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

Kieran O Leary <> Tue, 27 November 2018 17:44 UTC

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From: Kieran O Leary <>
Date: Tue, 27 Nov 2018 17:43:51 +0000
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To: Dave Rice <>
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Subject: Re: [Cellar] Mapping of MOV clean aperture values to Matroska
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Thanks for replying:

On Sun, 25 Nov 2018, 20:20 Dave Rice < wrote:

> Hi Kieran,
> > On Nov 23, 2018, at 4:56 AM, Kieran O Leary <>;
> wrote:
> >
> > Hi,
> >
> > Would I be correct in saying that ideally, Clean Aperture values in a
> > MOV/MPEG-4 file should map to the PixelCrop element in Matroska?
> Yes, they conceptually do the same thing but I’m a little confused about
> how to use the vert/horiz offset fractions of the clap atom to convert to
> the pixel crop atom. I tried in both QuickTime 7 and X, and while QuickTime
> X uses the cleanAperature w/h fractions to present a cropped image,
> QuickTime 7 doesn’t but appears to adjust the aspect ratio according to
> those values. QuickTime X does seem impacted by non-zero offset values not
> in a way that I consider coherent. Do you have any examples of un-centered
> aperatures? I was hoping that IMX files would, does the ones I have don’t
> use any aperature.

I don't think I've ever seen non-centered.. but it sounds like Matroska is
capable of a mapping,but the actual quicktime implementation sounds
inconsistent? I guess this could be specified explicitly by an archivist
who will hopefully be able to determine the intended rendering of the
original QuickTime file..
Also it looks like here's yet another related ticket:

> > I'm asking about this in regards to this ffmpeg ticket -
> >
> > As it appears that FFmpeg does not currently read the values in the
> > clap atom and it also does not map them to Matorksa files when
> > remuxing. It seems to make the most sense to map them to PixelCrop,
> > but I'd like to hear it from folks more knowledgable than me.
> Seems reasonable to me, but I’m not exactly certain what the mapping would
> be.

I was considering  doing some experiments with some common PAL scenarios (
crop to 703x576 with a PAR of 59:54, or crop to 704x576 with PAR of 12:11)
and using mkvpropedit to insert the metadata. I'll have to look beyond VLC
to other players that can display these crops and aspect ratio conversions
for the moment.

> > Another associated issue with the display of these kinds of values is
> > with VLC as it appears that
> >
> > 1) PixelCrop values in Matroska are not supported by VLC at the moment
> > -
> > 2) Clean aperture values are not supported by VLC at the moment
> >
> Sounds like these tickets are in my priority order as well.


Kieran O'Leary,

Irish Film Institute

> Dave Rice