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The Arithmetic of Printing Images

Printer resolutions are usually specified by the number of dots per inch (dpi) that they print. (Generally ppi-pixels per inch-refer to the image and display screen and dpi-dots per inch-refer to the printer and printed image. In this course we sometimes use them interchangeably)

For comparison purposes, monitors use an average of 72 ppi to display text and images, ink-jet printers range up to 1700 dpi or so, and commercial typesetting machines range between 1,000 and 2,400 dpi. Since image sizes are described in pixels and photographic prints in inches, you have to convert from pixels to inches. To do so, you divide the image's dimension in pixels by the resolution of the device in pixels per inch (ppi).

Understanding Pixels Per Inch

Normally you don't have to change the number of pixel's in an image to change the size of a printout. ......more

That task is handled by the software program you use to print the image. For example, if you place an image in a program such as QuarkXpress or PageMaker, its printed at the size you specify in those programs.

One thing to keep in mind is that if you enlarge a print too much, it won't be as sharp as you may desire. That's because a certain minimal number of dots per inch, usually about 300, are needed to get a good print. Pixels begin to show when the print is enlarged to a point where the pixels get get so big that the pixels per inch (ppi) fall too low.

If your printer can print a sharp image only at 300 or more pixels per inch, you need to determine if the size of the image you plan on printing will fall below this level. Let's say you have a scanned image and want to print it at a certain size. When you enlarge or reduce an image like this, the ppi change. To find out what the pixels (or dots) per inch becomes, you convert from the image's original size in pixels to its pixels per inch. For example, if you print an image that's 1600 pixels wide so the print is 10" wide, there are only 160 dots per inch (unless its resampled) (1600 pixels ÷ 10 inches = 160 pixels per inch). When you know the width of the image in pixels, you can divide that number by the printer's dpi to determine the largest possible print size in inches.

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