720 x 480 vs. 640 x 480

Steve Grisetti wrote on 10/16/2003, 9:14 AM

Can someone clear up my confusion with regards to the ideal dimensions for a still which will be used in a DV video?

I've always heard 720 x 480 is the standard size (This is, of course, assuming no special effects, such as a zoom) but, since TVs have a 4 x 3 aspect ratio, wouldn't 640 x 480 be a better fit?

In fact, I used 720 x 480 stills in a recent project and, while it looked fine on my TV, when I showed it on a video projector you could see edge showing above and below the still!

And, while we're doing math, how does that 480 pixel count vertically fit with the idea that a TV has 525 scan lines vertically?

Maybe I'd sleep better if I did sit up worrying about this stuff...

Comments

Former user wrote on 10/16/2003, 11:56 AM

It has something to do with square pixels vs. non-square. Digital is 720 x 480, analog is 640 x 480. There is a lot of info on the web about it if you do a search.

Dave T2

Chienworks wrote on 10/16/2003, 9:10 PM

DV pixels are 0.90909... times as wide as they are tall, so they're a little narrower than square. If you multiply 720 x 0.90909... you get 654.5454... not 640. This is because television screens are actually a tiny bit wider than 4x3. It's a little confusing, but in the long run it doesn't really matter too much.

When you use stills in your project, make them 654x480 or something proportional to that and they'll fit perfectly.

480 into 525? The vertical pixels get spread out over the scan lines to fill the picture. I guess, anyway.

discdude wrote on 10/16/2003, 9:33 PM

The visible picture area for NTSC is 480 lines. The remaining 45 lines make up the VBI (Vertical Blanking Interval) which is used for sync, closed captioning, etc. Digital video doesn't need those extra 45 lines, hence resolution is "only" 480 pixels.

Steve Grisetti wrote on 10/17/2003, 8:15 AM

Thanks, guys.

I did some on-line research on square pixels vs. .9 pixels, and it all just confused me more.

Chienworks bottom-lining really was very helpful. (And thanks, Dave, for chiming in too.)

So 654x480 it is!

Former user wrote on 10/17/2003, 10:26 AM

Yeah, I have read it over and over and still get confused. ;)

Dave T2

BrianJ wrote on 10/18/2003, 6:12 AM

I'm very interested in this thread. I've got a huge box of old family photos going back almost 100 years. They're all shapes and sizes, b&w and color, prints and 35mm slides. I plan to scan them and put together a slideshow that I can distribute to family members on DVDs. I have an Epson Perfection 2400 scanner that will scan at up to 2400 dpi. If I understand correctly, I can calculate the best dpi setting for each photo by dividing the longest dimension of the photo by 654 either or 480? e.g. for a 4" high x 6" wide photo, 654/6 = 109 dpi, or for a 6" high x 4" wide photo it would be 480/6 =80 dpi. Does this sound right? I'll have to do a little experimenting.

djcc wrote on 10/18/2003, 8:57 AM

Given the archival nature of your family photo project, you might want to consider scanning them at a higher resolution than TV can take advantage of.... afterall, once you have them scanned to digital form, you might want to print some out, create a PC screen saver, etc. Also, you never know what the future holds regarding TV resolution - so, since its a big effort anyway, why not just start with higher resolutions scans - using the same setting for all photos... If you are concerne with file sizes for this particular project, you can use any one of a number of utilities to batch reduce copies of your scans.

Chienworks wrote on 10/18/2003, 4:09 PM

BrianJ: you're on the right path, if you only want to use the scans in a video project.

In addition to what djcc said about using the scans at higher resolution for other purposes, you might want to scan larger than 654x480 anyway so that you can trim or zoom in while creating the video slideshow. Typically i tend to scan 6x4" prints at 300dpi giving an 1800x1200 image. This is still small enough so that VideoFactory (actually i use Vegas) doesn't get bogged down. It also allows me to zoom in closer on smaller details.