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Old 20 Oct 2003, 06:58 AM   #1
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Question Aspect Ratio and Borders

OK, this is somewhat technical. Because of Set-top DVD/TV setups using overburn, it's a good idea to encode a black border around the video, right? So... in TMPEGenc Plus 2.5x I go to the Clip frame dialog and use "Center - Custom Size" and "666 x 444" (Seems to make a pretty optimal border).

My video file is 640x360 (1:1 VGA / 16:9 Display). So the pixels are square, and the displayed image is 16 x 9 widescreen, right? I use the 16 x 9 encode with 720 x 480 (1.5:1 / 16:9). Theoretically, encoding full screen (640x360 -> 720x480) SHOULD be the same aspect ratio, right? However, if I select "Full Screen - Keep Aspect Ratio" It changes the output to 704x480 with black borders (8pix each, I guess) at the sides. I know 704x480 is Broadcast format or something like that...

My question is, if the source is 16x9, and 720x480 is SUPPOSED to be 16x9, why does it shrink to 704x480 when I tell it to keep aspect ratio? Shouldn't it be the same? What am I missing? Are all A/R calculations based on 704x480, and my border calculations flawed?

I'm going to be encoding some unusal aspect files (10:7 (display 4:3), and 20:11 (display Widescreen)) and I need to know what the right Keep A/R calculations are so I can encode a border without screwing with the A/R (I hate dropping data, plus I lose encoded (not floating) subtitle text if I don't).

Also, I've noticed that even with encoded borders on commercial DVDs, the image is slightly off-center. I lose 8-10 pixels on the right, no matter what. It's like I need to encode in an off-center box (bigger border on the right) to keep all video. I think that might be just my crappy DVD player though... Any thoughts?

(If someone can help, but needs more info, I can take some screenshots to show what I mean. Thanks!)
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Old 20 Oct 2003, 07:11 AM   #2
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Divide 720 by 480. It's 1.5.
Yeah, DVDs are a bit funny- the borders are seldom symmetrical.
And a 16:9 file on a DVD will be 720x480 with a total of 75 pixels of top and bottom border. 720x405 is 16:9. I'm pretty sure a DVD always has 720x480 pixels. I don't have a 16:9 TV so I don't know how it gets played back properly.
I'm also confused as to why the box says 16:9 AND 1.85:1 at the same time. And why the video actually turns out to be 1.53:1.
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Old 20 Oct 2003, 07:25 AM   #3
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Default To Metadan

"I use the 16 x 9 encode with 720 x 480 (1.5:1 / 16:9). Theoretically, encoding full screen (640x360 -> 720x480) SHOULD be the same aspect ratio, right?"

If you are under the impression that 640x360 is a 1.5:1 ratio, I'd suggest you carefully check your math...
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Old 20 Oct 2003, 07:41 AM   #4
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Quote:
Originally posted by setarip
"I use the 16 x 9 encode with 720 x 480 (1.5:1 / 16:9). Theoretically, encoding full screen (640x360 -> 720x480) SHOULD be the same aspect ratio, right?"

If you are under the impression that 640x360 is a 1.5:1 ratio, I'd suggest you carefully check your math...
No, no. 1.5:1 is for the DATA, not the display. IE, the data units are half again as tall as they are wide: 720 / 480 = 1.5, of course, the display isn't square, so that's kind of an abstract picture...

The 1.5 is just for calculating border and keeping the existing A/R. If you've already translated the 640x360 to 720x480 (with the same A/R: 16x9) They should display exactly the same, right? To add a border, I should knock off 1.5 pixels off the sides for every 1 off the top/bottom, and this will keep the A/R intact, right? Sorry, this is kind of confusing and I'm a little slow... Matrix Math isn't my forte, just a neccessary evil. Who came up with these ridiculous sizes, anyway? Who the hell said "Hey, 16:9 is a great aspect ratio for widescreen. Hey, I've got an idea! Let's encode a 16x9 ratio (1.777x) into a data file with a 3x2 (1.5) aspect ratio! It'll be confusing as hell!
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Old 20 Oct 2003, 08:44 AM   #5
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Quote:
Originally posted by colep
Divide 720 by 480. It's 1.5.
Yeah, DVDs are a bit funny- the borders are seldom symmetrical.
And a 16:9 file on a DVD will be 720x480 with a total of 75 pixels of top and bottom border. 720x405 is 16:9. I'm pretty sure a DVD always has 720x480 pixels. I don't have a 16:9 TV so I don't know how it gets played back properly.
I'm also confused as to why the box says 16:9 AND 1.85:1 at the same time. And why the video actually turns out to be 1.53:1.
The major problem here is that there's so much translation. There's a DATA A/R as well as a DISPLAY A/R. One has to do with the encoding of a file (always 1:1, really - data is dimensionless...) and the other, the way it's displayed.

The 720x480 DATA is used right up, as in the 75 pixel border is not encoded. The DATA is encoded at 1.5:1 (3:2). Right? Divide 720 into 480...
16:9 DISPLAY on the other hand, is a ratio of 1.77777:1 (divide 16 into 9, or your 720x405) So, two files have the same DISPLAY A/R, it doesn't matter how the data is encoded, it will DISPLAY with the same A/R, just change the effective unit/pixel SHAPE. I could display a 100 x 400 encoded video (1:4 DATA A/R) with a DISPLAY A/R of 4:3 (1.33333). It would be a WIDE square like your TV, there would just be less detail horizontally (wide-ass pixels).

SO! The MPEG FILE is 720x480: A/R 1.5 or 3:2, but DISPLAYED at 1.777 or 16:9. My original question is: this being the case, a file with a 16:9 DISPLAY A/R (whether it be 640x360 or 403x917 DATA), should translate directly to another 16:9 DISPLAY, using the full 720 x 480, and look the same, at least as far as Aspect Ratio is concerned, RIGHT? Why then does TMPGEnc encode at 704 x 480 with a 8pix border when I tell it to keep the A/R??

I have no idea where you're getting 1.85:1 or 1.53:1 from. GRRR!!! I'm about ready to say to hell with all DVD paraphernalia and attach a computer to my TV and use PowerDVD fullscreen so I don't lose any frickin' data! 'Course, that doesn't help me share with my buddies who all have DVD players...
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