avformat/matroskaenc: Redo handling of FlagDefault

Up until now, the Matroska muxer would mark a track as default if it had
the disposition AV_DISPOSITION_DEFAULT or if there was no track with
AV_DISPOSITION_DEFAULT set; in the latter case even more than one track
of a kind (audio, video, subtitles) was marked as default which is not
sensible.

This commit changes the logic used to mark tracks as default. There are
now three modes for this:
a) In the "infer" mode the first track of every type (audio, video,
subtitles) with default disposition set will be marked as default; if
there is no such track (for a given type), then the first track of this
type (if existing) will be marked as default. This behaviour is inspired
by mkvmerge. It ensures that the default flags will be set in a sensible
way even if the input comes from containers that lack the concept of
default flags. This mode is the default mode.
b) The "infer_no_subs" mode is similar to the "infer" mode; the
difference is that if no subtitle track with default disposition exists,
no subtitle track will be marked as default at all.
c) The "passthrough" mode: Here the track will be marked as default if
and only the corresponding input stream had disposition default.

This fixes ticket #8173 (the passthrough mode is ideal for this) as
well as ticket #8416 (the "infer_no_subs" mode leads to the desired
output).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

1 files changed, 19 insertions, 0 deletions

diff --git a/doc/muxers.texi b/doc/muxers.texi
index d341827a86..1a1cdb910a 100644
--- a/doc/muxers.texi
+++ b/doc/muxers.texi
@@ -1384,6 +1384,25 @@ A safe size for most use cases should be about 50kB per hour of video.
 
 Note that cues are only written if the output is seekable and this option will
 have no effect if it is not.
+@item default_mode
+This option controls how the FlagDefault of the output tracks will be set.
+It influences which tracks players should play by default. The default mode
+is @samp{infer}.
+@table @samp
+@item infer
+In this mode, for each type of track (audio, video or subtitle), if there is
+a track with disposition default of this type, then the first such track
+(i.e. the one with the lowest index) will be marked as default; if no such
+track exists, the first track of this type will be marked as default instead
+(if existing). This ensures that the default flag is set in a sensible way even
+if the input originated from containers that lack the concept of default tracks.
+@item infer_no_subs
+This mode is the same as infer except that if no subtitle track with
+disposition default exists, no subtitle track will be marked as default.
+@item passthrough
+In this mode the FlagDefault is set if and only if the AV_DISPOSITION_DEFAULT
+flag is set in the disposition of the corresponding stream.
+@end table
 @end table
 
 @anchor{md5}