Pixel


 * For the webcomic, see Pixel (webcomic).

A pixel (short for picture element, using the common abbreviation "pix" for "picture") is one of the many tiny dots that make up the representation of a picture in a computer's memory. Each such information element is not really a dot, nor a square, but an abstract sample. With care, pixels in an image can be reproduced at any size without the appearance of visible dots or squares; but in many contexts, they are reproduced as dots or squares and can be visibly distinct when not fine enough. The intensity of each pixel is variable; in color systems, each pixel has typically three or four dimensions of variability such and Red, Green and Blue, or Cyan, Magenta, Yellow and Black.

Technical
A pixel is generally thought of as the smallest complete sample of an image. The definition is highly context sensitive. For example, we can speak of pixels in a visible image (e.g. a printed page) or pixels carried by one or more electronic signal(s), or represented by one or more digital value(s), or pixels on a display device, or pixels in a digital camera (photosensor elements). This list is not exhaustive and depending on context there are several synonyms which are accurate in particular contexts, e.g. pel, sample, bytes, bits, dots, spots, superset, triad, stripe set, window, etc. We can also speak of pixels in the abstract, in particular when using pixels as a measure of resolution, e.g. 2400 pixels per inch or 640 pixels per line. Dots is often used to mean pixels, especially by computer sales and marketing people, and gives rise to the abbreviation DPI or dots per inch.

The more pixels used to represent an image, the closer the result can resemble the original. The number of pixels in an image is sometimes called the resolution, though resolution has a more specific definition. Pixels can be expressed as a single number, as in a "three-megapixel" digital camera, which has a nominal 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 colour samples 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 correspondence 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 some types of computer-generated art.

Native vs. logical pixels
Since the resolution of most computer displays can be adjusted from the computer's operating system, a display's pixel resolution may not be an absolute measurement.

Modern LCD computer displays are designed with a native resolution which refers to the perfect match between pixels and triads. (CRT displays also use red-green-blue phosphor triads, but these are not coincident with image pixels, and cannot therefore be said to be equivalent to pixels.)

The native resolution will produce the sharpest picture capable from the display. However since the user can adjust the resolution, the monitor must be capable of displaying other resolutions. Non-native resolutions have to be supported by approximate resampling in the LCD screen, using interpolation algorithms. This often causes the screen to look jagged or blurry. For example, a display with a native resolution of 1280×1024 will look best set at 1280×1024 resolution, will display 800×600 adequately by drawing each pixel with more physical triads, and may be unable to display in 1600×1200 at all due to the lack of physical triads.

Pixels can be 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 aspect ratios, such as those used in the PAL and NTSC variants of the CCIR 601 digital video standard, and the corresponding anamorphic widescreen formats.

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 can be described using its hue, saturation, and value, but is usually represented instead as red, green and blue intensities (see RGB).

Bits per pixel
The number of distinct colours that can be represented by a pixel depends on the number of bits per pixel (BPP). The maximum number of colors a pixel can take can be found by taking two to the power of the color depth. For example, common values are
 * 8 bpp [28=256; (256 colours)],
 * 16 bpp [216=65536; (65,536 colours, known as Highcolour)],
 * 24 bpp [224=16777216; (16,777,216 colours, known as Truecolour)].
 * 48 bpp [248=281474976710656; (281,474,976,710,656 colors, used in many flatbed scanners and for professional work)

Images composed of 256 colours or fewer are usually stored in the computer's video memory in chunky or planar format, where a pixel in memory is an index into a list of colours called a palette. These modes are therefore sometimes called indexed modes. While only 256 colours are displayed at once, those 256 colours are picked from a much larger palette, typically of 16 million colours. Changing the values in the palette permits a kind of animation effect. The animated startup logo of Windows 95 and Windows 98 is probably the best-known example of this kind of animation.

For depths larger than 8 bits, the number is the sum of the bits devoted to each 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 (the eye being more sensitive to green). 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 formats 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.

Subpixels
Many display and image-acquisition systems are, for various reasons, not capable of displaying or sensing the different colour channels at the same site. This approach is generally resolved by using multiple subpixels, each of which handles a single colour channel. For example, LCD displays typically divide each pixel horizontally into three subpixels. Most LED displays divide each pixel into four subpixels; one red, one green, and two blue. Most digital camera sensors also use subpixels, by using coloured filters. (CRT displays also use red-green-blue phosphor dots, but these are not aligned with image pixels, and cannot therefore be said to be subpixels.)

For systems with subpixels, two different approaches can be taken: The subpixels can be ignored, with pixels being treated as the smallest addressable imaging element, or the subpixels can be included in rendering calculations, which requires more analysis and processing time, but can produce apparently superior images in some cases.

The latter approach has been used to increase the apparent resolution of colour displays. The technique, referred to as subpixel rendering, uses knowledge of pixel geometry to manipulate the three coloured sub-pixels separately, and is most effective with flat-panel displays set to their native resolutions (because the pixel geometry of such displays is usually fixed and predictable). This is a form of anti-aliasing, and is mostly used to improve the appearance of text. Microsoft's ClearType, which is available in Windows XP, is an example of this.

Megapixel
A megapixel is 1 million pixels, and is usually used to express the resolution capabilities of digital cameras. For example, a camera that can take pictures with a resolution of 2048×1536 pixels is commonly said to have "3.1 megapixels" (2048 × 1536 = 3,145,728).

Digital cameras use photosensitive electronics, either Charge-coupled devices (CCDs) or CMOS sensors, which record brightness levels on a per-pixel basis. In most digital cameras, the CCD is covered with a patterned color filter having red, green, and blue regions in the Bayer filter mosaic arrangement, so that each sensor pixel can record the brightness of a single primary color. The camera interpolates the color information of neighboring pixels, through a process called de-mosaicing, to create the final image. Thus, an x-megapixel image from a digital camera can have as little as one quarter the color resolution of the same image as taken by a scanner. Thus, a picture of a blue or red object will tend to look fuzzy compared to the same object displayed in shades of grey. Green objects appear less fuzzy, since green is allocated more pixels (due to the eye's increased sensitivity for green). See for a more detailed discussion.

In a new development, the Foveon X3 CCD uses a tri-layer image sensor that detects red, green and blue intensity at each pixel location. This structure eliminates the need for de-mosaicing and eliminates the associated image artifacts, like color blurring around sharp edges.

Similar concepts
Several other types of objects derived from the idea of the pixel, such as the voxel (volume element), texel (texture element) and surfel (surface element), have been created for other computer graphics and image processing uses.