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What is a Digital Image?
A digital image is:
- a rectangular array of measurements (usually of light, but they may be measurements of things such as temperature, x-rays, the elevation of points on the surface of a planet, etc.) sampled at regular intervals and rounded off to the nearest whole number; and
- displayed according to a color Lookup Table (LUT), which translates each measurement into a pixel value that is then assigned a specific color or shade of gray.
In other words, a digital image is a long string of numbers representing different levels of brightness for grayscale images, and brightness and color for color images (such as RGB images). When the numbers are placed in the correct order (rows and columns), and a shade of gray or a color is assigned to each number by an LUT, a recognizable picture is formed. Look at the series of magnifications of the Antarctic ozone hole on the right to see how a digital image is really an array of numbers mapped to colors.
Pixels also correspond to distances and areas. For example, a length of twenty pixels in an image may represent one micron in an electron micrograph or one hundred kilometers in a satellite image. The more pixels there are in an image, the greater its resolution; an image with a high number of pixels and a high bit depth therefore more closely represents the actual thing that was measured. Bit depth tells you how much data is being used to define each pixel in an image. A bit is the smallest unit of data, and comes in two possible states: either 1 or 0 (or on/off, or black/white). A black-and-white image is a 1-bit image, since only one bit is needed to define each pixel; 8-bit and higher images give you a lot more information. Unlike graphics, where meaning comes only from the patterns formed by many pixels, each pixel in a digital image may contain meaningful information.
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