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

[Gyan Doshi <ffmpeg@gyani.pro>]

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. There's hardly any overhead here.
They google or do Ctrl-F on ffmpeg.html for rotate, and they get to the 
display matrix option entry.

Regards,
Gyan
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