DPI

DPI is an acronym for dots per inch, a measure of resolution used for computer peripherals such as printerss, scanners and monitors.

DPI always refers to a physical representation of pixels per length unit.

Purely computer generated images, such as from raytracing, digital paint programs, mathematical formulas (f.i. fractals) or from digital cameras, do not usually have a physical representation. Their resolution is virtual in the sense that there is no specification on how big the actual physical representation of the generated image would be.

For instance, a 4 Megapixel digital camera could create a 2000x1500 pixel image. Only when outputting this image to a physical medium with a certain size (say printing on to paper 20cm by 15cm) does the DPI get defined. Outputting the image to a different physical medium (such as a monitor) would result in a different DPI number.

The resulting DPI depend both on the resolution of the image (only so many pixels exist and can be put as "dots per inch") and the output device (printers, monitors etc. all have their technical limits on how many DPI they support).

Laser printers typically reach 300 DPI, though 600 DPI is becoming more common. Commercial typesetters are usually around 1200 DPI.

Ink jet printers often boost with very high DPI numbers. The reason for that is that they print each "pixel" by outputting a large (f.i. 5 or even 14) number of small ink droplets from a few colored inks. Only by taking each of these droplets as a dots such high DPI numbers exist.

This article was originally based on material from the Free On-line Dictionary of Computing, used with permission. Update as needed.
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