[FFmpeg-devel] [PATCH 2/3] avcodec/nvenc: Handle non-square pixel aspect ratios

[Philip Langdale]

On Sat, 17 Jan 2015 19:27:17 +0000
Kieran Kunhya <kierank at obe.tv> wrote:

> On 17 January 2015 at 18:14, Nicolas George <george at nsup.org> wrote:
> > Le septidi 27 nivôse, an CCXXIII, Philip Langdale a écrit :
> >> On Fri, 16 Jan 2015 22:17:56 +0100
> >> Nicolas George <george at nsup.org> wrote:
> >> Ok. I did this test and it produces correct results - SAR 133:221
> >> which yields the correct final aspect ratio,
> >
> > This is a good start.
> >
> >>                                        but if I try and do a real
> >> world case, like my PAL DVD, it goes wrong - and Timo's patch goes
> >> wrong too, with this same weird 1.02 (45/44) scale factor.
> >
> > The 45/44 value looks like 704/720: the sign that some people long
> > time ago in committees in the broadcast industry could not agree
> > whether the aspect ratio applies to the whole image or only the
> > part that is displayed on screen.
> 
> The active picture width is 702 pixels. I would read the following
> thread and ETSI TS 101 154 for more information about SAR/DAR etc
> https://lists.libav.org/pipermail/ffmpeg-user/2010-March/024551.html
> 
> I haven't quite followed what's going on to decide whether Nvidia or
> FFmpeg is correct.

For me, the problem is that the encoder is silently doing this aspect
ratio fiddling, whether I want it or not. If I want to transcode
material at the original size and aspect ratio, I should be able to. If
I, for whatever reason, want to adjust for active picture constraints
at encode time, I can set the SAR appropriately. So, ffmpeg applies
what you tell it to, the hardware adjusts what you tell it by 44/45 -
I'd call that wrong.

--phil