MKV - All streams marked as default when trying to remove all dispositions

[nheelyolis]

Summary of the bug

When trying to demux or encode an MKV file to another MKV file, all streams get marked as "default" when I'm trying to delete all dispositions in that file with -disposition[:stream_specifier] 0. You can't see it during the process itself, but if you do a ffprobe on the resulted file, you'll see that all its streams are marked as default.

Even if it's expected behavior (which doesn't look so to me), why doesn't it say that the output streams will be default during encoding/demuxing?

How to reproduce

1. Take any MKV file which has at least 1 stream marked as default.
2. Demux this file deleting all its dispositions.

   % ffmpeg -i in.mkv -map 0 -disposition:v:0 0 -c copy out.mkv
   ffmpeg version N-94959-g96f1d34d0c Copyright (c) 2000-2019 the FFmpeg developers
     built with gcc 9.1.0 (GCC)
     configuration: 
     libavutil      56. 35.100 / 56. 35.100
     libavcodec     58. 57.100 / 58. 57.100
     libavformat    58. 32.104 / 58. 32.104
     libavdevice    58.  9.100 / 58.  9.100
     libavfilter     7. 58.102 /  7. 58.102
     libswscale      5.  6.100 /  5.  6.100
     libswresample   3.  6.100 /  3.  6.100
   Input #0, matroska,webm, from 'in.mkv':
     Metadata:
       ENCODER         : Lavf58.29.100
     Duration: 00:14:48.06, start: -0.007000, bitrate: 3437 kb/s
       Stream #0:0(eng): Video: vp8, yuv420p(progressive), 1920x818, SAR 1:1 DAR 960:409, 24 fps, 24 tbr, 1k tbn, 1k tbc (default)
       Metadata:
         DURATION        : 00:14:48.055000000
       Stream #0:1(eng): Audio: opus, 48000 Hz, stereo, fltp
       Metadata:
         DURATION        : 00:14:48.061000000
       Stream #0:2(eng): Subtitle: webvtt
       Metadata:
         DURATION        : 00:10:29.807000000
   Output #0, matroska, to 'out.mkv':
     Metadata:
       encoder         : Lavf58.32.104
       Stream #0:0(eng): Video: vp8 (VP80 / 0x30385056), yuv420p(progressive), 1920x818 [SAR 1:1 DAR 960:409], q=2-31, 24 fps, 24 tbr, 1k tbn, 1k tbc
       Metadata:
         DURATION        : 00:14:48.055000000
       Stream #0:1(eng): Audio: opus ([255][255][255][255] / 0xFFFFFFFF), 48000 Hz, stereo, fltp
       Metadata:
         DURATION        : 00:14:48.061000000
       Stream #0:2(eng): Subtitle: webvtt
       Metadata:
         DURATION        : 00:10:29.807000000
   Stream mapping:
     Stream #0:0 -> #0:0 (copy)
     Stream #0:1 -> #0:1 (copy)
     Stream #0:2 -> #0:2 (copy)
   Press [q] to stop, [?] for help
   frame=22308 fps=13499 q=-1.0 Lsize=  372673kB time=00:14:48.04 bitrate=3437.8kbits/s speed= 537x    
   video:358428kB audio:13768kB subtitle:1kB other streams:0kB global headers:0kB muxing overhead: 0.127906%
   

3. Probe the resulted file and notice that all the streams are "default".

   % ffprobe out.mkv
   ffprobe version N-94959-g96f1d34d0c Copyright (c) 2007-2019 the FFmpeg developers
     built with gcc 9.1.0 (GCC)
     configuration: 
     libavutil      56. 35.100 / 56. 35.100
     libavcodec     58. 57.100 / 58. 57.100
     libavformat    58. 32.104 / 58. 32.104
     libavdevice    58.  9.100 / 58.  9.100
     libavfilter     7. 58.102 /  7. 58.102
     libswscale      5.  6.100 /  5.  6.100
     libswresample   3.  6.100 /  3.  6.100
   Input #0, matroska,webm, from 'out.mkv':
     Metadata:
       ENCODER         : Lavf58.32.104
     Duration: 00:14:48.06, start: -0.007000, bitrate: 3437 kb/s
       Stream #0:0(eng): Video: vp8, yuv420p(progressive), 1920x818, SAR 1:1 DAR 960:409, 24 fps, 24 tbr, 1k tbn, 1k tbc (default)
       Metadata:
         DURATION        : 00:14:48.062000000
       Stream #0:1(eng): Audio: opus, 48000 Hz, stereo, fltp (default)
       Metadata:
         DURATION        : 00:14:48.061000000
       Stream #0:2(eng): Subtitle: webvtt (default)
       Metadata:
         DURATION        : 00:10:29.814000000
   

[Balling]

What is default if all is default, BTW?

[nheelyolis]

Replying to Balling:

What is default if all is default, BTW?

Even when you have only one stream of each type, having disposition set to default does make a difference. It tells (in my understanding) that the stream absolutely should be used. For example, setting default disposition to subtitles forces the media player (I personally use mpv) to enable the subtitles. While normally you would have to enable them manually.

And when you have more than one stream of some type, setting all of them to default doesn't seem to make any sense. You obviously can't play two video streams at the same time, or two audio streams, etc. So it becomes up to media player to choose a stream, which leads to the same file being played differently by different media players. Because as far as I'm aware, this behavior is not defined anywhere. You are very welcome to correct me if I'm wrong.

[Balling]

You obviously can't play two video streams at the same time

You can and have to in Dolby Vision dual layer... Just saying. Of course it should be reconstructed actually to one stream...

two audio streams

Technically 7.1 EAC3 on Blu-rays is dual stream...

this behavior is not defined anywhere

Well, the logical choice would be to choose stream with smallest ID

[nheelyolis]

Replying to Balling:

You can and have to in Dolby Vision dual layer... Just saying. Of course it should be reconstructed actually to one stream...
Technically 7.1 EAC3 on Blu-rays is dual stream...

It's good to know, but it hardly has to do anything with dispositions. My point was that default disposition isn't meant to be set on multiple streams of the same type. And even if it's meant to, the user probably would want to do that explicitly.

Well, the logical choice would be to choose stream with smallest ID

It's logical, but it's still undefined. mpv for example does just that, while vlc tries to select subtitles with the same language as video (at least I think so, from my experiments).

[mkver]

Fixed in e3c54b549abf2798422245582c0ed775c894ddeb. Your aim can be achieved by removing all dispositions (at least the "default" bit) and using the passthrough mode.