A pixel (a contraction of picture element) is one of the many tiny dots that make up the representation of a picture in a computer's memory. 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. The colour and intensity of each dot is chosen individually by the computer to represent a small area of the picture. The example below shows a former Wikipedia logo with a portion greatly enlarged. The different shades of grey blend together to create the illusion of a smooth image. Note that sometimes (as in the example here) the edge pixels of text are reduced in shade to produce a less stepped look when viewed at normal size. This is called anti-aliasing.
The more pixels used to represent an image, the closer the result will resemble the original. The number of pixels in an image is called the resolution\. This can be expressed as a single number, as in a 'three megapixel' digital camera, which has three million pixels, or as a pair of numbers, as in a '640 by 480 display', which has 640 pixels from side to side and 480 from top to bottom (as in a VGA display), and therefore has a total number of 640 × 480 = 307,200 pixels.
The coloured dots that form a digitized image (such as a JPG file used on a web page) are also called pixels. Depending on how a computer displays an image, these may not be in one-to-one correspondance with screen pixels. In areas where the distinction is important, the dots in the image file may be called texels.
In computer programming, an image composed of pixels is known as a bitmapped image or a raster image. The word raster originates from analogue television technology. Bitmapped images are used to encode digital video and to produce computer-generated art.
The word resolution is also used in a slightly different sense to describe the physical size of the pixels, and is often quoted in pixels per inch. On most current monitors, a displayed pixel is quite small (about 1/72 to 1/96 of an inch, or 0.26 mm to 0.35 mm, a measurement known as the dot pitch). Pixels are either rectangular or square. A number called the aspect ratio describes the squareness of a pixel. For example, a 1.25:1 aspect ratio means that each pixel is 1.25 times wider than it is high. Pixels on computer monitors are usually square, but pixels used in digital video have non-square shapes, such as the D1 aspect ratio.
Each pixel in a monochrome image has its own brightness. Zero usually represents black, and the maximum value possible represents white. For example, in an eight-bit image, the maximum unsigned value that can be stored by eight bits is 255, so this is the value used for white. In a colour image, each pixel has its own brightness and colour, usually represented as a triplet of red, green and blue intensities (see RGB). On both full-colour LCD flat panels, and CRT monitors, each pixel is constructed from three sub-pixels for the three colours, spaced closely together. Each single-colour sub-pixel is brightened according to the triplicate number reference, and due to their proximity, they create an optical illusion of being one specially tinted pixel.
The number of distinct colours that can be represented by a pixel depends on the number of bits per pixel (BPP). Common values are:
- 8 bpp (256 colours)
- 16 bpp (65,536 colours, known as Highcolour)
- 24 bpp (16,777,216 colours, known as known as Truecolour).
For depths larger than 8 bits, the number is the total of the three RGB (red, green and blue) components. A 16-bit depth is usually divided into five bits for each of red and blue, and six bits for green (green gets more bits because the eye is more sensitive to that colour). A 24-bit depth allows 8 bits per component. On some systems, 32-bit depth is available: this means that each 24-bit pixel has an extra 8 bits to describe its opacity. On older systems, 4 bpp (16 colours) is also common.
When an image file is displayed on a screen, the number of bits per pixel is expressed separately for the raster file and for the display. Some raster file formatss have a greater bit-depth capability than others. The GIF format, for example, has a maximum depth of 8 bits, while TIFF files can handle 48-bit pixels. There are no displays that can display 48 bits of colour, so this depth is typically used for specialized professional applications with film scanners and printers. Such files are rendered on a screen with 24-bit depth.
A recent technique for increasing the apparent resolution of a colour display is to manipulate the three coloured sub-pixels separately. This is a form of anti-aliasing, and is mostly used to improve the appearance of text.