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Quesitons about DVD aspect ratio

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Bobby Hall
Quesitons about DVD aspect ratio
on May 13, 2016 at 11:58:18 pm
Last Edited By Bobby Hall on May 13, 2016 at 11:58:51 pm

I'm trying to better understand aspect ratios of DVDs and how they're displayed. I've read that all DVDs, whether 4:3 or 16:9, are 720x480 pixels.

If you have a fullframe pan & scan DVD and display it in its native 1.5:1 dimensions, how would this look? Does it look horizontally stretched? And is showing it in 4:3 just a matter of squishing it horizontally until it's 4:3?

And if you have a 4:3 letterboxed DVD, does it do the same thing as above and the only difference is black bars are encoded in the top and bottom of the frame?

If you have a 16:9 Anamorphic DVD, which is also 720x480 pixels, how does it look if displayed in its native 1.5:1 ratio? I don't quite understand this and I'm just curious how it would look compared to a 4:3 DVD.

Also, since pixels on a DVD are rectangular, I don't understand how the 720x480 ratio is 1.5:1. To me this only makes sense if the pixels are square because then the width and height would be the same.

If the width of a pixel is longer than its height, you can't divide them to get the aspect ratio, right? For instance, if you had a frame of 15 pixels wide and 10 pixels high, and if the pixels are 2 inches wide and 1 inch high (for the sake of argument), then the width of the frame would be 30 inches (15 pixels times 2 inches) and the height would be 10 inches (10 pixels times 1 inch). This would give an aspect ratio of 3:1. It would only be 1.5: 1 if the pixels were square. So how does a 720x480 DVD with rectangular pixels come out to 1.5:1?

Thanks to anyone who can clear this up for me!


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Jeff Pulera
Re: Quesitons about DVD aspect ratio
on May 26, 2016 at 3:01:53 pm

Your reference to a "1.5:1"" aspect ratio is incorrect because you are assuming square pixels with a 1.0 PAR (Pixel Aspect Ratio). DV video actually has a PAR of 0.9 (non-square pixels), while DV Widescreen uses a 1.2 PAR.

Year ago, 4:3 video on a computer was 640x480 with 1.0 PAR (square pixels). For whatever technical reason, with the advent of DVDs and digital video (miniDV cameras for instance), the format was decided as 720x480 with 0.9 PAR. The math doesn't come out perfect...a few extra lines were padded into the signal as I once read, but don't recall the specifics.

So basically, due to the 0.9 PAR, 720x480 still produces a video with a 4:3 aspect, just like a 640x480 source with a 1.0 PAR would.

Widescreen DVD uses a 1.2 PAR and if you multiply 720 x 1.2 you get 864. So in square pixels, the image would be 864x480 pixels which is 16:9. For computer/web viewing, you don't want non-square pixels since most video players assume 1.0 PAR. This means that your 720x480 widescreen image with 1.2 PAR would look like a 4:3 image because the computer would disregard the fact that the pixels ought to be displayed as rectangles and instead displays them as squares, changing the aspect of the entire image. When creating a widescreen video for the computer, one would encode as 864x480 with 1.0 PAR.

864/16 = 54, and 480/9=53.33 (close enough) so there is your 16:9 image. I know, the math is not perfect, but I didn't create these formats ;-)

http://www.miraizon.com/support/info_aspectratio.html

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

Hope this helps.

Jeff Pulera
Safe Harbor Computers


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Bobby Hall
Re: Quesitons about DVD aspect ratio
on May 26, 2016 at 10:27:22 pm

Thanks for the info Jeff!

How do all the pixels fit on a widescreen DVD? If they're longer than the pixels on a 4:3 DVD (1.2 compared to .9) I don't understand how that works. Could you theoretically have any kind of PAR you wanted as long as it was 720x480?

And what would the DVD look like on a TV screen if it didn't have the 4:3 or 16:9 flag encoded in it? I'm curious what 720x480 pixels would look like without the display aspect ratio encoded in it.

Also, you said that "...your 720x480 widescreen image with 1.2 PAR would look like a 4:3 image because the computer would disregard the fact that the pixels ought to be displayed as rectangles and instead displays them as squares..." But wouldn't it actually be 1.5:1 instead of 4:3 on a computer screen? Because if a computer interprets the pixels are square, then 720 divided by 480 is 1.5.

Thanks!


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Jeff Pulera
Re: Quesitons about DVD aspect ratio
on May 27, 2016 at 1:18:12 pm

"Because if a computer interprets the pixels are square, then 720 divided by 480 is 1.5."

I won't argue that point, but we were really talking about DVDs and not computer video ;-)

How do all the pixels fit on a widescreen DVD? What you have to remember is that it's all data...the DVD has 720x480 pixels for each frame regardless of whether they are 0.9 or 1.2 PAR. Same number of pixels taking up the exact same amount of space on the disc. The difference is in how those pixels are interpreted - how should they be displayed?

720 pixels across that are taller than they are wide make a narrower image that 720 pixels that are wider than tall. That's all it is.

And again, in regards to DVD, the aspect ratio is not 1.5 because we are not dealing with square pixels so that changes the equation.

Maybe you are overthinking the whole deal. Do we need to worry about the math going on "under the hood"? If making a 4:3 DVD, then encode as 0.9 PAR and for widescreen use the 1.2 preset. In regards to "can we use any PAR we want"...well...I dunno, I guess the image would be warped in some regard.

In recap, it's 0.9 or 1.2 for DVD. Period. For Blu-ray, which is 1080 or 720 HD, usually square pixels. Oh wait...here's a fun fact! Familiar with the old HDV camcorders? They recorded anamorphic HD, which is 1440x1080 pixels with 1.33 PAR. Guess what? 1440 x 1.33 = 1920, so there you get the 1920x1080 playback image. It is possible to create a 1080i Blu-ray using the 1440x1080 source, is part of the Blu-ray spec.

But for computers, encoded videos should ALWAYS use 1.0 PAR. Always.

Thanks

Jeff


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Bobby Hall
Re: Quesitons about DVD aspect ratio
on May 28, 2016 at 3:49:17 am

Thanks Jeff.

You mentioned a 1080i Blu-Ray. I thought all Blu-Rays were progressive. Can they also be interlaced?


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Jeff Pulera
Re: Quesitons about DVD aspect ratio
on May 31, 2016 at 2:54:28 pm

Hi Bobby,

Good question! Yes, one would assume Blu-ray would be progressive, however the options are limited.

Blu-ray can be 1080i or 1080p24. 1080p30 and 1080p60 are not supported.

For 720p, you can use 720p60 or 720p24. Check out the H.264 Blu-ray export presets in Premiere and these are what you will find.

Any export option you see with a 25 or 50 in it is for PAL countries.

https://en.wikipedia.org/wiki/Blu-ray

Thanks

Jeff Pulera
Safe Harbor Computers


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