Tip: Accurate integer ratio for NTSC DV still

Tech Diver wrote on 5/19/2006, 5:59 AM

By now most of us are familiar with the "optimal" pixel dimensions for still images used in NTSC DV to avoid scaling. Namely 655 by 480. Though rounded to an integer value of 655, the precise horizontal value is actually calculated as:
720 x 0.9091 = 654.552

If you have a highly detailed image and plan on zooming in, you will want an image with larger dimensions than 655x480. But what size? Do just double or triple 655 by 480? To answer this, let's calculate the ratio of the horizontal to the vertical using the precise values:
654.552 / 480 = 1.36365

This is a pretty nasty value as it can not be expressed PRECISELY with two "reasonably" sized integers. But it can be APPROXIMATED with a very high degree of accuracy. Without going into detail of how I went about determining the values using prime numbers, let me just state that the ratio I came up with is: 15 to 11 .

Remember how the precise ratio is 1.36365, well the approximation yields:
15 / 11 = 1.36363636 (repeating)
This approximation has an error only one part in 100,000 !!! In fact, this is significantly more accurate than the 655 / 480 ratio.

The practical use of the 15 to 11 ratio is that if you want zoom an image for example, somewhere around 150% you may want to use an image sized as 900x660 or 990x726 or 1065x781 or ... The possibilities go on and on. The point is that you are not limited to multiples of 655x480 and have a MUCH finer "granularity" of choices.

Comments

Chienworks wrote on 5/19/2006, 6:14 AM

Slight correction: it's not 0.9091, it's actually 0.909090909090909090909090909......, so the optimal size is 654.545454545454545454.....x480.
So 654.545454.../480 = 1.363636...

However, that really doesn't make any noticeable difference to your argument, especially since the correction makes 15/11 exactly right.

On the other hand, does it really matter? No matter what size image you bring in to Vegas, it will size it properly to maintain correct proportions. If you open up Pan/Crop and set to match output aspect then the image will fill the frame (instead of having the image fit the frame) and still maintain proportions.

Tech Diver wrote on 5/19/2006, 6:26 AM

Thanks for the correction, and I am pleasantly surprised to learn that 15/11 is 100% accurate.

Where this matters is when I am doing a montage of a great many stills and wish to keep the disk storage usages at a minimum. I don't want to use stills any larger than they need to be, and I like to be able to burn a CD (rather than a DVD) to archive the source material.

With my most recent project involving photos from the Hubble space telescope, where the images are huge and my zooming is great, reducing the image size even slightly has a significant impact on data storage.

Former user wrote on 5/19/2006, 6:26 AM

I have always used 720 x 540 because I would rather scale down than up. But I know mathematically this is not exact but has worked well for me.

Dave T2

Tech Diver wrote on 5/19/2006, 6:54 AM

If you're curious, an image at 720 x 528 would give you a "correct" ratio. But as Kelly mentioned, setting "match output aspect" in pan/zoom will always work no matter what your image size.

Chienworks wrote on 5/19/2006, 7:09 AM

Yes, resizing to the minimum dimensions necessary to allow the zoom level you want is a good idea.

However, if i had a 6400x4800 image that i wanted to zoom in about 3x, i'd resize it to 2000x1500. I wouldn't worry about cropping it to 2055x1507 or 2040x1496. That's too much work when Vegas takes care of those details for me.

Ain't computers great? :)

Former user wrote on 5/19/2006, 8:13 AM

I think I got the 720 x 540 from working with D1 which is 720 x 486 rez. I have a hard time shifting from D1 to DV in my head sometimes so I just locked in on the D1.

Dave T2