Determining Pixel Aspect Ratio (PAR)

[vampiredom]

UPDATE: I added the text and examples to #4 above.

They differ in less than or about 1 pixel, it is actutally the 702<->704 issue (702 no typo).

Yes, a negligible difference. The NTSC values, however, seem completely consistent here.

[vampiredom]

OK, it's pretty basic but it may be helpful (and avoid the hassle of having to look up the "standard" PARs.)

** EDIT: The GuessPAR() function is now a part of PARResizer.avsi ... see this thread.

The first parameter can be either a clip or a string in the format of "640x480", or "720x576", etc.

The second parameter, ws, specifies whether the widescreen PARs (ws=true) or standard 4:3 PARs (ws=false) should be assumed (since there is no way to automatically guess this).

GuessPAR() will return 1.0 if it cannot find a matching non-square pixel standard for the given dimensions.

Examples:

# load a widescreen .avi file
AviSource("MyFile.avi")
par = GuessPAR(last, ws=true)
Subtitle(String(par))

# Print an arbitrary PAR
BlankClip()
par = GuessPAR("704x480", false)
Subtitle(String(par))

[hello_hello]

Originally Posted by TheSkiller

Yes, as stated on the wiki page you linked, those are the official Rec.601 and "common digital" PARs.

Yeah but what about the info above the table?
"Note that sources differ on PARs for common formats – for example, 576 lines (PAL) displayed at 4:3 (DAR) corresponds to either PAR of 12:11 (if 704×576, SAR = 11:9), or a PAR of 16:15 (if 720×576, SAR = 5:4)."

That's what I'm still vague on, because both of those examples give you a PAR of exactly 4:3.
It seems to me most (PAL 4:3) DVDs use a PAR of 16:15 regardless of whether the extra 8 pixels each side contain image or not, but I'm still trying to understand this properly.

As a side note, a bit more browsing through the same wiki page led me to the sight below where they say the HDMI spec specifies an exact 4:3 or 16:9 aspect ratio and refers to the way anamorphic SD video is upscaled.
If it's true maybe it's another indication that exactly 16:9 and 4:3 are more standard than the official standards these days.
http://lurkertech.com/lg/video-syste...xelaspect_hdmi

[Sharc]

Originally Posted by hello_hello

.....It seems to me most (PAL 4:3) DVDs use a PAR of 16:15 regardless of whether the extra 8 pixels each side contain image or not, but I'm still trying to understand this properly.

Well, one simply has to accept that anamorphic DVD's can use the Generic or ITU PAR (or a mod-16 compliant approximation of it).
My experience: The majority of those (older?) DVDs which have about 8 pixels borders left and right follow the ITU PAR. This is no proof however, as the picture could also have been scaled to accommodate overscan. Those DVD with an active picture width of 720 (no left and right borders) are likely to follow the generic PAR.
One can try to do the "circle test" to conclude on the PAR (unfortunately a reliable circle may not be readily found in the source), or one can try to 'reverse-engineer' the PAR starting from the Movie Aspect Ratio (like 2.39, 2.35, 1.85, 1.7778 etc. as indicated on the cover or found on imdb).
What the playback device eventually does with it is a totally different story. For example, my standalone plays ITU DVDs via HDMI including the black borders left and right as 4:3 or 16:9, means the active picture is in fact 2.5% too narrow, wheras my old DVD player using the SCART interface to an analog TV plays these DVDs undistorted ...

[hello_hello]

Originally Posted by Sharc

Well, one simply has to accept that anamorphic DVD's can use the Generic or ITU PAR (or a mod-16 compliant approximation of it).

I don't recall ever stating they couldn't.

Originally Posted by Sharc

My experience: The majority of those (older?) DVDs which have about 8 pixels borders left and right follow the ITU PAR. This is no proof however, as the picture could also have been scaled to accommodate overscan. Those DVD with an active picture width of 720 (no left and right borders) are likely to follow the generic PAR.

Well I won't argue about older DVDs, but I'd certainly agree regarding most newer DVDs, which is the point I was making originally. The PARs posted at the beginning of this thread are all well and good, but if the majority of DVDs don't actually use them.....

Originally Posted by Sharc

What the playback device eventually does with it is a totally different story. For example, my standalone plays ITU DVDs via HDMI including the black borders left and right as 4:3 or 16:9, means the active picture is in fact 2.5% too narrow, wheras my old DVD player using the SCART interface to an analog TV plays these DVDs undistorted ...

I guess unless you tend to use your older player more often, it's probably a good thing most DVDs aren't ITU.
If newer equipment seems more likely to use generic 16:9 & 4:3 aspect ratios, and newer DVDs seem far more likely to have used them also, what's the motivation for studios/DVD manufacturers to suddenly do an about-face which makes returning to the official standards for DVDs likely?
Personally I think when it comes to understanding the "official" way to resize a DVD it's nice to know... but when it comes to instructing people how to "properly" resize a DVD or determine PARs etc, it's really probably time to let it go. Or at least explain both methods.... the one which is supposed to be used, and the one which is more likely to be used.

[Alex_ander]

I'd say that document-wise, there is no such a universal thing as 'ITU PAR', since there are 3 different ITU documents in
use for different cases:

1. BT.601 - for video capture and broadcasting - that's where the so-called 'ITU PAR' come from
2. H.262 (mpeg2) - is believed* to be used by DVD (digital storage)
3. H.264

* The DVD specification itself is not a widely quoted document.

#2 ITU document says that a standard mpeg2 decoder maps all the stored samples (like 720x480) to the AR (4:3, 16:9 or 2.21:1, unless it's SAR=1:1 - 'square sample') written in file header. Any other proportions can be only obtained using optional sequence_display_extension. There's also an example of usage SDE for possibly providing BT.601 proportions. But in practice (some people at videohelp looked through their collections), SDE is only used with pan&scan (of the stored samples, 540x480 are mapped to 4:3 then). This might explain why many (if not most) DVD's don't actually follow #1 document.

#3 ITU document is quite different and the first one to explicitely define standard SAR's (standard name for 'PAR' - Sample Aspect Ratio) for H264 encoded files (their codes are written in file header). There are 16 recommended values and all of them are calculated from 704 rather than 720 (no 702, no 711 and the like). For 720 and 480 'horizontal overscan' is mentioned in the table. It is actually 720/704 linear correction in direct calculations from 'resolution+AR' numbers and can be easily applied instead of referencing to any tables (making them absolutely useless). This apparently applies to SD content on BD, but leaves a doubt about about mpeg2 BD content (can it still follow #2 document or not).

[TheSkiller]

Originally Posted by hello_hello

Well I won't argue about older DVDs, but I'd certainly agree regarding most newer DVDs, which is the point I was making originally.

I agree with you that most (maybe even almost all) newer Hollywood film DVDs, for whatever reason, are using the non-ITU method (strictly talking about PAL discs here, don't know about NTSC ones), but it does not mean any new DVD is likely to do that because it also depends a lot on where the video originates from and what kind of "video" it is.
Footage that was shot with video cameras (in contrast to film cameras which requires a film->digital transfer) is much much more likely to follow ITU (yes, even today). In other words, footage that was shot to be viewed on a TV only, not in the cinema.
If you buy a DVD of a TV soap opera or something that was shot with SD(!) video cameras then you can be almost certain that it is following ITU, even if all 720 pixels of the DVD are filled (this is due to the fully digital recording and transfer to DVD). If it was shot using HD video cameras things depend on how it was downscaled, if all 720 pixels are containing picture then it is probably using non-ITU.
Of course, still, having all 720 pixels filled or not is not a fool proof method, but it's something to consider if you're willing to try and find out which one is correct.

[mnm-a]

Originally Posted by vampiredom

To convert a square pixel image to a different PAR is easy. Divide the width by the PAR like such:

640x480 @ 10/11 PAR
640 / (10/11) = 704
704x480

The above example is rather simplistic. In many cases, when converting from square pixels, we'll want to scale the image overall. First we need to determine the scaling factor. To accomplish this, divide the source height by the target height:

SourceHeight / TargetHeight = ScalingFactor

1280x720 -> 704x480 @ 40/33 PAR
480 / 720 = 0.66666666666666666666666666666667

Sorry, AFAIU this has a mistake.
It should be

TargetHeight / SourceHeight = ScalingFactor

[therube]

Originally Posted by hello_hello

Yoda's Resize Calculator
At least with the calculator handy it's not necessary to remember a table of PARs.

Link looks to be 404?
Videohelp hosts, Yoda's Resize Calculator 0.3.5.11.

(For reference, the thread here at doom9, Yoda's Resize Calculator.)

[hello_hello]

Originally Posted by therube

Link looks to be 404?
Videohelp hosts, Yoda's Resize Calculator 0.3.5.11.

(For reference, the thread here at doom9, Yoda's Resize Calculator.)

I think the links for the newer version in that thread are dead, so here's a new one for version 0.4.0.1.
I was never officially released, but it works fine and includes mpeg4 PARs as well as the old ITU PARs for NTSC and PAL. I'll see if I can get the VideoHelp page updated with the newer version at some stage, given Yoda has stopped development and the attachment in his post was never approved.
https://forum.doom9.org/showthread.p...51#post1825751

[StainlessS]

Originally Posted by hello_hello

I think the links for the newer version in that thread are dead

No not dead, simply awaiting attention of a vigilant moderator to approve Yoda's attachment. [it might take some time]

[hello_hello]

Originally Posted by StainlessS

No not dead, simply awaiting attention of a vigilant moderator to approve Yoda's attachment. [it might take some time]

Not to worry. The VideoHelp page has been updated with the newer version now.
https://www.videohelp.com/software/Y...ize-Calculator

[Logan9778]

Lol, here we go again.

THIS is what really helped me with BBC encodings (Doctor Who, The Young Ones, Fawlty Towers, etc.) I can't put it in enough posts.

BBC USES THEIR OWN UNIQUE SYSTEM!

Originally Posted by Ghitulescu

Good that you mentioned BBC.

BBC used a different approach.

The digital frame sizes are 788 and 1050 respectively (PAR=1), for they include also what BBC calls blanking. The extra pixels are usually painted black.

Resize then to 788/1050 then cut the black borders and you'll end with some strange number of pixels but quadratic.