FFmpeg fails to decode MKV attachment without an extra step of -dump_attachment

[Sun_Serega]

Summary of the bug:

I want to extract and immediately convert the "cover.webp" attachment to a BMP file.

It works if I first dump all attachments and then convert the needed one.

---

How to reproduce:

In my case the files with attachments are created with yt-dlp, for instance:

yt-dlp.exe -o 0.mkv --embed-thumbnail --merge-output-format mkv https://youtu.be/VQjKO5dEgBA

Trying to get info about the created "0.mkv" file:

ffprobe -analyzeduration "200M" -probesize "200M" -i 0.mkv
ffprobe version 2023-07-19-git-efa6cec759-full_build-www.gyan.dev Copyright (c) 2007-2023 the FFmpeg developers
  built with gcc 12.2.0 (Rev10, Built by MSYS2 project)
  configuration: --enable-gpl --enable-version3 --enable-static --disable-w32threads --disable-autodetect --enable-fontconfig --enable-iconv --enable-gnutls --enable-libxml2 --enable-gmp --enable-bzlib --enable-lzma --enable-libsnappy --enable-zlib --enable-librist --enable-libsrt --enable-libssh --enable-libzmq --enable-avisynth --enable-libbluray --enable-libcaca --enable-sdl2 --enable-libaribb24 --enable-libaribcaption --enable-libdav1d --enable-libdavs2 --enable-libuavs3d --enable-libzvbi --enable-librav1e --enable-libsvtav1 --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs2 --enable-libxvid --enable-libaom --enable-libjxl --enable-libopenjpeg --enable-libvpx --enable-mediafoundation --enable-libass --enable-frei0r --enable-libfreetype --enable-libfribidi --enable-libharfbuzz --enable-liblensfun --enable-libvidstab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-ffnvcodec --enable-nvdec --enable-nvenc --enable-d3d11va --enable-dxva2 --enable-libvpl --enable-libshaderc --enable-vulkan --enable-libplacebo --enable-opencl --enable-libcdio --enable-libgme --enable-libmodplug --enable-libopenmpt --enable-libopencore-amrwb --enable-libmp3lame --enable-libshine --enable-libtheora --enable-libtwolame --enable-libvo-amrwbenc --enable-libcodec2 --enable-libilbc --enable-libgsm --enable-libopencore-amrnb --enable-libopus --enable-libspeex --enable-libvorbis --enable-ladspa --enable-libbs2b --enable-libflite --enable-libmysofa --enable-librubberband --enable-libsoxr --enable-chromaprint
  libavutil      58. 14.100 / 58. 14.100
  libavcodec     60. 22.100 / 60. 22.100
  libavformat    60. 10.100 / 60. 10.100
  libavdevice    60.  2.101 / 60.  2.101
  libavfilter     9.  8.102 /  9.  8.102
  libswscale      7.  3.100 /  7.  3.100
  libswresample   4. 11.100 /  4. 11.100
  libpostproc    57.  2.100 / 57.  2.100
[matroska,webm @ 00000233b5e07400] Could not find codec parameters for stream 2 (Attachment: none): unknown codec
Consider increasing the value for the 'analyzeduration' (200000000) and 'probesize' (200000000) options
Input #0, matroska,webm, from '0.mkv':
  Metadata:
    ENCODER         : Lavf60.10.100
  Duration: 00:00:56.49, start: 0.000000, bitrate: 2200 kb/s
  Stream #0:0(eng): Video: vp9 (Profile 0), yuv420p(tv, bt709), 1920x1080, SAR 1:1 DAR 16:9, 29.97 fps, 29.97 tbr, 1k tbn (default)
    Metadata:
      DURATION        : 00:00:56.456000000
  Stream #0:1(eng): Audio: opus, 48000 Hz, stereo, fltp (default)
    Metadata:
      DURATION        : 00:00:56.488000000
  Stream #0:2: Attachment: none
    Metadata:
      filename        : cover.webp
      mimetype        : image/webp
Unsupported codec with id 0 for input stream 2

Or to extract the second stream directly:

ffmpeg -i 0.mkv -map 0:2 0.webp
ffmpeg version 2023-07-19-git-efa6cec759-full_build-www.gyan.dev Copyright (c) 2000-2023 the FFmpeg developers
  built with gcc 12.2.0 (Rev10, Built by MSYS2 project)
  configuration: --enable-gpl --enable-version3 --enable-static --disable-w32threads --disable-autodetect --enable-fontconfig --enable-iconv --enable-gnutls --enable-libxml2 --enable-gmp --enable-bzlib --enable-lzma --enable-libsnappy --enable-zlib --enable-librist --enable-libsrt --enable-libssh --enable-libzmq --enable-avisynth --enable-libbluray --enable-libcaca --enable-sdl2 --enable-libaribb24 --enable-libaribcaption --enable-libdav1d --enable-libdavs2 --enable-libuavs3d --enable-libzvbi --enable-librav1e --enable-libsvtav1 --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs2 --enable-libxvid --enable-libaom --enable-libjxl --enable-libopenjpeg --enable-libvpx --enable-mediafoundation --enable-libass --enable-frei0r --enable-libfreetype --enable-libfribidi --enable-libharfbuzz --enable-liblensfun --enable-libvidstab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-ffnvcodec --enable-nvdec --enable-nvenc --enable-d3d11va --enable-dxva2 --enable-libvpl --enable-libshaderc --enable-vulkan --enable-libplacebo --enable-opencl --enable-libcdio --enable-libgme --enable-libmodplug --enable-libopenmpt --enable-libopencore-amrwb --enable-libmp3lame --enable-libshine --enable-libtheora --enable-libtwolame --enable-libvo-amrwbenc --enable-libcodec2 --enable-libilbc --enable-libgsm --enable-libopencore-amrnb --enable-libopus --enable-libspeex --enable-libvorbis --enable-ladspa --enable-libbs2b --enable-libflite --enable-libmysofa --enable-librubberband --enable-libsoxr --enable-chromaprint
  libavutil      58. 14.100 / 58. 14.100
  libavcodec     60. 22.100 / 60. 22.100
  libavformat    60. 10.100 / 60. 10.100
  libavdevice    60.  2.101 / 60.  2.101
  libavfilter     9.  8.102 /  9.  8.102
  libswscale      7.  3.100 /  7.  3.100
  libswresample   4. 11.100 /  4. 11.100
  libpostproc    57.  2.100 / 57.  2.100
[matroska,webm @ 0000028a8c1517c0] Could not find codec parameters for stream 2 (Attachment: none): unknown codec
Consider increasing the value for the 'analyzeduration' (0) and 'probesize' (5000000) options
Input #0, matroska,webm, from '0.mkv':
  Metadata:
    ENCODER         : Lavf60.10.100
  Duration: 00:00:56.49, start: 0.000000, bitrate: 2200 kb/s
  Stream #0:0(eng): Video: vp9 (Profile 0), yuv420p(tv, bt709), 1920x1080, SAR 1:1 DAR 16:9, 29.97 fps, 29.97 tbr, 1k tbn (default)
    Metadata:
      DURATION        : 00:00:56.456000000
  Stream #0:1(eng): Audio: opus, 48000 Hz, stereo, fltp (default)
    Metadata:
      DURATION        : 00:00:56.488000000
  Stream #0:2: Attachment: none
    Metadata:
      filename        : cover.webp
      mimetype        : image/webp
[webp @ 0000028a8c18d540] Only WebP is supported
[out#0/webp @ 0000028a8ca3cec0] Could not write header (incorrect codec parameters ?): Invalid argument
Error opening output file 0.webp.
Error opening output files: Invalid argument

ffmpeg -i 0.mkv -map 0:2 0.bmp
ffmpeg version 2023-07-19-git-efa6cec759-full_build-www.gyan.dev Copyright (c) 2000-2023 the FFmpeg developers
  built with gcc 12.2.0 (Rev10, Built by MSYS2 project)
  configuration: --enable-gpl --enable-version3 --enable-static --disable-w32threads --disable-autodetect --enable-fontconfig --enable-iconv --enable-gnutls --enable-libxml2 --enable-gmp --enable-bzlib --enable-lzma --enable-libsnappy --enable-zlib --enable-librist --enable-libsrt --enable-libssh --enable-libzmq --enable-avisynth --enable-libbluray --enable-libcaca --enable-sdl2 --enable-libaribb24 --enable-libaribcaption --enable-libdav1d --enable-libdavs2 --enable-libuavs3d --enable-libzvbi --enable-librav1e --enable-libsvtav1 --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs2 --enable-libxvid --enable-libaom --enable-libjxl --enable-libopenjpeg --enable-libvpx --enable-mediafoundation --enable-libass --enable-frei0r --enable-libfreetype --enable-libfribidi --enable-libharfbuzz --enable-liblensfun --enable-libvidstab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-ffnvcodec --enable-nvdec --enable-nvenc --enable-d3d11va --enable-dxva2 --enable-libvpl --enable-libshaderc --enable-vulkan --enable-libplacebo --enable-opencl --enable-libcdio --enable-libgme --enable-libmodplug --enable-libopenmpt --enable-libopencore-amrwb --enable-libmp3lame --enable-libshine --enable-libtheora --enable-libtwolame --enable-libvo-amrwbenc --enable-libcodec2 --enable-libilbc --enable-libgsm --enable-libopencore-amrnb --enable-libopus --enable-libspeex --enable-libvorbis --enable-ladspa --enable-libbs2b --enable-libflite --enable-libmysofa --enable-librubberband --enable-libsoxr --enable-chromaprint
  libavutil      58. 14.100 / 58. 14.100
  libavcodec     60. 22.100 / 60. 22.100
  libavformat    60. 10.100 / 60. 10.100
  libavdevice    60.  2.101 / 60.  2.101
  libavfilter     9.  8.102 /  9.  8.102
  libswscale      7.  3.100 /  7.  3.100
  libswresample   4. 11.100 /  4. 11.100
  libpostproc    57.  2.100 / 57.  2.100
[matroska,webm @ 000002455fd517c0] Could not find codec parameters for stream 2 (Attachment: none): unknown codec
Consider increasing the value for the 'analyzeduration' (0) and 'probesize' (5000000) options
Input #0, matroska,webm, from '0.mkv':
  Metadata:
    ENCODER         : Lavf60.10.100
  Duration: 00:00:56.49, start: 0.000000, bitrate: 2200 kb/s
  Stream #0:0(eng): Video: vp9 (Profile 0), yuv420p(tv, bt709), 1920x1080, SAR 1:1 DAR 16:9, 29.97 fps, 29.97 tbr, 1k tbn (default)
    Metadata:
      DURATION        : 00:00:56.456000000
  Stream #0:1(eng): Audio: opus, 48000 Hz, stereo, fltp (default)
    Metadata:
      DURATION        : 00:00:56.488000000
  Stream #0:2: Attachment: none
    Metadata:
      filename        : cover.webp
      mimetype        : image/webp
Output #0, image2, to '0.bmp':
  Metadata:
    encoder         : Lavf60.10.100
  Stream #0:0: Attachment: none
    Metadata:
      filename        : cover.webp
      mimetype        : image/webp
Stream mapping:
  Stream #0:2 -> #0:0 (copy)
Press [q] to stop, [?] for help
[out#0/image2 @ 000002456064cec0] video:0kB audio:0kB subtitle:0kB other streams:0kB global headers:54kB muxing overhead: unknown
[out#0/image2 @ 000002456064cec0] Output file is empty, nothing was encoded
size=N/A time=N/A bitrate=N/A speed=N/A

But if I first dump it:

ffmpeg -dump_attachment:t "" -i 0.mkv
ffmpeg version 2023-07-19-git-efa6cec759-full_build-www.gyan.dev Copyright (c) 2000-2023 the FFmpeg developers
  built with gcc 12.2.0 (Rev10, Built by MSYS2 project)
  configuration: --enable-gpl --enable-version3 --enable-static --disable-w32threads --disable-autodetect --enable-fontconfig --enable-iconv --enable-gnutls --enable-libxml2 --enable-gmp --enable-bzlib --enable-lzma --enable-libsnappy --enable-zlib --enable-librist --enable-libsrt --enable-libssh --enable-libzmq --enable-avisynth --enable-libbluray --enable-libcaca --enable-sdl2 --enable-libaribb24 --enable-libaribcaption --enable-libdav1d --enable-libdavs2 --enable-libuavs3d --enable-libzvbi --enable-librav1e --enable-libsvtav1 --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs2 --enable-libxvid --enable-libaom --enable-libjxl --enable-libopenjpeg --enable-libvpx --enable-mediafoundation --enable-libass --enable-frei0r --enable-libfreetype --enable-libfribidi --enable-libharfbuzz --enable-liblensfun --enable-libvidstab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-ffnvcodec --enable-nvdec --enable-nvenc --enable-d3d11va --enable-dxva2 --enable-libvpl --enable-libshaderc --enable-vulkan --enable-libplacebo --enable-opencl --enable-libcdio --enable-libgme --enable-libmodplug --enable-libopenmpt --enable-libopencore-amrwb --enable-libmp3lame --enable-libshine --enable-libtheora --enable-libtwolame --enable-libvo-amrwbenc --enable-libcodec2 --enable-libilbc --enable-libgsm --enable-libopencore-amrnb --enable-libopus --enable-libspeex --enable-libvorbis --enable-ladspa --enable-libbs2b --enable-libflite --enable-libmysofa --enable-librubberband --enable-libsoxr --enable-chromaprint
  libavutil      58. 14.100 / 58. 14.100
  libavcodec     60. 22.100 / 60. 22.100
  libavformat    60. 10.100 / 60. 10.100
  libavdevice    60.  2.101 / 60.  2.101
  libavfilter     9.  8.102 /  9.  8.102
  libswscale      7.  3.100 /  7.  3.100
  libswresample   4. 11.100 /  4. 11.100
  libpostproc    57.  2.100 / 57.  2.100
[matroska,webm @ 000002c5d1e6b540] Could not find codec parameters for stream 2 (Attachment: none): unknown codec
Consider increasing the value for the 'analyzeduration' (0) and 'probesize' (5000000) options
Input #0, matroska,webm, from '0.mkv':
  Metadata:
    ENCODER         : Lavf60.10.100
  Duration: 00:00:56.49, start: 0.000000, bitrate: 2200 kb/s
  Stream #0:0(eng): Video: vp9 (Profile 0), yuv420p(tv, bt709), 1920x1080, SAR 1:1 DAR 16:9, 29.97 fps, 29.97 tbr, 1k tbn (default)
    Metadata:
      DURATION        : 00:00:56.456000000
  Stream #0:1(eng): Audio: opus, 48000 Hz, stereo, fltp (default)
    Metadata:
      DURATION        : 00:00:56.488000000
  Stream #0:2: Attachment: none
    Metadata:
      filename        : cover.webp
      mimetype        : image/webp
[aist#0:2/none @ 000002c5d25f4680] Wrote attachment (55132 bytes) to 'cover.webp'
At least one output file must be specified

Now suddenly it can be decoded:

ffmpeg -i cover.webp 0.bmp
ffmpeg version 2023-07-19-git-efa6cec759-full_build-www.gyan.dev Copyright (c) 2000-2023 the FFmpeg developers
  built with gcc 12.2.0 (Rev10, Built by MSYS2 project)
  configuration: --enable-gpl --enable-version3 --enable-static --disable-w32threads --disable-autodetect --enable-fontconfig --enable-iconv --enable-gnutls --enable-libxml2 --enable-gmp --enable-bzlib --enable-lzma --enable-libsnappy --enable-zlib --enable-librist --enable-libsrt --enable-libssh --enable-libzmq --enable-avisynth --enable-libbluray --enable-libcaca --enable-sdl2 --enable-libaribb24 --enable-libaribcaption --enable-libdav1d --enable-libdavs2 --enable-libuavs3d --enable-libzvbi --enable-librav1e --enable-libsvtav1 --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs2 --enable-libxvid --enable-libaom --enable-libjxl --enable-libopenjpeg --enable-libvpx --enable-mediafoundation --enable-libass --enable-frei0r --enable-libfreetype --enable-libfribidi --enable-libharfbuzz --enable-liblensfun --enable-libvidstab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-ffnvcodec --enable-nvdec --enable-nvenc --enable-d3d11va --enable-dxva2 --enable-libvpl --enable-libshaderc --enable-vulkan --enable-libplacebo --enable-opencl --enable-libcdio --enable-libgme --enable-libmodplug --enable-libopenmpt --enable-libopencore-amrwb --enable-libmp3lame --enable-libshine --enable-libtheora --enable-libtwolame --enable-libvo-amrwbenc --enable-libcodec2 --enable-libilbc --enable-libgsm --enable-libopencore-amrnb --enable-libopus --enable-libspeex --enable-libvorbis --enable-ladspa --enable-libbs2b --enable-libflite --enable-libmysofa --enable-librubberband --enable-libsoxr --enable-chromaprint
  libavutil      58. 14.100 / 58. 14.100
  libavcodec     60. 22.100 / 60. 22.100
  libavformat    60. 10.100 / 60. 10.100
  libavdevice    60.  2.101 / 60.  2.101
  libavfilter     9.  8.102 /  9.  8.102
  libswscale      7.  3.100 /  7.  3.100
  libswresample   4. 11.100 /  4. 11.100
  libpostproc    57.  2.100 / 57.  2.100
Input #0, webp_pipe, from 'cover.webp':
  Duration: N/A, bitrate: N/A
  Stream #0:0: Video: webp, yuv420p(tv, bt470bg/unknown/unknown), 1280x720, 25 fps, 25 tbr, 25 tbn
Stream mapping:
  Stream #0:0 -> #0:0 (webp (native) -> bmp (native))
Press [q] to stop, [?] for help
Output #0, image2, to '0.bmp':
  Metadata:
    encoder         : Lavf60.10.100
  Stream #0:0: Video: bmp, bgr24(pc, gbr/unknown/unknown, progressive), 1280x720, q=2-31, 200 kb/s, 25 fps, 25 tbn
    Metadata:
      encoder         : Lavc60.22.100 bmp
[image2 @ 000001d812ec6d40] The specified filename '0.bmp' does not contain an image sequence pattern or a pattern is invalid.
[image2 @ 000001d812ec6d40] Use a pattern such as %03d for an image sequence or use the -update option (with -frames:v 1 if needed) to write a single image.
[out#0/image2 @ 000001d8129ee9c0] video:2700kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
frame=    1 fps=0.0 q=-0.0 Lsize=N/A time=00:00:00.00 bitrate=N/A speed=   0x

[mkver]

webp is not in the list of image tags in lavf/matroskadec.c. Adding it would fix this; but a better solution is to use the mime types of the relevant AVCodecDescriptor. I'll take care of this.

[Sun_Serega]

Replying to mkver:

webp is not in the list of image tags in lavf/matroskadec.c. Adding it would fix this; but a better solution is to use the mime types of the relevant AVCodecDescriptor. I'll take care of this.

Thanks. Is there maybe any better workaround, than -dump_attachment? Like maybe specifying the codec manually?

[Sun_Serega]

Replying to Sun_Serega:

I couldn't get the - (default pipe) to work with -dump_attachment, but now I tried pipe:1 and it worked:

ffmpeg -dump_attachment:2 pipe:1 -i 0.mkv | ffmpeg -i - 0.png

(:2 is a stream index)

Still 2 commands, but at least, this is a better workaround than dumping the attachment to a file...