aspect ratio for web

dvideo wrote on 2/27/2007, 6:58 AM
i know its common for 720 * 480 to be switched to 640 *480, also 360 *240 is commonly rendered instead as 320 *240. what would be a common file for the web, the next size down from 360*240?

Also, subtracting 640 from 720 is 80, and subtracting 320 from 360 is 40. How is it determented to drop 80 from the bigger file type, but only 40 from the smaller in order to keep the aspect ratio as desired for the web?

thanks

Comments

Former user wrote on 2/27/2007, 7:44 AM
In order to get 360 x 240 out of 720 x 480, you divide by 2.

So if the difference in the first is 80, divide by 2 (40).

Remember these are ratios, basically fractions with the prime being 4 x 3 for the correct TV aspect.

Dave T2
dvideo wrote on 2/27/2007, 8:12 AM
what i'm wondering is when dividing by 2, you get 360, so my question is, why subtract 40 to give 320 * 240 ? how do you know to subtract 40?
thanks
Former user wrote on 2/27/2007, 8:19 AM
640 x 480 is true 4 x 3 square pixels (such as on a computer). 720 x 480 is 4 x 3 using non-square pixels (DV video format.

If you divide 640 by 2, you get 320. 320 x 240 is true 4 x 3 square pixels.

Dave T2
John_Cline wrote on 2/27/2007, 8:37 AM
Standard DV is based on rectangular pixels. It is 720x480 (which is a ratio of 1.5:1) but it displays on a TV at a ratio of 4:3, which is 1.3333:1. Computer displays are based on square pixels, so a DV video which is 720x480 needs to be scaled to 640x480 so it displays correctly on a computer. When resizing for a computer display, take the vertical size, in this case 480, and multiply by 1.3333. If you want the vertical size to be 240, multiply that by 1.3333, which ends up being 320.

Or if you want a specific final horizontal size, take the desired horizonal size and divide it by 1.333 to get the vertical size.

Actually, to maintain an absolutely correct aspect ratio, 640x480 should really be 655x480 and 320x240 should be 327.5x240, but most codecs like Divx or WMV prefer that the image dimensions are equally divisible by 8, some prefer its divisible by 16. So it's 640x480, 512x384, 320x240, 160x120...

John
mikkie wrote on 2/27/2007, 11:25 AM
Every great once in a while being old has it perks. ;?}

Looooong time back analog video was captured to PC &/or workstation at 640 x 480 (NTSC). One company went & upped the scanning rate so more data per line of video was captured, & this became the 720 used for D1 etc... About the same time PAR was born.

There's a difference in the frame widths both because of PAR & because there's more data per line in the 720 version. Where we get 655 & 640 for square, 720 & 704 for common non-square like DVD.
FWIW...
For editing & display, what's often most important is what hardware & software expect; put 640 or 655 on a DVD, & the player will stretch it one way or the other (NTSC/PAL). Just about as important: going from 720 to square pixel 655 or 640, even if an intermediate step, means you lose picture data. To a PC a pixel is square, & an original 720 frame has 720 pixels per line. Converting to 655 you lose the difference - that's made up by interpolation converting back.