Pixel Definition

A pixel (a contraction of picture element) is any of the many tiny dots that make up the representation of bitmap images (i.e., patterns of dots) which are stored in a computer's memory or shown on a display monitor.

The number of pixels in an image is called the resolution. The more pixels used to represent an image, the less visible the individual pixels and the closer the result will resemble the original (photograph, drawing, etc.). Usually the dots are so small and so numerous that, when printed on paper or displayed on a computer monitor, they appear to merge into a smooth image.

Resolution is commonly expressed as a pair of numbers, the first of which represents the number of pixels horizontally and the second of which represents the number of pixels vertically. These numbers are largely standardized, with common resolutions including 640 by 480 pixels, 800 by 600 pixels and 1600 by 1200 pixels.

Resolution is also often expressed as a single number, particularly in the case of digital cameras and image-sensing semiconductor chips. Thus, for example, a five megapixel camera would have the ability to create images with approximately five million pixels, which is the same as a 2560 by 1920 pixel resolution. Another way of describing resolution is the number of pixels per unit of length, usually pixels per inch.

In the most simple monochrome images, each pixel can have only one of two values, usually either black or white. In grayscale monochrome images, each pixel can have any of a number of levels of brightness, with intermediate levels producing various shades of gray in the case of black and white monochrome. Modern computers generally permit a minimum of one byte (i.e., eight bits) to be assigned to each pixel, thereby allowing each pixel to have any of 256 levels of brightness, ranging from zero for black to 255 for white.

In color images, each pixel consists of three dots or subpixels, each of which can express any of multiple levels of brightness for one of the three primary colors (i.e., red, blue and green). The number of distinct colors that can be represented by a pixel depends on the number of bits per pixel. Common values are 8bpp (bits per pixel), which can produce 256 colors, 16bbp, which can produce 65,536 colors, and 24bpp, which can produce approximately 16.78 million colors.

Major trends with regard to pixels have been to make them smaller and to include more per sensing element and display screen. At the same time, the continued drop in storage and memory costs is making it easier to store and use the ever-higher resolution images.

The vast majority of digital images are stored and generated as bitmaps, usually as JPEG (Joint Photographic Experts Group) and GIF (graphics interchange format) files. The main alternative is vector graphics, which uses mathematical formulae to describe shapes, colors and placement. Among the advantages of vector graphics are smaller file sizes, ease of scaling and ease of modification. SVG (scalable vector graphics) is an increasingly popular W3C (World Wide Web Consortium) standard for using XML (extensible markup language) to describe vector grapics for web and other computer applications. Despite this difference in the way that they are stored and generated, vector graphics are ultimately displayed as a pattern of pixels.

Created January 25, 2006.
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