Re: [FFmpeg-devel] [PATCH 02/18] fftools/ffmpeg_filter: refactor setting input timebase

From: Anton Khirnov <anton@khirnov.net>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
Subject: Re: [FFmpeg-devel] [PATCH 02/18] fftools/ffmpeg_filter: refactor setting input timebase
Date: Sun, 10 Mar 2024 23:24:30 +0100
Message-ID: <171010947057.7287.8642196154964262055@lain.khirnov.net> (raw)
In-Reply-To: <20240310192147.GQ6420@pb2>

Quoting Michael Niedermayer (2024-03-10 20:21:47)
> On Sun, Mar 10, 2024 at 07:13:18AM +0100, Anton Khirnov wrote:
> > Quoting Michael Niedermayer (2024-03-10 04:36:29)
> > > On Fri, Mar 08, 2024 at 06:34:36AM +0100, Anton Khirnov wrote:
> > > > Quoting Michael Niedermayer (2024-03-07 21:37:39)
> > > > > On Wed, Mar 06, 2024 at 12:03:03PM +0100, Anton Khirnov wrote:
> > > > > > Treat it analogously to stream parameters like format/dimensions/etc.
> > > > > > This is functionally different from previous code in 2 ways:
> > > > > > * for non-CFR video, the frame timebase (set by the decoder) is used
> > > > > >   rather than the demuxer timebase
> > > > > > * for sub2video, AV_TIME_BASE_Q is used, which is hardcoded by the
> > > > > >   subtitle decoding API
> > > > > > 
> > > > > > These changes should avoid unnecessary and potentially lossy timestamp
> > > > > > conversions from decoder timebase into the demuxer one.
> > > > > > 
> > > > > > Changes the timebases used in sub2video tests.
> > > > > > ---
> > > > > >  fftools/ffmpeg_filter.c               |  17 ++-
> > > > > >  tests/ref/fate/sub2video_basic        | 182 +++++++++++++-------------
> > > > > >  tests/ref/fate/sub2video_time_limited |   8 +-
> > > > > >  3 files changed, 106 insertions(+), 101 deletions(-)
> > > > > 
> > > > > breaks:
> > > > > 
> > > > > ./ffmpeg -i \[a-s\]_full_metal_panic_fumoffu_-_01_-_the_man_from_the_south_-_a_hostage_with_no_compromises__rs2_\[1080p_bd-rip\]\[BBB48A25\].mkv  -filter_complex '[0:s:1]scale=800:600' -t 15 -qscale 2 -y a.avi
> > > > > 
> > > > 
> > > > Use a constant framerate.
> > > 
> > > why not automatically choose a supported timebase  ?
> > > 
> > > "[mpeg4 @ 0x55973c869f00] timebase 1/1000000 not supported by MPEG 4 standard, the maximum admitted value for the timebase denominator is 65535"
> > 
> > Because I don't want ffmpeg CLI to have codec-specific code for a codec
> > that's been obsolete for 15+ years. One could also potentially do it
> > inside the encoder itself, but it is nontrivial since the computations
> > are spread across a number of places in mpeg4videoenc.c and
> > mpegvideo_enc.c. And again, it seems like a waste of time - there is no
> > reason to encode mpeg4 today.
> 
> This is not mpeg4 specific, its just a new additional case that fails

The case you reported is mpeg4 specific.

> ./ffmpeg -i mm-small.mpg test.dv
> [dvvideo @ 0x7f868800f100] Found no DV profile for 80x60 yuv420p video. Valid DV profiles are:

There is no mechanism for an encoder to export supported time bases.

> IMHO ffmpeg should be able to select supported parameters if the user
> indicated thats what he wants
> 
> We also do this with pixel formats and not fail and require the user to manually
> specify it

AFAIK only a tiny number of obsolete encoders place restrictions on the
timebase, so I see little point in spending effort on making them work
automagically.

-- 
Anton Khirnov
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