Re: [FFmpeg-devel] [PATCH 2/3] avisynth: use AviSynth+'s frame properties to set various fields

[Stephen Hutchinson <qyot27@gmail.com>]

On 8/24/22 1:04 PM, Steinar Apalnes wrote:
> tir. 8. feb. 2022 kl. 12:03 skrev Stephen Hutchinson <qyot27@gmail.com>:
> 
>> * Field Order
>> * Chroma Location
>> * Color Transfer Characteristics
>> * Color Range
>> * Color Primaries
>> * Matrix Coefficients
>>
>> The existing TFF/BFF detection is retained as a fallback for
>> older versions of AviSynth that can't access frame properties.
>> The other properties have no legacy equivalent to detect them.
>>
>> Signed-off-by: Stephen Hutchinson <qyot27@gmail.com>
>>
> .......
>   Hi Stephen,
> 
> Would it be possible to add support for "_SARum" and "_SARDen" so that
> ffmpeg could also recognize the sample aspect ratio in avs scripts?
> 

I'm a bit hesitant to do so, namely because the _SARNum/Den properties
are much more likely to need to have been changed due to operations
in-script, and unless the user is studious about updating those
properties after even just a basic resizing operation, then _SARNum/Den
will still be set to the original values populated by the source filter,
and will be wrong, leading to encodes ending up wrong and potentially
bug reports to Trac which aren't actually the fault of the demuxer.

This is partially coming from the fact that even the color-based
properties that were already added have experienced some level of
backlash because of the requirement for users to ensure the properties
are correctly updated if they've done any changes to those factors
(as best as I'm aware, the filters in the AviSynth+ core still only pass
through the existing properties, but they don't update them if they
pertain to that property's functionality; I believe some external
filters do update them, however).  I would be fairly confident in
betting that users resizing video is far more common than them doing
color correction ops that would require updating the frameprops FFmpeg
can currently read.

One mitigation to that, IMO, would be to flag that as an experimental
feature, making it to where FFmpeg won't read _SARNum/Den unless
the -strict option has been used.
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