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Old October 30th, 2007, 03:21 PM   #1
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SD video playback on computer - double check

Ok, SD video is 720x480 - when played back on video devices, right?

SD video played back on a computer is 640x480, right?

So if I'm resizing video for distribution on the web, I work off of 640x480?

To use 480 (640 x 75%) as my width, then my height would be 360, right? (480 x 75%) Or am I totally in the weeds?

Initially I thought the height would be 320. But that was when I was working off of a 720x480 frame size.

Sorry for the simple question, that I may have answered myself, but I just need to be sure that I've done this the correct way. I'd rather asked a simple question here than get it wrong somewhere else...

Thanks for the help...

Kevin
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Old October 30th, 2007, 03:44 PM   #2
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Computer monitors have square pixels, so the image must have the exact same proportions as its aspect ratio - either 16:9 or 4:3. So the formulas are:

16:9 video
------------
16 / 9 X height = width
or
9 / 16 X width = height

4:3 video
-----------
4 / 3 x height = width
or
3 / 4 X width = height
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Old October 30th, 2007, 03:50 PM   #3
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Thanks.

I thought that's what I had forgot to figure in, square pixels...

Kevin
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Old October 30th, 2007, 06:23 PM   #4
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Quote:
Originally Posted by Kevin Randolph View Post
Ok, SD video is 720x480 - when played back on video devices, right?
480i/480p SD video is 720x480, period.
Quote:
Originally Posted by Kevin Randolph View Post
SD video played back on a computer is 640x480, right?
SD video is 720x480.
Quote:
Originally Posted by Kevin Randolph View Post
So if I'm resizing video for distribution on the web, I work off of 640x480?
Depends. Many players do not check video aspect ratio, many codecs do not set it properly, and many encoding types do not have aspect ratio field at all assuming 1:1 pixel aspect ratio (PAR). Resizing video to 1:1 PAR for computer playback is the easiest way to avoid problems.
Quote:
Originally Posted by Boyd Ostroff
Computer monitors have square pixels
Not true. We have problems with non-square PAR partly because many programmers don't care to check for device caps and for window or image aspect ratio. Every PC-based system has methods for checking device aspect ratio, GetDeviceCaps() comes to mind. Another problem is that only some image/video formats include PAR field, like GIF, but this information is largely ignored or not properly set.

In the PC world 1:1 PAR was introduced with VGA standard, when was it, 1988? 640x480 gives 1.3(3), exactly the proportions of 4:3 monitor. Prior EGA standard had 640x350, CGA was capable of 320x200 or 640x200, all these settings produced proper picture on a standard 4:3 monitor.

This is an interesting entry: http://www.microsoft.com/windows/win...xelFrames.aspx

Quote:
AVI files do not contain pixel aspect ratio information, so many players, including Windows Media Player, render all AVI files with square pixels. Therefore, the frame appears stretched or squeezed horizontally when the file is played back.
How Microsoft could be so careless? Windows dates back to 1983, it worked on all kind of trash unknown in Mac world. How could Microsoft create a video container that does not contain PAR info?

Last edited by Michael Jouravlev; October 30th, 2007 at 06:59 PM.
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