Why is the 2.21:1 flag never used?

[MrTroy]

I recently started to wonder:
The 2.21:1 flag is official part of the DVD standard, which means that all DVD players that comply to the standard must support it (i.e. be able to scale it to 16:9 / 4:3 displays).

Then why is it never used on commercial DVDs? 2.35:1 movies stored with the 2.21:1 flag could fill 94% of the resolution, instead of the 76% they do with the 16:9 flag. And a 2.20:1 movie like Lawrence of Arabia could be stored in almost the entire 576i (/ 480i) resolution!

[Wilbert]

Then why is it never used on commercial DVDs? 2.35:1 movies stored with the 2.21:1 flag could fill 94% of the resolution, instead of the 76% they do with the 16:9 flag.
Because people don't have 2.21:1 TVs.

[MrTroy]

Because people don't have 2.21:1 TVs.
I see what you mean: CRT tv's can't do anything with the extra resolution. But increasingly more people have LCD tv / projectors / etc. Their PQ will improve with the extra vertical lines.

[SeeMoreDigital]

It is possible to generate encodes at say, 720x576 pixels and add an appropriate level of aspect ratio signalling "within the video stream" so they can be displayed at say, 2.35:1 - or any other movie/theatrical aspect ratio!

Meaning (in the above mentioned instance) all of the 720x576 pixels would make-up the image pixels and the black mattes would be introduced automatically by the decoder to fill the rest of the screen.

It's easy to do this with MPEG-4 streams. Many stand-alone players can handle the encodes too ;)

Here's a 10 second video only MPEG-4 720x576 @ 2.35:1 sample (http://81.98.148.105/Uploaded_Files/Doom9_Forum_files/MPEG-4_720x576@2.35.1.7z).


Cheers

[mpucoder]

The 2.21:1 flag is official part of the DVD standard
No, it's not. While mpeg-2 can use 1:1, 4:3, 16:9, and 2.21:1 DVD can use only 4:3 and 16:9. There are many more places where the full range of values of mpeg are not permitted.

[MrTroy]

No, it's not. While mpeg-2 can use 1:1, 4:3, 16:9, and 2.21:1 DVD can use only 4:3 and 16:9. There are many more places where the full range of values of mpeg are not permitted.
Oh, I thought it was. DVD Lab Pro allows you to author 2.21:1 DVDs anyway.

[Sulik]

Technically, 2.21:1 AR signaling is not allowed in MPEG-2 Main Profile.

[zambelli]

I question the quality advantage of doing such a large horizontal stretch anyway. Interpolating from 704x480 to 854x480 is only a 21% horizontal stretch - not a big deal at all. Interpolating from 704x480 to 1064x480 is a 51% stretch.

In the case of PAL, resizing 704x576 to 1024x576 is already a large 45% stretch. But 1280x576 would be a 82% horizontal stretch! With a format of rather limited resolution such as DVD - a 1.82x stretch is not necessairly the best way to enhance your video.

Edit: Adjusted numbers for standard ITU.601 resolutions (704, not 720).

[SeeMoreDigital]

I can't disagree with your logic there Zambelli ;)

We PAL DVD users suffer from quite a significant pixel stretch compared to our NTSC DVD cousins.

Makes you wonder how/why people use SVCD resolutions, such as PAL 480x576 and NTSC 480x480.


Cheers

[MrTroy]

There's no difference in horizontal stretch between NTSC and PAL. Their horizontal resolutions are the same. If your display is 1800 pixels wide, the 720 pixels will be stretched to 1800, no matter if it's PAL or NTSC. In both cases there will be a 250% stretch.

There's a difference in vertical stretch however, and actually PAL does better than NTSC in that. 'Cause in your hypothetic 1800x1012 display, PAL needs a vertical stretch of 176%, whereas NTSC needs 211%!

If there would be 1.5:1 displays, NTSC would indeed have an advantage as it could use square pixels, but in 16:9 displays PAL is undoubtedly better.

[Slogra]

Wow, that is a great idea. I never thought of it. You would gain a lot of resolution by doing this. It's too bad it isn't supported by DVD. It would look great on high resolutions screens.
It's also too bad that widescreen TVs don't support it. It could work like those 4:3 tvs with a widescreen mode which shows anamorpic DVDs at full resolution by squeezing the picture.

I don't really get what the problem would be with stretching...

[zambelli]

There's no difference in horizontal stretch between NTSC and PAL. Their horizontal resolutions are the same. If your display is 1800 pixels wide, the 720 pixels will be stretched to 1800, no matter if it's PAL or NTSC. In both cases there will be a 250% stretch.
I think you missed my point. I wasn't talking about stretching to final display sizes. I was talking about stretching to meet the DAR.
If your 720x480 NTSC DVD is widescreen enhanced, it will get resized to ~852x480 by the decoder/player in order to achieve square pixels in a 16:9 aspect ratio. What it actually gets resized to to fit your screen is a completely unrelated story.
A 720x576 PAL widescreen enhanced DVD needs to be stretched to 1024x576 to meet the 16:9 aspect ratio. Divide 1024 by 704 and you'll see that every PAL DVD gets stretched horizontally 1.4545x.

Edit: Adjusted numbers for standard ITU.601 resolutions (704, not 720).

[MrTroy]

What it actually gets resized to to fit your screen is a completely unrelated story.

I don't know what you mean. Of course it's true that PAL has to be stretched more (because NTSC is 1.5:1 natively, and PAL 1.25:1). But would this actually be visible on screen, if the end resolution (our hypothetical 1800px) is the same in both cases?

[zambelli]

I don't know what you mean. Of course it's true that PAL has to be stretched more (because NTSC is 1.5:1 natively, and PAL 1.25:1). But would this actually be visible on screen, if the end resolution (our hypothetical 1800px) is the same in both cases?
What matters is the pixel aspect ratio of the display. If you have display with square pixels, which is most likely, the first imperative is to resize the DVD to square pixels. As you noted, DVDs have the same resolution regardless of whether they're 4:3 or 16:9, so that means their pixels are non-square pixels of variable pixel aspect ratio.

To get square pixels from DVDs, one needs to multiply their horizontal resolution by these pixel expansion ratios:
DVD NTSC fullscreen 10:11
DVD NTSC widescreen 40:33
DVD PAL fullscreen 12:11
DVD PAL widescreen 16:11

If 2.21:1 DAR flags were supported, you'd have to multiply the horizontal resolution by these pixel expansion ratios:
DVD NTSC ultrawidescreen 1.50681818
DVD PAL ultrawidescreen 1.808181818
in order to get square pixels.

[foxyshadis]

ie, the decoder resizes first, then the renderer resizes at the end. Especially if the decoder uses a lame resize, it can look worse than a single bicubic resize.

[Mug Funky]

umm... 2.21:1 isn't DVD compliant. mpeg-2 supports it, but DVD is a subset of mpeg-2. it doesn't support a lot of stuff mpeg-2 does.

all you get in DVD is 4:3 and various 16:9 displaying methods (pan-scan or letterbox or a combination).

[bugmenotwillyou]

kikddd

[SeeMoreDigital]

Just a thought...

It might be kinda interesting if someone were able to create a tool that could change the PAR of MPEG-2 (and even MPEG-1) streams.... Offering the same kind of flexibility MPEG4 Modifier offers us for MPEG-4 streams....


Cheers

[mpucoder]

mpeg-2 does not have a PAR, only the Display Aspect Ratio, which is completely independant of the frame size. Changing AR in DVD video streams has no effect on playback, it is the AR information in the ifo file that is used.

[SeeMoreDigital]

mpeg-2 does not have a PAR, only the Display Aspect Ratio, which is completely independant of the frame size. Changing AR in DVD video streams has no effect on playback, it is the AR information in the ifo file that is used.In this particular instance I was not thinking about placing the MPEG-2 stream within a VOB container or having it parsed by an IFO file.

Is it totally impossible to assign "weird and wonderful" aspect ratio signalling values to MPEG-2 bit-stream?

[mpucoder]

mpeg-2 has only 4 display aspect ratios - 1:1, 4:3, 16:9, and 2.21:1