Microsoft KB Archive/139421

When It’s Important to Know the DPI and When It Doesn’t Matter

PSS ID Number: Q139421 Article last modified on 03-28-1996

3.50

IRIX

= SUMMARY =

This article describes the difference between DPI (dots per inch) and size (in pixels) and explains why you sometimes need to know the DPI.

= MORE INFORMATION =

DPI gives the number of dots packed into a one-inch space on a single line. This doesn’t means that an image with a higher DPI will be smaller; it just means that the image will have more definition because it uses more dots (or pixels) to define the same aera.

A dot is not the same thing as a pixel. Dots are always discussed in relation to a unit of measurement (an inch). Dots per inch are relative units of measurement (dots relative to inches), whereas pixels are absolute units of measurement. DPI tells you how good the resolution of the image was while it was scanned, and pixels tell you how big the actual image is or how much space it will take up on your computer screen.

Pixels define the size of the image. For example 1024 by 768 pixels means 1024 pixels wide by 768 pixels high.

DPI tells you how many pixels of that same image you would need to exactly match the size of the original image if you were to print that same image.

For example, if you have two images, both 1024 by 768 pixels, but one image is at 85.33 DPI and the other is at 128 DPI, you can think of the one with the higher DPI value as having its pixels packed tighter together than the one with a lower DPI. In fact, the first image (85.33 DPI) was scanned at field 12 and the second one at field 8. This means that 1024 divided by 85.33 equals approximately 12 (or field 12). So, if you scan a field 12 at 85.33 DPI, you will get an image that is 1024 pixels wide. In the same manner, 1024 divided by 128 equals 8 (or field 8).

Better resolution equals higher resolution values and higher DPI and the more dots you pack in a one-inch space, the higher the DPI will be. However, this doesn’t mean that the bigger the size of the image (say 1024 by 768 pixels instead of 640 by 480), the higher the resolution and the higher the DPI.

DPI is directly related to the scanned field. It doesn’t affect the size of the image. The relationship between field size in traditional animation and DPI can be defined as “For the same output size, the smaller the field, the higher the DPI.” Why? Because to keep the same file size, each pixel in the image needs to represent a smaller area, thus packing them tighter.

For example, think of a file as a sardine can. Each sardine represents one pixel. Some cans have a few big sardines, while others may have many more sardines packed in the same can size. Of course, to pack more sardines into the same can size, you have to take smaller sardines because the can itself is the same size. Dots are like the sardines and files are like the cans that contain sardines. The more dots you pack in, the higher the resolution will be.

The confusion comes from the fact that you need to know the DPI (or field size) when you scan in an image but not when you output to video or film. Most of the time the DPI value is irrelevent when you output. When you output to video or film what is important is the file size itself: 640 by 480 pixels or 720 by 486 pixels for video or 2K (2048 by 1536 pixels) or 4K (4096 by 3072 pixels) for 35mm film.

Differences Between Video and Film Output
In video, what’s important is the number of lines. PAL, SeCAM, and NTSC video use different numbers of lines but the width is the same. That’s why in Toonz you hear discussions about the vertical fit for TV. The DPI question is irrelevent.

In film, though, what’s important is the number of pixels that you want to have on a single line. As with TV, the DPI question is irrelevent because whatever you scanned in will never give you good results if you try to match the DPI. That’s why in Toonz you hear discussions about the horizontal fit for film.

NOTE: Softimage Inc. is a wholly owned subsidiary of Microsoft Corporation.

Additional reference words: 3.50 KBCategory: kbgraphic KBSubcategory: ============================================================================= Copyright Microsoft Corporation 1996.