On Fri, Jan 19, 2024 at 7:54 PM Andrea paz via Cin <cin@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 > -- > Cin mailing list > Cin@lists.cinelerra-gg.org > https://lists.cinelerra-gg.org/mailman/listinfo/cin -- Cin mailing list Cin@lists.cinelerra-gg.org https://lists.cinelerra-gg.org/mailman/listinfo/cin
