[FFmpeg-user] Changing DAR ?

[Ronald F. Guilmette]

In message <CAPYw7P7xVbCcpHLZvd_C2tS_Yp2UYPT2PDbzAb7da4=17ucMPQ at mail.gmail.com>
Paul B Mahol <onemda at gmail.com> wrote:

>> Now I am totally flumoxed!  What would be the difference between these
>> two video filters?
>>
>>      setdar=dar=16/9
>>      setdar=ratio=16/9

I don't think that I saw any answer to this yet, so I'm still wondering
about it.

>> and also, what would happen if I just said:
>>
>> 	-vf setdar=16/9
>> ?
>>
>> (I believe this last thing above is actually what I tried to use in my
>> earlier experiments.  I don't remember seeing any errors or warning when
>> trying to use that.  Is it in fact erroneous?)
>>
>> Obviously, "-vf" is an option for ffmpeg.  To my way of thinking, that
>> makes "setdar=" what I would call a "sub-option".  But now it appears
>> that that sub-option has its own specific, special, and peculiar set of
>> sub-sub-options (e.g.  "dar", "ratio", "max").
>
>Perculiar?

Well, since you asked, yes, "peculiar" is the right word.

I find it peculiar that a sub-option, suich as "setdar" has its own
sub-sub-option which allows the user to specifically request setting
of the DAR, i.e.:

	setdar=dar=<something>
               ^^^

I mean seriously... WTF?

To anybody who finds this notation NON-peculiar... please raise your
hand.

>What you propose instead?

At the moment, I have no specific change proposal because I still am
very much less than clear about _either_ what the various sub-sub-options
of setdar and/or setsar do actually do, _or_ what they are intended
to do (which may perhaps not in fact be what they are actually doing
at present).

And to be clear, when I say that I don't know... and that online docu-
mentation currently fails to adequately explain... what any of these
things actually do, or what they are intended to do, I am saying that
with respect to *numerous* different input and/or output video formats,
each of which is likely to have different inherent capabilities and
thus, each of which may be affecetd by these sub-options and sub-sub-
options in different ways... perhaps radically different.

For example, even though it is an entirely different option that the
ones that have been discussed in this exchange, I've been playing around
also with the -aspect option and so far I have found that it works great
when the video stream is MPEG-4/10, but otherwise, may not do anything
at all, e.g. when used on a file which is a WVM container containing a
WMV9 video stream.

I think that it is safe to say that all this aspect ratio stuff is a big
muddle, and at present, neither the online ffmpeg documentation nor ffmpeg
itself is making any of it any less muddled.  Of course, it is not ffmpeg's
fault in any sense that we humans have repeatedly elected to erect Towers
of Bable in the form of numerous incompatible languages and dialects, even
when it comes to video stream formats (and also container formats for same)
but as I say, the lack of clear documentation about the relevant ffmpeg
options, sub-options, and sub-sub-options, when combined with ffmpegs
clearly inconsistant actual behavior (e.g. when the -aspect option is
used with various file formats) isn't making the Tower of Babel any more
cognizable by ordinary humans.

I hate to have to do this, but it appears that if I personally want to
actually understand either ffmpeg as a tool and/or the set of things I
can and can't do to manipulate various file formats I'm probably going
to have to reach down to first principals, and even though I'm not
familiar with any of this stuff I'm gonna have to grub around down in
the code to find the various relevant header formats and fields, e.g.
for MPEG-4/10, MPEG-4/2, WMV9, WMV8, WMV7, MPEG-2, etc.  Maybe once 
I gather all this info and begin to unsderstand it I may then be able
to understand why sometimes (e.g. when working with WVM material) I can't
seem to get ffmpeg to diddle the DAR without also diddling the PAR and
vise versa, and maybe I'll then be able to understand other ffmpeg
behaviorial quirks I've encountered also.

Sigh.  Oh well.  I didn't actually have that much else to do this decade.
;-)