5 Replies Latest reply on Jun 24, 2009 4:39 PM by Stan Jones

# Whats the math behind converting from square pixels to non-square...?

Does anyone know the math behind resizing a video from square pixel to non-square?

Say if I have a 720x480 4:3 video and I render it out as square pixels. So then I need to resize it down to non-square so that it will no longer be stretched....I know I can just use premiere or quicktime to scale it properly for me but what is the math behind it? I'll admit im no mathematician, all i know is that square pixels = 1 and dv = .9, how you go from there is beyond me.

Reason I ask is because I have a 800x600 video in square pixels and just want to know how in the world it makes sense that you change it to 755x600 and the stretch is gone. How does do you get to that number?

• ###### 1. Re: Whats the math behind converting from square pixels to non-square...?

720 x 480 using square pixels would be 640 x 480.

800 x 600 should have no stretch to it.

• ###### 2. Re: Whats the math behind converting from square pixels to non-square...?

H x PAR = SH

Explanation: Horizontal resolution (H) times PAR (Pixel Aspect Ratio) equals SH, Square pixel horizontal resolution.

• ###### 3. Re: Whats the math behind converting from square pixels to non-square...?

so using that math

720 x .9 = 648 not 640....so why is 720x480 nonsquare = 640x480 square...?

• ###### 4. Re: Whats the math behind converting from square pixels to non-square...?

This is because we are (to a certain extent) comparing apples to oranges. Just because you have 720x480 @ ~0.9 and 640x480 @ 1.0 does NOT mean that they represent the same area. They don't, in fact:

The 4x3 or 16x9 "area" of standard NTSC or PAL video is approximately the innermost 704 pixels. So…

720 - 704 = 16

You must crop 8 pixels from each side prior to scaling.

720 - 16 = 704

704 * (10/11) = 640

• ###### 5. Re: Whats the math behind converting from square pixels to non-square...?

I don't have CS4, and have only been scanning the CS4 forums; so there may be better links regarding this in this forum.  CS4 changed the par to a more "correct" option.  It is not .9 now but .91.  (See Dan's little "about" before the .9.)  And when you are dealing with 800x600, there may be other issues.

I like the  AE/Meyers explanations of all this (so also see the regular help for Premiere):

Also see the free Meyers video on Lynda.com

http://www.lynda.com/home/Player.aspx?lpk4=40550

EDIT: I missed the more relevant thing in Dan's: see the 10/11 as the multiplier rather than .9?  10/11 is the "new" .909090 = .91.

Message was edited by: Stan Jones