Reconciling ITU-R BT.601 AR with "known" theatrical AR

[bkman]

Hi. I know that the ITU standard calculated AR is the "correct" way to do it with DVD's, but why is it that after autocrop on some films the resulting AR doesn't exactly match the theatrical AR on the box, and on websites, etc. Is the DVD mastered incorrectly? Should the DVD AR be corrected to the theatrical AR?

Thanks.

[CWR03]

To begin with, some websites are just plain wrong. It also depends on the selected output resolution - since most programs that auto-select crop require the use of MOD-16, the resolution of a video file can be off quite a bit. I've had files that should be 1.85:1 try to come out as 16:9 without my intervention.

The DVD AR should be corrected so that round objects appear round on the screen.

[bkman]

I mostly said autocrop as a shorthand for cropping away all black around a full-frame. Just assume I'm talking about mod2 cropping for the purposes of this topic.

[CWR03]

Even so, wouldn't you want round objects to appear round on your screen? That's all that matters to me when creating a video file from any source.

[bkman]

Well, yeah, that is the point. It's hard to find and measure perfectly round objects in the video, however, so I'm asking in this topic for some professional advice on a general rule of thumb or possible exceptions.

[jggimi]

Have you read the 40 posts in http://forum.doom9.org/showthread.php?t=42708 yet? :rolleyes:

[bkman]

Have you read the 40 posts in http://forum.doom9.org/showthread.php?t=42708 yet? :rolleyes:

Actually, I have, but as far as I can see it doesn't answer my question.

[CWR03]

If you can't tell that the aspect is wrong just by looking at the video, then you won't be able to tell that the aspect is wrong when you're watching it. :)

I mostly use Gordian Knot, and it's rarely wrong for movies, unless it's severely and noticeably wrong. At those times it's usually the "Input Pixel Aspect Ratio" 16:9 or 4:3 settings that's wrong. I have yet to need to force aspect with AutoGK.

[bkman]

It is the question of which is "more right" (between the ITU and the theatrical AR) that I am interested in, and in what circumstances. I'd appreciate a more definitive answer.

[unskinnyboy]

Just when you think you can have nothing left to discuss about ITU-R BT.601 resizing, here we go again...!

Why are you comparing ITU-R BT.601 AR and the theatrical AR? ITU-R comes into question only if the DVD was authored with ITU-R honored. If it was, then the avi should be resized with ITU-R checked. If not, then it should be unchecked. There is no definite way to know for sure, although from experience, pillarboxing of ~8 pixels on both sides indicates the presence of ITU-R (usually). 99% of the recent Hollywood movies come without ITU-R resizing, but the same can't be said for other DVDs from other regions.

The reason why everyone is giving you the advice of checking for round objects is because that's the only way to know for sure. If you really want to become anal about it, eyeball the whole movie, find a round/spherical object, copy/paste the frame into an image editor and measure the diameters top-to-bottom, left-to-right. Do this with ITU-R checked and unchecked. With one of them, the diameters will exactly match (or closely match), and that's your AR. The situation is complicated in that most directors use a lot of angles when shooting, and so something you perceive as bad AR might be something director-intended. Not to mention the difficulty of finding a well-defined, proper round object without shadows to compare.

Theatrical AR is the AR of the movie as it was shown in the theater. During a DVD transfer, it can change or remain the same.

Everything said, if your eyes can't make out the difference between ITU-R resizing and non-ITU-R resizing, it doesn't matter anyway.

[check]

When circles look most square with ITU resizing - the correct resizing is ITU. Vice versa also applies :)

[bkman]

No way to tell for sure short of measuring circles?

Well that's irritating. Has anyone gone to the effort of compiling a database? And would it be useful at all to pick the closest AR to the theatrical OAR out of standard and ITU?

[unskinnyboy]

I don't know how to explain this any more clearly. Yes, no way other than measuring circles to tell for sure.

Compiling a database of what? Movies which needs ITU-R resizing and which don't? Well, that's a total waste of time, considering the different the DVD editions of the same movie - WS, FS, Rated, Unrated, Special Edition, Unrated Special Edition, Ultimate Edition, Limited Edition, Superbit, Director's Cut, R-rated Director's Cut, 10th Anniversary Special, Nothing-special-here-but-just-another-cut-to-get-more-money cut etc, different region versions, different releases, all of which may or may not have the same AR/resizing across all the DVDs.

Do the following as a rule of thumb: keep it unchecked and flip thru the movie in VirtualDubMoD to see if everything looks OK. If it does, good. If not, check it.

[Brother John]

Don't measure circles! you need spheres. Take a perfectly round and flat cake plate (or something) as an example. Apart from all AR issues it only appears perfectly round if the camera is located directly above the plate's centre point and looks straight down. All other angles will not produce a round shape!
To be on the safe side you need a sphere because it's equally round regardless of the camera angle. But where to get perfect spheres? There are not too many of those in your average movie.

What I'm getting at is: Even with careful measuring you can hardly ever be 100% sure. If you speak German, have a look here: "Die Bedeutung der ITU-R BT.601 für das PAR" (http://encodingwissen.brother-john.net/spezial/itur-bt601.html), where I elaborated a little on the subject. An interesting fact is that we're arguing about a deviation of 2,56% for PAL and 1,29% for NTSC. If it wasn't called by the "magic name" ITU-R BT.601 we wouldn't even waste a second thought on it. E.g. Gordian Knot only warns about (resizing) aspect errors of more than 3,5%. At the end of the day "ITU or no ITU" is a highly academic discussion with only little practical value.

99% of the recent Hollywood movies come without ITU-R resizing
What makes you so sure? If it's just because of missing vertical bars, that's a weak argument. I could think of a number of ways for ITU without bars.

[unskinnyboy]

What makes you so sure? If it's just because of missing vertical bars, that's a weak argument. I could think of a number of ways for ITU without bars.
No, it's not because of the lack of pillarboxing of the kind I mentioned few posts above (even that doesn't conclusively indicate lack of ITU-R, as I myself had noted). My observation was a result of my experience in encoding a lot of them released in recent times + whatever my friends told me about the ones they encoded. But, of course, I haven't encoded every single one of them movies ever out on DVD, so it was more of an observation than a universal truth. YMMV.

I don't agree that ITU-R resizing is of no (or little) pratical value. I strive to get the correct resizing whenever possible, because most of the times the difference sticks out like a sore thumb for me. But maybe it's just me.