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

Kieran O Leary <kieran.o.leary@gmail.com> Sun, 02 December 2018 16:18 UTC

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From: Kieran O Leary <kieran.o.leary@gmail.com>
Date: Sun, 2 Dec 2018 16:17:57 +0000
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Subject: Re: [Cellar] Mapping of MOV clean aperture values to Matroska
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Hi Steve,

On Sun, Dec 2, 2018 at 3:44 PM Steve Lhomme <slhomme@matroska.org>; wrote:

> Hello,
>
> For VLC the values are read but they are likely lost along the way to
> the display by the decoder or the video output. You may try this file
> to check if your video output in VLC supports cropping:
> http://v2v.cc/~j/theora_testsuite/offset_test.ogv
>
> That's pretty cool. ffplay and mpv seem to display the full image, but VLC
displays a crop. At Teasdale in my home Ubuntu 18.04 desktop.


> This illustrates the issue with cropping at the container level (and
> ffmpeg) the container may have some values set but some codecs also
> have such info and may overwrite this value. And there's not really
> any way to tell who is write. It may depend on the codec and the
> container.


I"m mostly familiar with uncompressed video within MOV, which is then
compressed to FFV1/MKV, and in these cases you would only have values in
the container. I hadn't thought of the other codecs that could have
stream-level cropping.


> 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


> Is it in the supposedly visible area or a
> supposedly padded area (which may not correspond in ffmpeg to the
> padding Quicktime would use) ?
>

Unfortunately I don't fully understand what you mean here by supposedly
visible and padded. PAL video has padded black pillarboxing (i have seen
expections to this) that is not intended to be viewed. These clean aperture
values specify the area that is to be displayed to the viewer. These values
are also essential to generating the correct aspect ratio. If the 720x576
pixels is transformed with a PAR of the PAL standard - 59:54, then you get
786x576 aka 1.364:1, but if the 59:54 value is applied with the clean
aperture values taken into consideration, then you will get the correct 4:3
aspect ratio.

Best,

Kieran O'Leary,
Irish FIlm Institute.