On Mon, 16 Apr 2018, Timo Teras wrote:
On Sun, 15 Apr 2018 16:42:01 +0200 (CEST)
Marton Balint <c...@passwd.hu> wrote:
On Sun, 15 Apr 2018, Timo Teräs wrote:
> Calculate DAR with assumed SAR 1:1 when SAR is undefined. Same
> assumption is done in ffplay to create the play window. Usually
> DAR is more useful metadata than SAR when e.g. choosing which
> media of multiple versions to use to fit the display.
I don't think it's good idea to generally assume 1:1
display_aspect_ratio for every undefined sample aspect ratio. It
depends heavily on your actual use case. If MOV/MP4 specifies that
1:1 SAR should be used, then maybe you should fix
av_guess_sample_aspect_ratio instead, and return 1:1 there if the
format context is MOV/MP4. You may add a demuxer (AVFMT) flag to
signal that such behaviour should be used, and
av_guess_sample_aspect_ratio can check for that demuxer flag.
Looking at code, av_guess_sample_aspect_ratio() is used only in ffplay
and ffprobe.
ffplay implicitly assumes undefined SAR is 1:1 to create the playback
window properly; this happens in calculate_display_rect() when
"bad" aspect_ratio is reset to 1.0.
I would expect same logic would have been useful in ffprobe. This would
help to report back to user what ffplay is going to do with the video.
Or at least give a hint on how to categorize the clip. SAR 1:1 is
pretty good guess for most formats.
I really don't see why don't you fix your application instead which parses
ffprobe output? If you see N/A aspect ratio, use 1:1.
To be frank, I am not sure if ffprobe should use av_guess_aspect_ratio
when it displays stream metadata. It is only there now to av_reduce the
aspect ratios and to sanitize some invalid aspect ratios to 0/1. FFprobe's
job is to return stream metadata as is, not to make guesses.
For this reason, my preferred solution was to patch ffprobe so we can
give a guess for all files. If the above patch is not a good idea, how
about adding new "effective_{sample,display}_aspect_ratio" fields? Or
just a flag "aspect_ratio_guessed" to tell if it's not defined in the
file?
Effective is not a good name here. Use best_effort_* or
guessed_* if you have to, but I don't see the much benefit in adding extra
fields to ffprobe output for such simple and trivial heuristics. Like I
said, it's not ffprobe's job. The next guy will want to add a
guessed_color_primaries based on resolution, and I can think of more
complicated guesses :)
I would prefer not to do any file type specific special handling if
possible. However, if that's the only acceptable solution, I'm happy to
implement that too. But then I'd prefer to have a 'no default SAR of
1:1' flags so file formats can inhibit the assumption instead of
explicitly needing to enable it. Is there any formats where this would
be useful? Or how about just making av_guess_sample_aspect_ratio()
return 1:1 in case nothing better exists? It's called "guess" after all
and is used in ffplay/ffprobe only...
Opt in for MOV/MP4 is required because *the specs says* that renderers
should behave this way. So the aspect ratio guess is based on
something, and not a wild guess. I think most of the other format specs
does not say such things.
Regards,
Marton
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