Image Processing Reference
In-Depth Information
1.3.2 Resolution
Pixel images are rectangular images made up of little squares made up of
image dots, or pixels. The density of the dots contained in any given pixel
image is called its resolution . Resolution is normally measured in dots per inch
(dpi ). In the metric system, dpi is the number of dots per 2.54 centimeters. You
can also refer to an image's resolution in pixels per centimeters (the standard
measurement in most European four-color printing companies). Although
dpi seems to calculate only the length or width of an image, changing the
resolution of an image will influence its height as well. For example, doubling
the resolution of an image will result in a fourfold increase of the number of
An image's size (the dimension in inches, millimeters, or pixels) is directly
dependent on its resolution. If you transform an image with a resolution of
300 dpi to a resolution of 72 dpi using GIMP, the image size (width x height)
increases more than fourfold, even though without interpolation the number
of image dots remains the same.
300 dpi will produce an image of quality resolution. 300 dpi is
recommended for a scanned image, especially if you intend to edit and print
the image at a 1:1 scale.
If you want to enlarge an image, you'll want to scan it at a higher resolution.
As a rule of the thumb, if you plan to double the image size (width or height),
scan at twice the resolution desired for the final image. If you simply want to
reduce an image's dimensions, the visible image quality will usually stay the
same or get better, so you need not worry about increasing it.
Four-color printing uses various standard resolutions (e.g., 150, 300,
600, or 1200 dpi). These are indicative values.
Images on the Internet often use lower resolutions, mostly 72 or 96
dpi, values that correspond to the standard resolution of PC monitors. A low
resolution keeps the file sizes of images small enough for the images to be
efficiently and quickly transmitted over the Internet. Low-resolution images
will still yield good-quality printouts on inkjet printers.
Bottom line : A higher resolution (i.e., higher quantity of finer dots) will
result in an excellent image that can be enlarged to a certain extent without
compromising quality. On the other hand, if you reduce the resolution of
an image without reducing its dimensions, the image quality will drop. It is
important to make a copy of the original image when experimenting with size
and resolution because the process cannot be reversed.
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