ffprobe using lowercase or uppercase for format tags

Question

I am trying to understand the behavior of ffprobe with regards to using either uppercase or lowercase convention for format tags.

Here are the steps to reproduce. Create an MP4, AVI and a MKV container from an existing MP4 movie:

$ ffmpeg -y -i input.mp4 -c:a copy -c:v copy -metadata title="This is my title" -metadata date="1234" -metadata genre="erneg" output.mp4
$ ffmpeg -y -i input.mp4 -c:a copy -c:v copy -metadata title="This is my title" -metadata date="1234" -metadata genre="erneg" output.avi
$ ffmpeg -y -i input.mp4 -c:a copy -c:v copy -metadata title="This is my title" -metadata date="1234" -metadata genre="erneg" output.mkv

Now using, either

% ffprobe -v error -hide_banner -show_format -print_format json -i output.mp4

or:

% ffprobe -v error -hide_banner -show_format -print_format json -i output.avi

I get respectively:

    "tags": {
        "major_brand": "isom",
        "minor_version": "512",
        "compatible_brands": "isomiso2mp41",
        "title": "This is my title",
        "date": "1234",
        "encoder": "Lavf58.20.100",
        "genre": "erneg"
    }

and

    "tags": {
        "date": "1234",
        "genre": "erneg",
        "title": "This is my title",
        "encoder": "Lavf58.20.100"
    }

but with MKV container, I get the uppercase convention (except for 'title'):

    "tags": {
        "title": "This is my title",
        "DATE": "1234",
        "MAJOR_BRAND": "isom",
        "MINOR_VERSION": "512",
        "COMPATIBLE_BRANDS": "isomiso2mp41",
        "GENRE": "erneg",
        "ENCODER": "Lavf58.20.100"
    }

What is the difference in between the lowercase and uppercase convention for format tags in ffprobe ?

For reference, system is Debian/buster.

% ffmpeg -version 
ffmpeg version 4.1.6-1~deb10u1 Copyright (c) 2000-2020 the FFmpeg developers
built with gcc 8 (Debian 8.3.0-6)
configuration: --prefix=/usr --extra-version='1~deb10u1' --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --arch=amd64 --enable-gpl --disable-stripping --enable-avresample --disable-filter=resample --enable-avisynth --enable-gnutls --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libcodec2 --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libjack --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librsvg --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzmq --enable-libzvbi --enable-lv2 --enable-omx --enable-openal --enable-opengl --enable-sdl2 --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-chromaprint --enable-frei0r --enable-libx264 --enable-shared
libavutil      56. 22.100 / 56. 22.100
libavcodec     58. 35.100 / 58. 35.100
libavformat    58. 20.100 / 58. 20.100
libavdevice    58.  5.100 / 58.  5.100
libavfilter     7. 40.101 /  7. 40.101
libavresample   4.  0.  0 /  4.  0.  0
libswscale      5.  3.100 /  5.  3.100
libswresample   3.  3.100 /  3.  3.100
libpostproc    55.  3.100 / 55.  3.100

user_4663670 · Answer

There is no convention for format tags in ffprobe. The difference comes from the muxer:

The TagName SHOULD always be written in all capital letters and contain no space.

Source: Matroska Tagging

Simple tags will therefore be written in uppercase in the resulting mkv file.

title is handled differently in the muxer and is converted to a Title element. Since it's not saved as a regular tag it will be shown as lowercase.

For instance mkvinfo reveals:

% mkvinfo /tmp/output.mkv
[...]
|+ Segment information
| + Timestamp scale: 1000000
| + Title: This is my title
[...]
|+ Tags
| + Tag
|  + Targets
|  + Simple
|   + Name: MAJOR_BRAND
|   + String: isom
|  + Simple
|   + Name: MINOR_VERSION
|   + String: 512
|  + Simple
|   + Name: COMPATIBLE_BRANDS
|   + String: isomiso2mp41
|  + Simple
|   + Name: GENRE
|   + String: erneg
|  + Simple
|   + Name: DATE
|   + String: 1234
|  + Simple
|   + Name: ENCODER
|   + String: Lavf58.20.100
[...]

ffprobe using lowercase or uppercase for format tags

1 Answers