[FFmpeg-user] Matroska encode specifies SAR/DAR when no aspect ratio info is present in source .mov

[Kieran O Leary]

Hi Carl,

Thanks for getting back to me so quick.

On Wed, Jul 13, 2016 at 9:37 AM, Carl Eugen Hoyos <cehoyos at ag.or.at> wrote:
> Kieran O Leary <kieran.o.leary <at> gmail.com> writes:
>
>> A .mov source with no SAR/DAR information (listed as #sar 0: 0/1)
>> in the framemd5, has specific SAR information when remuxed as mkv
>> (#sar 0: 1/1). Obviously these values are also available via
>> ffmpeg -i input.
>
> Not really:
> The muxer does not write a value if sar == 0/1 (the field is not
> mandatory) but the demuxer always provides a value to the world.
> You could read the mkv specification in a way that this is
> correct (because a sar of 1/1 is the default for mkv).
>
> Does the demuxing behaviour hit you?

It only means that I have to come up with a different way (ignore the
commented lines in framemd5 output) of diffing input/output framemd5
files when transcoding, which I should be doing anyway.

> It could be changed but this is isn't completely trivial:
> For 3D files, the SAR is 2/1 (or 1/2) by default, so in this
> case, the demuxer would still have to output a sar although
> not specified in the file.

I understand. I don't know how essential it is, seeing as no SAR would
mean 1:1 anyhow. The only case that I can think of is if an archivist
was really desperate to preserve as many properties of the original
file when remuxing to another container.  Seeing as PAR/SAR is a
significant property of the source, its declaration, or
non-declaration possibly should be preserved, even if players will
treat the file the same way regardless. I can probably live without it
as I'd prefer if the SAR was declared in the source to begin with.

>
> Carl Eugen
>

Best,

Kieran.