Resolution, Pixels, and DPI

July 2008

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Resolution, Pixels and DPI

CADCAM Software for All Woven Textiles

In the field of computer graphics images, the "resolution" of an image is an area, which seems to create a lot of confusion. By definition, the resolution of an image is simply the number of pixels (dots) within a measured area (usually an inch). This gives a value of "dots per inch" for the image. The term "dots per inch" is commonly abbreviated to "dpi" as in the following examples -

100 dpi = 100 dots, or pixels, per inch
720 dpi = 720 dots, or pixels, per inch

The resolution can be used together with the size in pixels of the image to give the physical size of the image. For example, if an image is 400 pixels wide by 550 pixels high then it will have a different physical size depending upon the resolution of the image -

400 x 550 pixels @ 100 dpi => 400/100 x 550/100 = 4 inches x 5.5 inches
400 x 550 pixels @ 400 dpi => 400/400 x 550/400 = 1 inch x 1.375 inches

To calculate the physical size, simply divide the size in pixels by the resolution.

Device Resolution
A computer screen has its own resolution, which is governed by the density of phosphor dots or LCD elements on the screen. Generally it is accepted that a computer screen has a resolution of 72 dots per inch, but this is not always the case so it should not be assumed.

Colour printers have their own native resolution. This depends upon the size of the ink particles that are deposited on the paper and how close together these ink particles are. Printer resolutions are much higher than screen resolutions - typically 600, 720, 1200, or even up to 2880 dots per inch.

Scanners also have their own resolution. If you scan a one-inch square image at 300 dpi then the resulting image will be 300 x 300 pixels. If you scan the same one-inch square at 1200 dpi then the resulting image will be 1200 x 1200 pixels in size.

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