[Cin] DAR, PAR and SAR, again

[Andrew Randrianasulu]

On Fri, Jan 19, 2024 at 7:54 PM Andrea paz via Cin
<cin at lists.cinelerra-gg.org> wrote:
>
> I am opening a new post because the previous one, for me, was becoming
> too confusing.
>
> I quote Terje's latest email:
>
> ==============================================================
>
> Well, I have prioritized more to understand what happends than
> interprete the somewhat ambiguous definitions.
> (the order of the factors is indifferent).
> https://stackoverflow.com/questions/18877243/why-ffmpeg-print-sar-instead-of-par
>
> I think of SAR in the FFmpeg output as a correction factor for the
> recordered picture frame format resolution (Wp), to get the desired
> output display resolution (expanded Wd with square px) at a given DAR,
> (or in opposite order).
>
> I found a useful table at ffmpeg-user that contains information for
> practically all SD and HD video formats.
> Maybe something for the manual?
> https://www.mail-archive.com/ffmpeg-user@ffmpeg.org/msg27522.html
>
>
>                display     DAR       picture     PAR      SAR = DAR/PAR
>              ===========  ====     ===========  ====     =====
> 16:9-2160:  3840 x 2160  16:9  :  3840 x 2160  16:9  :   1:1
>   4:3-2160:  2880 x 2160   4:3  :  2880 x 2160   4:3  :   1:1
> 16:9-1080:  1920 x 1080  16:9  :  1920 x 1080  16:9  :   1:1
>   4:3-1080:  1440 x 1080   4:3  :  1440 x 1080   4:3  :   1:1
>   16:9-576:  1024 x 576   16:9  :   720 x 576    5:4  :  64:45
>    4:3-576:   768 x 576    4:3  :   720 x 576    5:4  :  16:15
>   16:9-480:   853 x 480   16:9  :   720 x 480    3:2  :  32:27
>    4:3-480:   640 x 480    4:3  :   720 x 480    3:2  :   8:9
> [...]
>
> ===========================================================
>
> My God, I feel like giving up....
> The following is my opinion (indeed, my delirium!).
> The table is wrong (for ffmpeg!) because it uses PAR. As is also
> stated in the thread you cited:
>
> "> PAR (picture aspect ratio [1]) [noun]: 1, The horizontal-to-vertical
>  > size [3] ratio (H:V, e.g. 5:4, 3:2) for pictures.
> get rid of this one altogether, because nothing in FFmpeg use it, and
> rightly, because nobody should use it."
>
> Instead of calling it PAR, it should be considered simply the aspect
> ratio of W x H of the input. That is, the W/H of the formula DAR=W/H x
> SAR (and not DAR=PAR x SAR). PAR and SAR cannot be different since
> they are the same thing (i.e. the shape of a single pixel, as seen
> from the image https://stackoverflow.com/questions/18877243/why-ffmpeg-print-sar-instead-of-par
> that you mentioned). In practice, in my opinion, the PAR in the table
> is nothing more than the SAR meant as Storage aspect ratio; in fact,
> the author of the table defines it as a Picture aspect ratio, which is
> a frame aspect ratio and not a pixel aspect ratio. It seems to me that
> in an effort to be precise, it creates even more confusion. Instead,
> the SAR in the table is nothing more than the S(ample)AR ( =
> P(ixel)AR). If you think I am crazy to keep up with these definitions,
> you are right!
>
> The formula DAR = PAR x SAR was convenient and simple, however, ffmpeg
> decided to use "samples" instead of pixels and so PAR should no longer
> be used. That is why I was thinking of removing all reference to PAR.
> The trouble is that if one searches around one always finds PAR and
> never SAR meant as "sample a.r."; including many tutorials on ffmpeg.
>
> The sample concept is derived from analog video signals and is encoded
> in the standards BT. 601 and Digital. It refers to horizontal scan
> lines (in MHz) and not simply pixels. From Wikipedia: "analog video
> does not have pixels, but rather a raster scan, and thus has a
> well-defined vertical resolution (the lines of the raster), but not a
> well-defined horizontal resolution, since each line is an analog
> signal." S(ample)AR was created precisely to adapt the analog concept
> to the digital world.
>
> To further explore the concept of sample:
> https://web.archive.org/web/20140816103129/http://lipas.uwasa.fi/~f76998/video/conversion/
>
> Your definition of SAR as a correction factor is right and I will use
> it in txt. Thank you for the cue.
>
> To summarize: in the digital domain we can use the concept of
> S(ample)AR when we are dealing with digital anamorphic formats (for
> example, when transcoding to a format that has different aspect
> ratio). But the most frequent use is in "standard-definition
> television or with DV, HDV and a few other formats. SAR is used via
> the formula DAR = SAR x (W/H), where SAR is the conversion factor
> between anamorphic Input and Output.
>
> Perhaps the clearest treatment is as follows:
> https://encodingwissen.de/hintergrund/videobild/anamorph/itu-r-bt601/
>
> It is in German and therefore needs to be translated (the Italian
> translation is not very good...). They seem to me to be similar
> concepts to Raffaella Traniello's guide.
>
> As an additional (my) confusion, in CinGG we also have the W/H Ratio
> parameters, which are simple multipliers but, if set only one of them
> (with values of the classic aspect ratios, 4:3; 16:9, etc.) and
> leaving the other one at 1, they act as a real SAR even though they
> are not...
>
> @Andrew
> In CinGG/Info-->Detail is the data reported taken using ffmpeg?

yes, by same equations as  ffprobe does, hopefully

> In other words: does CinGG use SAR like ffmpeg or does it have an even
> different way?

It only written/named like this there because we must interface with ffmpeg ...

> PS: In my opinion, it would be better to change W/H Ratio to "W/H multiplier"

if whole line still fit window .... why not. Just ... you see we have
new explainer right there. Old tutorials will be confusing if we
change this element

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