On Mon, Apr 16, 2018 at 09:06:55AM +0300, 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. > > 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? > > 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...
unknown is not the same as 1:1 the user (of av_guess_sample_aspect_ratio) may want to know if theres a 1:1 actually stored in some place or just nothing. applications like ffplay cannot use "unknown" so they have to pick 1:1 but why is "unknown" a problem for ffprobe if thats how it is ? [...] -- Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB Everything should be made as simple as possible, but not simpler. -- Albert Einstein
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