eng): Video: h264 (High), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], 25 fps, 25 tbr, 1k tbn, 50 tbc (default)
    Stream #0:1: Attachment: mjpeg
    Metadata:
      filename        : out.jpg
      mimetype        : image/jpeg
Output #0, matroska, to 'out2.mkv':
  Metadata:
    encoder         : Lavf54.59.106
    Stream #0:0(eng): Video: h264 (H264 / 0x34363248), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], q=2-31, 25 fps, 1k tbn, 1k tbc (default)
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
Press [q] to stop, [?] for help
frame= 5287 fps=0.0 q=-1.0 Lsize=   57601kB time=00:03:31.36 bitrate=2232.5kbits/s    
video:57559kB audio:0kB subtitle:0 global headers:0kB muxing overhead 0.071764%

Note:

• In the above examples, substituting -dump_attachment:t for -dump_attachment doesn't make a difference.
• The defect is the same in the Git master head.

[cehoyos]

http://ffmpeg.org/ffmpeg.html#Main-options

Patch sent, try "ffmpeg -dump_attachment:t "" -i out.mkv"

[slhck]

Replying to cehoyos:

http://ffmpeg.org/ffmpeg.html#Main-options

Patch sent, try "ffmpeg -dump_attachment:t "" -i out.mkv"

Yes, this does the job just fine. Thank you!

The only remaining issue is that there's no indicator of a successful command here—FFmpeg will still say that no output file has been specified and you'll have to look in your folder for the written attachments. Maybe it could be a little more verbose?

[cehoyos]

Patch applied.