Re: [FFmpeg-devel] [PATCH v2 2/4] ffmpeg: Add display_matrix option

[Anton Khirnov <anton@khirnov.net>]

Quoting Gyan Doshi (2022-08-17 12:53:11)
> 
> 
> On 2022-08-17 02:35 pm, Anton Khirnov wrote:
> > Quoting Gyan Doshi (2022-08-17 10:50:43)
> >>
> >> On 2022-08-17 01:48 pm, Anton Khirnov wrote:
> >>> Quoting Thilo Borgmann (2022-08-16 20:48:57)
> >>>> Am 16.08.22 um 16:10 schrieb Anton Khirnov:
> >>>>> Quoting Thilo Borgmann (2022-08-15 22:02:09)
> >>>>>> $subject
> >>>>>>
> >>>>>> -Thilo
> >>>>>>    From fe2ff114cb004f897c7774753d9cf28298eba82d Mon Sep 17 00:00:00 2001
> >>>>>> From: =?UTF-8?q?Jan=20Ekstr=C3=B6m?= <jeebjp@gmail.com>
> >>>>>> Date: Mon, 15 Aug 2022 21:09:27 +0200
> >>>>>> Subject: [PATCH v2 2/4] ffmpeg: Add display_matrix option
> >>>>>>
> >>>>>> This enables overriding the rotation as well as horizontal/vertical
> >>>>>> flip state of a specific video stream on the input side.
> >>>>>>
> >>>>>> Additionally, switch the singular test that was utilizing the rotation
> >>>>>> metadata to instead override the input display rotation, thus leading
> >>>>>> to the same result.
> >>>>>> ---
> >>>>> I still don't see how it's better to squash multiple options into a
> >>>>> single option.
> >>>>>
> >>>>> It requires all this extra infrastructure and in the end it's less
> >>>>> user-friendly, because user-understandable things like rotation or flips
> >>>>> are now hidden under "display matrix". How many users would know what a
> >>>>> display matrix is?
> >>>> FWIW I think Gyan's request to do this all in one option that effect one thing (the display matrix) is valid.
> >>> I don't.
> >>>
> >>> It may be one thing internally, but modeling user interfaces based on
> >>> internal representation is a sinful malpractice. More importantly, I see
> >>> no advantage from doing it - it only makes the option parsing more
> >>> complicated.
> >> It's not based on ffmpeg's 'internal representation'. All transform
> >> attributes are stored as a composite in one mathematical object.
> > Keyword "stored". It is internal representation. Users should not care
> > how it is stored, the entire point point of our project is to shield
> > them from that as much as possible.
> >
> >> Evaluating the matrix values will need to look at all sources of
> >> contribution. So gathering and presenting all these attributes in a single
> >> option (+ docs) makes it clearer to the user at the cost of an initial
> >> learning curve.
> > Are you seriously expecting all users who want to mark a video as
> > rotated or flipped to learn about display matrices?
> 
> They don't need to know how to encode or decode the matrix if they don't 
> want to. Only that it is the container.
> 
> The difference is between
> 
>   -rotate:v:0 90 -hflip:v:0 1 -scale:v:0 2
> 
> and
> 
>   -display_matrix:v:0 rotate=90:hflip=1:scale=2
> 
> The latter syntax is all too familiar to users from AVFrame filters and 
> BSFs.

The syntax similarity is misleading - filters are applied in the order
you list them, while these options are always applied in fixed order.
The analogous filters are also called rotate, [vf]flip, and scale -
there is no display_matrix filter.

-- 
Anton Khirnov
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