SAR / PAR confusion

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SAR (Segment Aspect Ratio) has been changed to PAR (Pixel Aspect Ratio) next to Input on the GUI up right, but it still shows the numeric value for SAR. Those abbreviation don’t stand for the same thing, they are not interchangeable without changing the associated values. SAR is specified in the MPEG-2 ISO specs (which is what ffmpeg refers to), whereas PAR is not.

Unfortunately, the Wiki about Pixel Aspect Ratio contains false information, from my point of view. The information here is correct:

https://ffmpeg.org/pipermail/ffmpeg-user/2020-September/050282.html

Don’t get me wrong, I don’t ask for showing the correct value for PAR, better would be to show SAR and DAR, because that’s what people will see when checking their files with ffmpeg/ffprobe.

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The problem is that documents use the same abbreviations (SAR in this case) for different measurements. SAR can be Sample Aspect Ratio or Storage Aspect Ratio (not seen Segment Aspect Ratio before).

I think the devs have switched over to how the video standards use PAR and DAR, while avoiding SAR because of the differing uses. I believe SAR in the TVAI gui was Sample Aspect Ratio which does equal PAR (and how ffmpeg uses SAR).

You have to be careful reading that ffmpeg link because some of that info looks a bit confused.

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I agree that there is a lot of misinformation on the internet about that matter. SAR (Sample Aspect Ratio) is defined in the MPEG-2 specs ISO/IEC 13818-2:2013. Gladly I found a read sample for free on the internet, it says:

3.114

sample aspect ratio

SAR

This specifies the relative distance between samples. It is defined (for the purposes of Rec. ITU-T H.262 | ISO/IEC 13818-2), as the vertical displacement of the lines of luminance samples in a frame divided by the horizontal displacement of the luminance samples. Thus, its units are (metres per line) ÷ (metres per sample).

FFmpeg refers to those specs and PAR doesn’t exist there. I think that everybody seriously involved in video conversion is familiar with SAR and DAR, I doubt that this is true for PAR.

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Back on this with one additional note.

The authors of the articles I linked to agree in this equitation: PAR = DAR / SAR.

If PAR would be equal to SAR, as apparently assumed by those who made the change, DAR would always be 1, which obviously is false.

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That equation is for Storage Aspect Ratio, hence alot of confusion in that article (it’s also in the wiki you linked).

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Using the DVD video in your screenshot as an example:

PAR = DAR / SAR (in this equation SAR is Storage Aspect Ratio)

1.067 = 4/3 / (720/576)

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Fine. Your equitation shows that PAR isn’t equal to SAR, i.e. you cannot change the name without changing the value, which is what happened on the GUI.

And, here comes the real problem: You say (according to Wiki) SAR = 720/576 = 1.25, but ffprobe tells this for the media in my example:

Stream #0:1[0x1e0]: Video: mpeg2video (Main), yuv420p(tv, top first), 720x576 [SAR 16:15 DAR 4:3], 25 fps, 25 tbr, 90k tbn

SAR = 1.667 and not 1.25

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Your maths is wrong, 16:15 is 1.067, which equals PAR.

This illustrates my point. The articles are talking about two different measurements, but both are abbreviated to SAR.

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Excuses, typo. Of course ffprobe shows SAR is 1,067 but you said PAR is 1.067 and SAR is 1.25.

However, with all respect, what you promote is renaming the technically well defined parameter SAR into something else.

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Your missing the point that SAR does equal PAR if SAR is the Sample Aspect Ratio (1.067).

You objected to the gui change from SAR to PAR in your first post because the numeric value wasn’t changed, but clearly it doesn’t need to change because PAR = SAR = 1.067.

The SAR in the equation PAR = DAR / SAR is a different measurement, the Storage Aspect Ratio, which is just the ratio of the pixel dimensions (720/576).

I haven’t renamed anything. All this came from the articles you linked. I personally don’t care whether they use SAR or PAR in the gui, but whatever they use, the numeric value is correct and doesn’t need altering.

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Don’t think that more arguments will clarify the situation. Anyway, I made my point, which is:
Topaz please replace PAR (introduced with 4.1.2) with DAR and SAR, as it they are shown in ffprobe and ffmpeg, e.g. for a PAL DVD, like so:

720x576 [SAR 16:15 DAR 4:3]

Because it makes more sense to an average user presenting it that way, as a simplified key values.

Nothing against simplifying things, if possible, but what has been done there is swapping values. I believe, at the first place, any used values must be defined somewhere and they have to be correct.

As mentioned already, for a PAL DVD, PAR is 720/576 = 5:4 = 1.25 and not 1.067 as shown on the GUI.

No, PAR is 1.067 as shown in the gui.

720/576 is the Storage Aspect Ratio.

Unfortunately, that is wrong, at least for those who believe that what FFmpeg shows is correct.

Read the wiki carefully. It is correct. FFmpeg is also correct.

https://en.wikipedia.org/wiki/Pixel_aspect_ratio

Sample Aspect Ratio is 1.067 and equals PAR.
Storage Aspect Ratio is 720/576.

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I have been dealing with anamorphic media for over three decades, so I have a firm perspective on that matter. Perhaps, we can agree that we disagree?

Most definitely.

but it’s containerized in a 4:3 for the SD TVs, that what changes it from the RAW formula.

Yes, that’s what DAR specifies, basically. However PAL DVD supports two different aspect ratios (DAR), 4:3 and 16:9, both have the same resolution (720x576) and thus the same PAR, but different SAR.