More about DPI

nolonemo wrote on 6/5/2007, 3:27 PM
Assuming you were scanning a 50x37.5" image for your project (that would be quite some scanner), I would think you would first find the smallest area you wanted to zoom in on. Assuming the aspect ratio of that area was 3:4, you would want that area in your scanned image to be 720 pixels along the long edge. Assuming that that area is 6x8 inches on the 50x37.5" image, for example, you would want to scan the 50x37.5 image at 90 dpi (6" image becomes 720 pixels long). That should give you the minimum size full image while keeping maximum quality on the zoom in. I have no idea what your total image size would be, but it would be pretty $%@#& large.

Comments

johnmeyer wrote on 6/5/2007, 6:21 PM
I posted this earlier today in another thread, and that post, in turn, was a actually copied from something I wrote several years ago. I think it will help you. Here it is:Dots per inch (dpi) isn't useful, as Kelly has already stated, but it CAN be converted to something useful. What is useful? The measure of the total pixels in your video frame, still photo file, or other image.

There are two common ways to express the total number of pixels:

1. Give the number of pixels in the horizontal direction and also the number in the vertical direction. For instance, NTSC DV 4:3 video is 720 pixels by 480 pixels.

2. Give the total number of pixels. This is simply the result of multiplying the two numbers given in #1: 720 x 480 = 0.346 megapixels. Digital still photo cameras are usually specified in this terminology.

Now, in printing, dots per inch is often used, because to make a picture look good on a piece of paper, you have to stuff a certain number of dots into each square inch of paper. If you don't, the picture looks grainy, like newspaper. Based on how your eye perceives things, and on the technology of printing, it turns out that pictures start to look pretty good when you put about a 150 dots (horizontal) by 150 dots (vertical) into each square inch.

Now, with that as background, here is how you can convert dots per inch into one of the two measures (#1 and #2) that are useful when dealing with video: you simply multiply the dpi times the size of the image. So, if you scan a physical photograph (using a flatbed scanner, for instance) and you scan at 200 dpi, and the picture is a 4x6 photo, then the total pixels is 200 times 4 = 800 by 200 time 6 = 1200. Thus, the resulting file will be 800x1200 (or 1200x800, depending on how you rotate the image) and the total pixels will 800x1200 = 0.960 megapixels.

Thus endeth today's lesson. There will be a test at start of class tomorrow.
Coursedesign wrote on 6/5/2007, 6:56 PM
Nolonemo, thanks, that umm, helps...

At least as long as everybody realizes that Vegas has no opinion on how closely the pixels should be positioned in video.

It literally doesn't care if you show the same video pixels tightly squeezed on an iPod or projected a lot farther apart on a 12' diagonal Home Theater screen.

DPI are just a way here to get from the human-friendly "inches across" to the machine-friendly "pixels across".