[Ffmpeg-devel] DAR and SAR FFmpeg option confusion

[Måns Rullgård]

Roman Shaposhnik said:
> Hi
>
> On Tue, 2007-03-13 at 09:27 +0000, M?ns Rullg?rd wrote:
>> >
>> >   I see your point now and it does make sense. That said, I guess
>> > my biggest concern would be that starting to demand DAR being 15:11
>> > for 4:3 DV (or anything else for that matter) might be problematic from
>> > a usability standpoint plus it might present a problem as far as
>> > flagging 4:3 in various containers is concerned.
>> >
>> >   But I guess, if admitting DAR being 15:9 instead of 4:3 is our
>> > official "party line" I would protest too loudly ;-)
>>
>> The problem with DV appears to be that it is outright lying about the
>> aspect ratios.
>
>   What is lying in your opinion ? The codec ? The SPEC ? The
> information presented here ?
>        http://www.mir.com/DMG/aspect.html
>   Please be more specific.

You can't have SAR=10:11, DAR=4:3 and size=720x480, all at the same time.
The maths simply don't work out.

>   Also, you say that its just DV, not really -- SVCD for example
> would also have the same problem.

I said DV because that was what was being discussed.

>>  Given two of DAR, SAR and size, the third can be
>> easily calculated.  The numbers you gave are inconsistent.  We can't
>> easily "fix" it as mathematics won't bend very easily.
>
>    Which numbers and why do you think they are inconsistent ? Once
> again, please go read: http://www.mir.com/DMG/aspect.html and if
> you find yourself in disagreement with what's written there then we
> can talk about it meaningfully. The numbers Robert and I quoted are
> very consistent with what's written as part of the resource mentioned
> above.

If you have encoded 720x480 pixels, and want to display it with DAR=4:3
and SAR=10:11, you'll have to crop the image.  Alternatively, you could
use a wider DAR to keep the full image and SAR, or use a different SAR
to fit all the pixels in DAR=4:3.  The web page you reference states that
cropping is the correct thing to do.

IMHO it is correct of FFmpeg to do calculations based on mathematical truths,
not what some random spec tells us to pretend.

-- 
M?ns Rullg?rd
mans at mansr.com