Pixel - AbsoluteAstronomy.com

Digital imaging

Digital imaging or digital image acquisition is the creation of digital images, typically from a physical scene. The term is often assumed to imply or include the processing, compression, storage, printing, and display of such images...

, a pixel, or pel, (picture element) is a single point in a raster image

Raster graphics

In computer graphics, a raster graphics image, or bitmap, is a data structure representing a generally rectangular grid of pixels, or points of color, viewable via a monitor, paper, or other display medium...

, or the smallest addressable screen element in a display device

Display device

A display device is an output device for presentation of information in visual or tactile form...

; it is the smallest unit of picture that can be represented or controlled.

Each pixel has its own address. The address of a pixel corresponds to its coordinates. Pixels are normally arranged in a two-dimensional grid, and are often represented using dots or squares. Each pixel is a sample

Sampling (signal processing)

In signal processing, sampling is the reduction of a continuous signal to a discrete signal. A common example is the conversion of a sound wave to a sequence of samples ....

of an original image; more samples typically provide more accurate representations of the original. The intensity

Intensity (physics)

In physics, intensity is a measure of the energy flux, averaged over the period of the wave. The word "intensity" here is not synonymous with "strength", "amplitude", or "level", as it sometimes is in colloquial speech...

of each pixel is variable. In color image systems, a color is typically represented by three or four component intensities such as red, green, and blue

RGB color model

The RGB color model is an additive color model in which red, green, and blue light is added together in various ways to reproduce a broad array of colors...

, or cyan, magenta, yellow, and black

CMYK color model

The CMYK color model is a subtractive color model, used in color printing, and is also used to describe the printing process itself. CMYK refers to the four inks used in some color printing: cyan, magenta, yellow, and key...

.

In some contexts (such as descriptions of camera sensors), the term pixel is used to refer to a single scalar element of a multi-component representation (more precisely called a photosite in the camera sensor context, although the neologism sensel is sometimes used to describe the elements of a digital camera's sensor), while in others the term may refer to the entire set of such component intensities for a spatial position. In color systems that use chroma subsampling

Chroma subsampling

Chroma subsampling is the practice of encoding images by implementing less resolution for chroma information than for luma information, taking advantage of the human visual system's lower acuity for color differences than for luminance....

, the multi-component concept of a pixel can become difficult to apply, since the intensity measures for the different color components correspond to different spatial areas in a such a representation.

The word pixel is based on a contraction of pix ("pictures") and el (for "element"); similar formations with el for "element" include the words voxel

Voxel

A voxel is a volume element, representing a value on a regular grid in three dimensional space. This is analogous to a pixel, which represents 2D image data in a bitmap...

and texel

Texel (graphics)

A texel, or texture element is the fundamental unit of texture space, used in computer graphics. Textures are represented by arrays of texels, just as pictures are represented by arrays of pixels....

Etymology

The word "pixel" was first published in 1965 by Frederic C. Billingsley

Frederic C. Billingsley

Frederic Crockett Billingsley was an American engineer, who spent most of his career developing techniques for digital image processing in support of American space probes to the moon, to Mars, and to other planets....

of JPL

Jet Propulsion Laboratory

Jet Propulsion Laboratory is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. The facility is headquartered in the city of Pasadena on the border of La Cañada Flintridge and Pasadena...

, to describe the picture elements of video images from space probes to the Moon and Mars. However, Billingsley did not coin the term himself. Instead, he got the word "pixel" from Keith E. McFarland, at the Link Division of General Precision in Palo Alto

Palo Alto, California

Palo Alto is a California charter city located in the northwest corner of Santa Clara County, in the San Francisco Bay Area of California, United States. The city shares its borders with East Palo Alto, Mountain View, Los Altos, Los Altos Hills, Stanford, Portola Valley, and Menlo Park. It is...

, who did not know where the word originated. McFarland said simply it was "in use at the time" (circa 1963).

The word is a combination of picture and element, via pix. The word pix appeared in Variety

Variety (magazine)

Variety is an American weekly entertainment-trade magazine founded in New York City, New York, in 1905 by Sime Silverman. With the rise of the importance of the motion-picture industry, Daily Variety, a daily edition based in Los Angeles, California, was founded by Silverman in 1933. In 1998, the...

magazine headlines in 1932, as an abbreviation for the word pictures, in reference to movies. By 1938, "pix" was being used in reference to still pictures by photojournalists.

The concept of a "picture element" dates to the earliest days of television, for example as "Bildpunkt" (the German word for pixel, literally 'picture point') in the 1888 German patent of Paul Nipkow. According to various etymologies, the earliest publication of the term picture element itself was in Wireless World
Wireless World
Wireless World was the pre-eminent British magazine for radio and electronics enthusiasts. It was one of the very few "informal" journals which were tolerated as a professional expense.- History :...

magazine in 1927, though it had been used earlier in various U.S. patents filed as early as 1911.

Some authors explain pixel as picture cell, as early as 1972. In video processing, pel is often used instead of pixel. For example, IBM used it in their Technical Reference for the original PC.

Words with similar etymologies

Texel
Texel (graphics)
A texel, or texture element is the fundamental unit of texture space, used in computer graphics. Textures are represented by arrays of texels, just as pictures are represented by arrays of pixels....

(texture element) and luxel (lux
Lux
The lux is the SI unit of illuminance and luminous emittance, measuring luminous flux per unit area. It is used in photometry as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface...

element) are words used to describe a pixel when it is used in specific context (texturing and light mapping respectively)
A voxel
Voxel
A voxel is a volume element, representing a value on a regular grid in three dimensional space. This is analogous to a pixel, which represents 2D image data in a bitmap...

is a volume element, the 3D analogue of a 2D pixel.
Surfel
Surfel
Surfel is an abbreviation of "surface element". In 3D computer graphics, the use of surfels is an alternative to polygonal modeling. An object is represented by a dense set of points or viewer-facing discs holding lighting information. Surfels are well suited to modeling dynamic geometry, because...

s (surface elements) have the same naming pattern as pixels, but share more similarities with shrunken triangles than expanded pixels.
A resel
Resel
A resel is a resolution element - a concept used in image analysis. It describes the actual spatial image resolution in an image .The number of resels in the image will be lower or equal to the number of pixel/voxels in the image....

is a resolution element, used to describe the discrete number of resolvable elements of a continuous signal (e.g. in optical imaging).

Technical

thought of as the smallest single component of a digital image. However, the definition is highly context-sensitive. For example, there can be "printed pixels" in a page, or pixels carried by electronic signals, or represented by digital values, or pixels on a display device, or pixels in a digital camera

Digital camera

A digital camera is a camera that takes video or still photographs, or both, digitally by recording images via an electronic image sensor. It is the main device used in the field of digital photography...

(photosensor elements). This list is not exhaustive, and depending on context, there are several terms that are synonymous in particular contexts, such as pel, sample, byte, bit, dot, spot, etc. The term "pixels" can be used in the abstract, or as a unit of measure, in particular when using pixels as a measure of resolution, such as: 2400 pixels per inch, 640 pixels per line, or spaced 10 pixels apart.

The measures dots per inch

Dots per inch

Dots per inch is a measure of spatial printing or video dot density, in particular the number of individual dots that can be placed in a line within the span of 1 inch . The DPI value tends to correlate with image resolution, but is related only indirectly.- DPI measurement in monitor...

(dpi) and pixels per inch

Pixels per inch

Pixels per inch or pixel density is a measurement of the resolution of devices in various contexts; typically computer displays, image scanners, and digital camera image sensors....

(ppi) are sometimes used interchangeably, but have distinct meanings, especially for printer devices, where dpi is a measure of the printer's density of dot (e.g. ink droplet) placement. For example, a high-quality photographic image may be printed with 600 ppi on a 1200 dpi inkjet printer. Even higher dpi numbers, such as the 4800 dpi quoted by printer manufacturers since 2002, do not mean much in terms of achievable resolution.

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

Image resolution

Image resolution is an umbrella term that describes the detail an image holds. The term applies to raster digital images, film images, and other types of images. Higher resolution means more image detail....

, though resolution has a more specific definition. Pixel counts 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

Video Graphics Array

Video Graphics Array refers specifically to the display hardware first introduced with the IBM PS/2 line of computers in 1987, but through its widespread adoption has also come to mean either an analog computer display standard, the 15-pin D-subminiature VGA connector or the 640×480 resolution...

display), and therefore has a total number of 640 × 480 = 307,200 pixels or 0.3 megapixels.

The pixels, or color samples, that form a digitized image (such as a JPEG

JPEG

In computing, JPEG . The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality....

file used on a web page) may or may not be in one-to-one correspondence

Bijection

A bijection is a function giving an exact pairing of the elements of two sets. A bijection from the set X to the set Y has an inverse function from Y to X. If X and Y are finite sets, then the existence of a bijection means they have the same number of elements...

with screen pixels, depending on how a computer displays an image. In computing, an image composed of pixels is known as a bitmapped image
Bitmap
In computer graphics, a bitmap or pixmap is a type of memory organization or image file format used to store digital images. The term bitmap comes from the computer programming terminology, meaning just a map of bits, a spatially mapped array of bits. Now, along with pixmap, it commonly refers to...

or a raster image
Raster graphics
In computer graphics, a raster graphics image, or bitmap, is a data structure representing a generally rectangular grid of pixels, or points of color, viewable via a monitor, paper, or other display medium...

. The word raster originates from television scanning

Raster scan

A raster scan, or raster scanning, is the rectangular pattern of image capture and reconstruction in television. By analogy, the term is used for raster graphics, the pattern of image storage and transmission used in most computer bitmap image systems...

patterns, and has been widely used to describe similar halftone

Halftone

Halftone is the reprographic technique that simulates continuous tone imagery through the use of dots, varying either in size, in shape or in spacing...

printing and storage techniques.

Sampling patterns

For convenience, pixels are normally arranged in a regular two-dimensional grid

Regular grid

A regular grid is a tessellation of n-dimensional Euclidean space by congruent parallelotopes . Grids of this type appear on graph paper and may be used in finite element analysis as well as finite volume methods and finite difference methods...

. By using this arrangement, many common operations can be implemented by uniformly applying the same operation to each pixel independently. Other arrangements of pixels are possible, with some sampling patterns even changing the shape (or kernel) of each pixel across the image. For this reason, care must be taken when acquiring an image on one device and displaying it on another, or when converting image data from one pixel format to another.

For example:

LCD screens
Liquid crystal display
A liquid crystal display is a flat panel display, electronic visual display, or video display that uses the light modulating properties of liquid crystals . LCs do not emit light directly....

typically use a staggered grid, where the red, green, and blue components are sampled at slightly different locations. Subpixel rendering
Subpixel rendering
Subpixel rendering is a way to increase the apparent resolution of a computer's liquid crystal display or Organic Light Emitting Diode display by rendering pixels to take into account the screen type's physical properties...

is a technology which takes advantage of these differences to improve the rendering of text on LCD screens.
The vast majority of color digital cameras use a Bayer filter
Bayer filter
A Bayer filter mosaic is a color filter array for arranging RGB color filters on a square grid of photosensors. Its particular arrangement of color filters is used in most single-chip digital image sensors used in digital cameras, camcorders, and scanners to create a color image...

, resulting in a regular grid of pixels where the color of each pixel depends on its position on the grid.
A clipmap
Clipmap
Clipmapping is a method of clipping a mipmap to a subset of data pertinent to the geometry being displayed. This is useful for loading as little data as possible when memory is limited, such as on a graphics processing unit.- External links :* * *...

uses a hierarchical sampling pattern, where the size of the support
Support (mathematics)
In mathematics, the support of a function is the set of points where the function is not zero, or the closure of that set . This concept is used very widely in mathematical analysis...

of each pixel depends on its location within the hierarchy.
Warped grids are used when the underlying geometry is non-planar, such as images of the earth from space.
The use of non-uniform grids is an active research area, attempting to bypass the traditional Nyquist limit
Nyquist rate
In signal processing, the Nyquist rate, named after Harry Nyquist, is two times the bandwidth of a bandlimited signal or a bandlimited channel...

.
Pixels on computer monitors are normally "square" (this is, having equal horizontal and vertical sampling pitch); pixels in other systems are often "rectangular" (that is, having unequal horizontal and vertical sampling pitch – oblong in shape), as are digital video
Digital video
Digital video is a type of digital recording system that works by using a digital rather than an analog video signal.The terms camera, video camera, and camcorder are used interchangeably in this article.- History :...

formats with diverse aspect ratios
Pixel aspect ratio
Pixel aspect ratio is a mathematical ratio that describes how the width of a pixel in a digital image compares to the height of that pixel....

, such as the anamorphic widescreen
Anamorphic widescreen
Anamorphic widescreen, when applied to DVD manufacture, is a video process that horizontally squeezes a widescreen image so that it can be stored in a standard 4:3 aspect ratio DVD image frame. Compatible playback equipment can then re-expand the horizontal dimension to show the original widescreen...

formats of the Rec. 601 digital video standard.

Resolution of computer monitors

Computers can use pixels to display an image, often an abstract image that represents a GUI

Gui

Gui or guee is a generic term to refer to grilled dishes in Korean cuisine. These most commonly have meat or fish as their primary ingredient, but may in some cases also comprise grilled vegetables or other vegetarian ingredients. The term derives from the verb, "gupda" in Korean, which literally...

. The resolution of this image is called the display resolution and is determined by the video card

Video card

A video card, Graphics Card, or Graphics adapter is an expansion card which generates output images to a display. Most video cards offer various functions such as accelerated rendering of 3D scenes and 2D graphics, MPEG-2/MPEG-4 decoding, TV output, or the ability to connect multiple monitors...

of the computer. LCD monitors also use pixels to display an image, and have a native resolution

Native resolution

The native resolution of a LCD, LCoS or other flat panel display refers to its single fixed resolution. As an LCD display consists of a fixed raster, it cannot change resolution to match the signal being displayed as a CRT monitor can, meaning that optimal display quality can be reached only when...

. Each pixel is made up of triads

Triad (computers)

In CRT or computer terminology, a triad is a group of three phosphor dots coloured red, green, and blue on the inside of the CRT display of a computer monitor or television set. By directing differing intensities of electron beams onto the three phosphor dots, the triad will display a colour by...

, with the number of these triads determining the native resolution. On some CRT

Cathode ray tube

The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...

monitors, the beam sweep rate may be fixed, resulting in a fixed native resolution. Most CRT monitors do not have a fixed beam sweep rate, meaning they do not have a native resolution at all - instead they have a set of resolutions that are equally well supported.
To produce the sharpest images possible on an LCD, the user must ensure the display resolution of the computer matches the native resolution of the monitor.

Bits per pixel

The number of distinct colors that can be represented by a pixel depends on the number of bits per pixel (bpp). A 1 bpp image uses 1-bit for each pixel, so each pixel can be either on or off. Each additional bit doubles the number of colors available, so a 2 bpp image can have 4 colors, and a 3 bpp image can have 8 colors:

1 bpp, 2¹ = 2 colors (monochrome)
2 bpp, 2² = 4 colors
3 bpp, 2³ = 8 colors

...

8 bpp, 2⁸ = 256 colors
16 bpp, 2¹⁶ = 65,536 colors ("Highcolor" )
24 bpp, 2²⁴ ≈ 16.8 million colors ("Truecolor")

For color depths of 15 or more bits per pixel, the depth is normally the sum of the bits allocated to each of the red, green, and blue components. Highcolor, usually meaning 16 bpp, normally has five bits for red and blue, and six bits for green, as the human eye is more sensitive to errors in green than in the other two primary colors. For applications involving transparency, the 16 bits may be divided into five bits each of red, green, and blue, with one bit left for transparency. 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

Opacity (optics)

Opacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shielding material, glass, etc...

(for purposes of combining with another image).

Subpixels

Many display and image-acquisition systems are, for various reasons, not capable of displaying or sensing the different color channels

Channel (digital image)

Color digital images are made of pixels, and pixels are made of combinations of primary colors. A channel in this context is the grayscale image of the same size as a color image, made of just one of these primary colors. For instance, an image from a standard digital camera will have a red, green...

at the same site. Therefore, the pixel grid is divided into single-color regions that contribute to the displayed or sensed color when viewed at a distance. In some displays, such as LCD, LED, and plasma displays, these single-color regions are separately addressable elements, which have come to be known as subpixels. For example, LCDs typically divide each pixel horizontally into three subpixels. When the square pixel is divided into three subpixels, each subpixel is necessarily rectangular. In the display industry terminology, subpixels are often referred to as pixels, as they are the basic addressable elements in a viewpoint of hardware, and they call pixel circuits rather than subpixel circuits.

Most digital camera image sensor

Image sensor

An image sensor is a device that converts an optical image into an electronic signal. It is used mostly in digital cameras and other imaging devices...

s use single-color sensor regions, for example using the Bayer filter

Bayer filter

A Bayer filter mosaic is a color filter array for arranging RGB color filters on a square grid of photosensors. Its particular arrangement of color filters is used in most single-chip digital image sensors used in digital cameras, camcorders, and scanners to create a color image...

pattern, and in the camera industry these are known as pixels just like in the display industry, not subpixels.

For systems with subpixels, two different approaches can be taken:

The subpixels can be ignored, with full-color 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.

This latter approach, referred to as subpixel rendering

Subpixel rendering

Subpixel rendering is a way to increase the apparent resolution of a computer's liquid crystal display or Organic Light Emitting Diode display by rendering pixels to take into account the screen type's physical properties...

, uses knowledge of pixel geometry

Pixel geometry

The components of the pixels in an image sensor or display can be ordered in different patterns, called pixel geometry....

to manipulate the three colored subpixels separately, producing a slight increase in the apparent resolution of color displays. While CRT

Cathode ray tube

displays use red-green-blue-masked phosphor areas, dictated by a mesh grid called the shadow mask, it would require a difficult calibration step to be aligned with the displayed pixel raster, and so CRTs do not currently use subpixel rendering.

The concept of subpixels is related to samples

Sample (graphics)

In computer graphics, a sample is an intersection of channel and a pixel.The diagram below depicts a 24-bit pixel, consisting of 3 samples for Red, Green, and Blue....

Megapixel

A megapixel (MP or Mpx) is one million pixels, and is a term used not only for the number of pixels in an image, but to express the number of image sensor

Image sensor

An image sensor is a device that converts an optical image into an electronic signal. It is used mostly in digital cameras and other imaging devices...

elements of digital camera

Digital camera

s or the number of display elements of digital display

Computer display

A monitor or display is an electronic visual display for computers. The monitor comprises the display device, circuitry, and an enclosure...

s. For example, a camera with an array of 2048 × 1536 sensor elements is commonly said to have "3.1 megapixels" (2048 × 1536 = 3,145,728).

Digital cameras use photosensitive electronics, either charge-coupled device

Charge-coupled device

A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...

(CCD) or complementary metal–oxide–semiconductor

CMOS

Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...

(CMOS) image sensor

Image sensor

An image sensor is a device that converts an optical image into an electronic signal. It is used mostly in digital cameras and other imaging devices...

s, consisting of a large number of single sensor elements, each of which records a measured intensity level. In most digital cameras, the sensor array is covered with a patterned color filter mosaic having red, green, and blue regions in the Bayer filter

Bayer filter

arrangement, so that each sensor element can record the intensity of a single primary color of light. The camera interpolates the color information of neighboring sensor elements, through a process called demosaicing

Demosaicing

A demosaicing algorithm is a digital image process used to reconstruct a full color image from the incomplete color samples output from an image sensor overlaid with a color filter array...

, to create the final image. These sensor elements are often called "pixels", even though they only record 1 channel (only red, or green, or blue) of the final color image. Thus, two of the three color channels for each sensor must be interpolated and a so-called N-megapixel camera that produces an N-megapixel image provides only one-third of the information that an image of the same size could get from a scanner. Thus, certain color contrasts may look fuzzier than others, depending on the allocation of the primary colors (green has twice as many elements as red or blue in the Bayer arrangement).

Display resolution

The display resolution

Display resolution

The display resolution of a digital television or display device is the number of distinct pixels in each dimension that can be displayed. It can be an ambiguous term especially as the displayed resolution is controlled by all different factors in cathode ray tube , flat panel or projection...

of a display device

Display device

A display device is an output device for presentation of information in visual or tactile form...

(such as a digital television

Digital television

Digital television is the transmission of audio and video by digital signals, in contrast to the analog signals used by analog TV...

) is the number of distinct pixels in each dimension that can be displayed. It can be an ambiguous term especially as the displayed resolution is controlled by different factors in cathode ray tube

Cathode ray tube

(CRT) and flat panel or projection displays using fixed-element arrays.

External links

A Pixel Is Not A Little Square: Microsoft Memo by computer graphics pioneer Alvy Ray Smith.
Pixels and Me: Video of a history talk at the Computer History Museum
Computer History Museum
The Computer History Museum is a museum established in 1996 in Mountain View, California, USA. The Museum is dedicated to preserving and presenting the stories and artifacts of the information age, and exploring the computing revolution and its impact on our lives.-History:The museum's origins...

.
Square and non-Square Pixels: Technical info on pixel aspect ratios of modern video standards (480i,576i,1080i,720p), plus software implications.
120 Megapixel is here now: A lot of information about MegaPixel and Gigapixel.

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Etymology

Words with similar etymologies

Technical

Sampling patterns

Resolution of computer monitors

Bits per pixel

Subpixels

Megapixel

Display resolution

See also

External links