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

From: James Almer <jamrial@gmail.com>
To: ffmpeg-devel@ffmpeg.org
Subject: Re: [FFmpeg-devel] [PATCH 02/18] fftools/ffmpeg_filter: refactor setting input timebase
Date: Sun, 10 Mar 2024 19:29:27 -0300
Message-ID: <7d011f1c-f131-412c-85ce-318c71efba0f@gmail.com> (raw)
In-Reply-To: <171010947057.7287.8642196154964262055@lain.khirnov.net>

On 3/10/2024 7:24 PM, Anton Khirnov wrote:
> 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.

Could it be added as an extension to AVProfile, or AVCodec?

> 
>> 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.
> 
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