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

Jerome Martinez <jerome@mediaarea.net> Sun, 02 December 2018 16:26 UTC

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From: Jerome Martinez <jerome@mediaarea.net>
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Date: Sun, 2 Dec 2018 17:26:00 +0100
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Subject: Re: [Cellar] Mapping of MOV clean aperture values to Matroska
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On 02/12/2018 16:44, Steve Lhomme wrote:
> [...] some codecs also
> have such info and may overwrite this value.

Which one?
e.g. AVC crop values are more for handling widths/heights not multiple 
of the macroblock size, and output from the decoder is the one cropped, 
then the clean aperture from the container is applied (BTW, this is 
sometimes not what the user wants :(,  I got complains that I show the 
clean aperture size and not the encoded size in MediaInfo).


> [...]
>
> As for the MOV atom, is there are spec somewhere ? Or is it clear that
> it's some cropping ? Is it in the supposedly visible area or a
> supposedly padded area (which may not correspond in ffmpeg to the
> padding Quicktime would use) ?

https://developer.apple.com/library/archive/documentation/QuickTime/QTFF/QTFFChap2/qtff2.html

"The clean aperture defines the part of the encoded pixels to be 
displayed. The pixel aspect ratio is the aspect ratio of the encoded 
pixels. Conceptually the encoded pixels are decompressed, stretched (or 
shrunk) based on the pixel aspect ratio, and extra pixels are cropped 
off according to the clean aperture."