[FFmpeg-user] Changing DAR ?

[Paul B Mahol]

On 10/8/13, Ronald F. Guilmette <rfg at tristatelogic.com> wrote:
>
> In message
> <CAPCGgdS_JFurFUCyyUJ8vX8bxZvSUiqa8BxrRFrQy7gGkBbC-g at mail.gmail.com>
> Anatol <anatol2002 at gmail.com> wrote:
>
>>Check:
>>http://ffmpeg.org/ffmpeg-filters.html#setdar_002c-setsar
>
> Thank you.  The information I found there has proven to be somewhat
> helpful, however...
>
> 1)  It's relly too bad that a link to the page you just pointed me to
> isn't found under the description of the -vf option on this page:

-vf sets list of video filters in filtergraph.

Each filter may have own options.

see:

ffmpeg -h filter=setdar

>
>      http://www.ffmpeg.org/ffmpeg.html#Video-Options
>
> or even here:
>
>       http://www.ffmpeg.org/ffmpeg.html#filter_005foption
>
> 2)  The descriptions of both setdar= and setsar= sub-options at the
> location
> you just pointed me to are extraordinarily unclear.  More specifically,
> what, exactly is that equation near the beginning supposed to represent?
>
> The verbage at the top of this section seems to be saying that setdar=
> will cause the *SAR* to be changed!
>
>       "The setdar filter sets the Display Aspect Ratio ...
>
>       This is done by changing the specified Sample (aka Pixel) Aspect
>       Ratio..."
>
> Huh?  Obviously, what is said here is not what was actually meant.  And
> also, of course, the true meaning and significance of the equation shown
> is nowhere explained.
>
> Then there is this:
>
>    The filters accept the following options:
>
>    'r, ratio, dar (setdar only), sar (setsar only)'
>
> Huh??  Is that a literal 'r'?  Should that be written with or without the
> surrounding single quotes?
>
> My impression is that 'r' in this context is supposed to represent an
> actual literal (floating point?) number, such as "1.7777", but that sure
> is not clear.  Likewise, I guess that "ratio" in this context really means
> something like 16/9, however even that is quite confusing.  Earlier, I had
> tried using setdar=16:9 because that (colon) notation seems to be supported
> by other parts of ffmpeg.  However I got neither any error nor even any
> warning, as far as I could see, and yet there was no effect produced when
> I tried using -vf setdar=16:9".  So I guess that ffmpeg demands different
> notations for the same ratio in different context, yes?
>
> Nothing like a bit of inconsistancy to confuse the users!

Please elaborate what is wrong with documentation so it can be improved.
Or even provide fixed text.

>
> I also have no idea what context, if any, the 'max' sub-parameter might be
> useful in.  Do people ever really try something foolish like this?
>
>       -vf setdar=10000/1
>
> And if they do, and things go haywire, don't they get what they deserve?
>
> Last but not least, we have these two examples in the following section:
>
>     setdar=dar=16/9
>     setdar=ratio=16/9:max=1000
>
> Now I am totally flumoxed!  What would be the difference between these
> two video filters?
>
>      setdar=dar=16/9
>      setdar=ratio=16/9
>
> 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? What you propose instead?

>
> To say that this is all made somewhat less than clear by the documentation
> would be an understatement, I think.
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