Subject: Re: [Cellar] Mapping of MOV clean aperture values to Matroska

From: Kieran O Leary <[email-masked]>
Date: Tue, 5 May 2020 13:13:27 +0100

Hi all,

I'm not sure if Neil is subscribed to this listserv, but if not - here's
the rest of this thread:
https://mailarchive.ietf.org/arch/browse/cellar/?q=clean%20aperture
Just to flag that Neil Birkbeck is working on adding support to ffmpeg that
will:
map clap values from mov to mov using side data, as well as also mapping
clap values to PixelCrop values within Matroska.
Here's the thread:
http://ffmpeg.org/pipermail/ffmpeg-devel/2020-April/261465.html
Here's my testing - my mail settings are weird at the moment  so I broke
the threading on ffmpeg-devel which is embarrassing - here's my reply:
http://ffmpeg.org/pipermail/ffmpeg-devel/2020-May/262155.html
And seeing as the source mov file as created by Media Express has clap
width values of 41472/59 , this was rounded up to an even 8 pixelcrop on
each side of the image in the MAtroska file, creating a display output of
704/576. Whereas the source mov file would have been displayed as 703/576.
I think that Neil is going to fix this, but I'm wondering if there's some
restriction within PixelCrop so that only even values can be used?
This reply by Neil showed the rounding issue:
http://ffmpeg.org/pipermail/ffmpeg-devel/2020-April/261592.html

Best,

Kieran.

On Mon, Dec 3, 2018 at 7:50 AM Steve Lhomme <[email-masked]> wrote:

> > >> In Matroska we assume that we override what the codec says.
> > >> But I don't think that's how it's handled in ffmpeg. Also there's the
> > >> question of what these values apply to: on the decoded surface, which
> > >> may include padding on the right/bottom or the area that's supposed to
> > >> be visible. In Matroska the "PixelWidth/PixelHeight" correspond to the
> > >> visible area, on which we apply extra cropping. (this is currently not
> > >> written anywhere).
> > >>
> > >> As for the MOV atom, is there are spec somewhere ? Or is it clear that
> > >> it's some cropping ?
> > >
> > >
> > > I think this is as good of a spec that is freely available - the 'clap
> section'
> https://developer.apple.com/library/archive/documentation/QuickTime/QTFF/QTFFChap3/qtff3.html#//apple_ref/doc/uid/TP40000939-CH205-125850
> >
> > OK, so we should support it in VLC (which we don't) and that seems to
> > map more or less to Matroska values.
>
> I see you submitted a feature request already, excellent
> https://trac.videolan.org/vlc/ticket/21179
>