Digital image
Encyclopedia
A digital image is a numeric representation (normally binary
) of a two-dimensional image
. Depending on whether or not the image resolution
is fixed, it may be of vector
or raster
type. Without qualifications, the term "digital image" usually refers to raster images also called bitmap images.
values, called picture elements or pixel
s. The digital image contains a fixed number of rows and columns of pixels. Pixels are the smallest individual element in an image, holding quantized values that represent the brightness of a given color at any specific point.
Typically, the pixels are stored in computer memory as a raster image
or raster map, a two-dimensional array of small integers. These values are often transmitted or stored in a compressed
form.
Raster images can be created
by a variety of input devices and techniques, such as digital camera
s, scanner
s, coordinate-measuring machines, seismographic profiling, airborne radar, and more. They can also be synthesized from arbitrary non-image data, such as mathematical functions or three-dimensional geometric models; the latter being a major sub-area of computer graphics
. The field of digital image processing
is the study of algorithms for their transformation.
the number and nature of those samples:
The term digital image is also applied to data associated to points scattered over a three-dimensional
region, such as produced by tomographic
equipment. In that case, each datum is called a voxel
.
Some digital camera
s give access to almost all the data captured by the camera, using a raw image format
. The Universal Photographic Imaging Guidelines (UPDIG) suggests these formats be used when possible since raw files produce the best quality images. These file formats allow the photographer and the processing agent the greatest level of control and accuracy for output. Their use is inhibited by the prevalence of proprietary information (trade secrets) for some camera makers, but there have been initiatives such as OpenRAW
to influence manufacturers to release these records publicly. An alternative may be Digital Negative (DNG)
, a proprietary Adobe product described as “the public, archival format for digital camera raw data”. Although this format is not yet universally accepted, support for the product is growing, and increasingly professional archivists and conservationists, working for respectable organizations, variously suggest or recommend DNG for archival purposes.
). In mathematical terms, a vector consists of point that has both direction and length.
Often, both raster and vector elements will be combined in one image; for example, in the case of a billboard with text (vector) and photographs (raster).
software displays images. Web browser
s can display standard internet image formats including GIF, JPEG
, and PNG. Some can show SVG
format which is a standard W3C format.
Some viewers offer a slideshow
utility to display a sequence of images.
Photographic competitions run by photographic societies have historically displayed prints, or slides, submitted by photographers, which are then judged. Increasingly, slide competitions are being replaced by PDI competitions, with photographers submitting suitably-sized digital images which are then projected onto a screen using display software on a computer. The digital images may have originated as images captured on film, then scanned, but are now more likely to have been captured by a digital camera
.
Each competition provides its own specification for the images, depending on the software and video projector
used. A typical digital image projector
conforms to SXGA+ resolution, and the competition will normally accept JPEG
images with sRGB color profile at 1400 pixels wide by 1050 pixels high. Another (older) common format, because it uses cheaper projectors, is XGA, at 1024 pixels wide by 768 pixels high.
preceded digital cameras and computers by decades.
The first picture to be scanned, stored, and recreated in digital pixels was displayed on the Standards Eastern Automatic Computer at NIST. The advancement of digital imagery continued in the early 1960s, alongside development of the space program and in medical
research. Projects at the Jet Propulsion Laboratory
, MIT, Bell Labs
and the University of Maryland
, among others, used digital images to advance satellite imagery
, wirephoto standards conversion, medical imaging
, videophone
technology, character recognition, and photo enhancement.
Rapid advances in digital imaging began with the introduction of microprocessor
s in the early 1970s, alongside progress in related storage and display technologies. The invention of computerised axial tomography (CAT scanning), using x-ray
s to produce a digital image of a "slice" through a three-dimensional object, was of great importance to medical diagnostics. As well as origination of digital images, digitization of analog images allowed the enhancement and restoration of archaeological
artefacts and began to be used in fields as diverse as nuclear medicine
, astronomy
, law enforcement
, defence and industry
.
Advances in microprocessor technology paved the way for the development and marketing of charge-coupled device
s (CCDs) for use in a wide range of image capture
devices and gradually displaced the use of analog film
and tape
in photography and videography towards the end of the 20th century. The computing power necessary to process digital image capture also allowed computer-generated digital images to achieve a level of refinement close to photorealism
.
Binary numeral system
The binary numeral system, or base-2 number system, represents numeric values using two symbols, 0 and 1. More specifically, the usual base-2 system is a positional notation with a radix of 2...
) of a two-dimensional image
Image
An image is an artifact, for example a two-dimensional picture, that has a similar appearance to some subject—usually a physical object or a person.-Characteristics:...
. Depending on whether or not the image 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....
is fixed, it may be of vector
Vector graphics
Vector graphics is the use of geometrical primitives such as points, lines, curves, and shapes or polygon, which are all based on mathematical expressions, to represent images in computer graphics...
or raster
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...
type. Without qualifications, the term "digital image" usually refers to raster images also called bitmap images.
Raster
Raster images have a finite set of digitalDigital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
values, called picture elements or pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
s. The digital image contains a fixed number of rows and columns of pixels. Pixels are the smallest individual element in an image, holding quantized values that represent the brightness of a given color at any specific point.
Typically, the pixels are stored in computer memory as 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 raster map, a two-dimensional array of small integers. These values are often transmitted or stored in a compressed
Image compression
The objective of image compression is to reduce irrelevance and redundancy of the image data in order to be able to store or transmit data in an efficient form.- Lossy and lossless compression :...
form.
Raster images can be created
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...
by a variety of input devices and techniques, such as 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...
s, scanner
Image scanner
In computing, an image scanner—often abbreviated to just scanner—is a device that optically scans images, printed text, handwriting, or an object, and converts it to a digital image. Common examples found in offices are variations of the desktop scanner where the document is placed on a glass...
s, coordinate-measuring machines, seismographic profiling, airborne radar, and more. They can also be synthesized from arbitrary non-image data, such as mathematical functions or three-dimensional geometric models; the latter being a major sub-area of computer graphics
Computer graphics
Computer graphics are graphics created using computers and, more generally, the representation and manipulation of image data by a computer with help from specialized software and hardware....
. The field of digital image processing
Digital image processing
Digital image processing is the use of computer algorithms to perform image processing on digital images. As a subcategory or field of digital signal processing, digital image processing has many advantages over analog image processing...
is the study of algorithms for their transformation.
Raster image types
Each pixel of a raster image is typically associated to a specific 'position' in some 2D region, and has a value consisting of one or more quantities (samples) related to that position. Digital images can be classified according tothe number and nature of those samples:
- binaryBinary imageA binary image is a digital image that has only two possible values for each pixel. Typically the two colors used for a binary image are black and white though any two colors can be used. The color used for the object in the image is the foreground color while the rest of the image is the...
- grayscaleGrayscaleIn photography and computing, a grayscale or greyscale digital image is an image in which the value of each pixel is a single sample, that is, it carries only intensity information...
- colorColor imageA color image is a digital image that includes color information for each pixel.For visually acceptable results, it is necessary to provide three samples for each pixel, which are interpreted as coordinates in some color space...
- false-color
- multi-spectralMulti-spectral imageA multispectral image is one that captures image data at specific frequencies across the electromagnetic spectrum. The wavelengths may be separated by filters or by the use of instruments that are sensitive to particular wavelengths, including light from frequencies beyond the visible light range,...
- thematicThematic imageThematic images are usually image products of classification processing of multispectral images of the earth surface. The classification process differentiates types of surface such as land, water, forest, lake, structure etc...
- picture functionPicture functionA picture function is a mathematical representation of a two-dimensional image as a function of two spatial variables. The function f describes the intensity of the point at coordinates ....
The term digital image is also applied to data associated to points scattered over a three-dimensional
Volume
Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....
region, such as produced by tomographic
Tomography
Tomography refers to imaging by sections or sectioning, through the use of any kind of penetrating wave. A device used in tomography is called a tomograph, while the image produced is a tomogram. The method is used in radiology, archaeology, biology, geophysics, oceanography, materials science,...
equipment. In that case, each datum is called 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...
.
Raster file formats
Most users come into contact with raster images through digital cameras.Some 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...
s give access to almost all the data captured by the camera, using a raw image format
RAW image format
A camera raw image file contains minimally processed data from the image sensor of either a digital camera, image scanner, or motion picture film scanner. Raw files are so named because they are not yet processed and therefore are not ready to be printed or edited with a bitmap graphics editor...
. The Universal Photographic Imaging Guidelines (UPDIG) suggests these formats be used when possible since raw files produce the best quality images. These file formats allow the photographer and the processing agent the greatest level of control and accuracy for output. Their use is inhibited by the prevalence of proprietary information (trade secrets) for some camera makers, but there have been initiatives such as OpenRAW
OpenRAW
OpenRAW was an initiative to raise awareness of a serious problem with top-end digital photography and to help solve that problem. The problem concerns long-term access and viewing of the raw images often used by professional and experienced amateur photographers...
to influence manufacturers to release these records publicly. An alternative may be Digital Negative (DNG)
Digital Negative (file format)
Digital Negative is an open raw image format owned by Adobe used for digital photography. It was launched on September 27, 2004. The launch was accompanied by the first version of the DNG specification, plus various products including a free of charge DNG Converter utility...
, a proprietary Adobe product described as “the public, archival format for digital camera raw data”. Although this format is not yet universally accepted, support for the product is growing, and increasingly professional archivists and conservationists, working for respectable organizations, variously suggest or recommend DNG for archival purposes.
Vector
Vector images resulted from mathematical geometry (vectorVector
Vector, a Latin word meaning "carrier", may refer in English to:-In computer science:*A one-dimensional array**Vector , a data type in the C++ Standard Template Library...
). In mathematical terms, a vector consists of point that has both direction and length.
Often, both raster and vector elements will be combined in one image; for example, in the case of a billboard with text (vector) and photographs (raster).
Image viewing
Image viewerImage viewer
An image viewer or image browser is a computer program that can display stored graphical image; it can often handle various graphics file formats...
software displays images. Web browser
Web browser
A web browser is a software application for retrieving, presenting, and traversing information resources on the World Wide Web. An information resource is identified by a Uniform Resource Identifier and may be a web page, image, video, or other piece of content...
s can display standard internet image formats including GIF, 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....
, and PNG. Some can show SVG
Scalable Vector Graphics
Scalable Vector Graphics is a family of specifications of an XML-based file format for describing two-dimensional vector graphics, both static and dynamic . The SVG specification is an open standard that has been under development by the World Wide Web Consortium since 1999.SVG images and their...
format which is a standard W3C format.
Some viewers offer a slideshow
Slideshow
A slide show is a display of a series of chosen information or pictures, done for artistic or instructional purposes. Slide shows are conducted by a presenter using an apparatus, such as a carousel slide projector, an overhead projector or in more recent years, a computer running presentation...
utility to display a sequence of images.
Projected Digital Image (PDI)
"Projected Digital Image" or "PDI" is a digital image used for projection, often in the context of a photographic competition comprising such images. For example, some competitions run by the Photographic Alliance of Great Britain are PDI competitions.Photographic competitions run by photographic societies have historically displayed prints, or slides, submitted by photographers, which are then judged. Increasingly, slide competitions are being replaced by PDI competitions, with photographers submitting suitably-sized digital images which are then projected onto a screen using display software on a computer. The digital images may have originated as images captured on film, then scanned, but are now more likely to have been captured by 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...
.
Each competition provides its own specification for the images, depending on the software and video projector
Video projector
A video projector is an image projector that receives a video signal and projects the corresponding image on a projection screen using a lens system. All video projectors use a very bright light to project the image, and most modern ones can correct any curves, blurriness, and other...
used. A typical digital image projector
Image projector
An image projector is an optical device that projects an image onto a surface, commonly a projection screen.Most projectors creates an image by shining a light through a small transparent image, but some newer types of projectors can project the image directly, by using lasers...
conforms to SXGA+ resolution, and the competition will normally accept 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....
images with sRGB color profile at 1400 pixels wide by 1050 pixels high. Another (older) common format, because it uses cheaper projectors, is XGA, at 1024 pixels wide by 768 pixels high.
History
Early Digital fax machines such as the Bartlane cable picture transmission systemBartlane cable picture transmission system
Bartlane cable picture transmission system was a technique invented in 1920 to transmit images over cable lines. It was named after its inventors Harry G. Bartholomew and Maynard D. McFarlane and was first used to transmit a picture across the Atlantic in 1921. Using the Bartlane system, images...
preceded digital cameras and computers by decades.
The first picture to be scanned, stored, and recreated in digital pixels was displayed on the Standards Eastern Automatic Computer at NIST. The advancement of digital imagery continued in the early 1960s, alongside development of the space program and in medical
Medicine
Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....
research. Projects at the Jet Propulsion Laboratory
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...
, MIT, Bell Labs
Bell Labs
Bell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...
and the University of Maryland
University of Maryland, College Park
The University of Maryland, College Park is a top-ranked public research university located in the city of College Park in Prince George's County, Maryland, just outside Washington, D.C...
, among others, used digital images to advance satellite imagery
Satellite imagery
Satellite imagery consists of photographs of Earth or other planets made by means of artificial satellites.- History :The first images from space were taken on sub-orbital flights. The U.S-launched V-2 flight on October 24, 1946 took one image every 1.5 seconds...
, wirephoto standards conversion, medical imaging
Medical physics
Medical physics is the application of physics to medicine. It generally concerns physics as applied to medical imaging and radiotherapy, although a medical physicist may also work in many other areas of healthcare...
, videophone
Videophone
A videophone is a telephone with a video screen, and is capable of full duplex video and audio transmissions for communication between people in real-time...
technology, character recognition, and photo enhancement.
Rapid advances in digital imaging began with the introduction of microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
s in the early 1970s, alongside progress in related storage and display technologies. The invention of computerised axial tomography (CAT scanning), using x-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s to produce a digital image of a "slice" through a three-dimensional object, was of great importance to medical diagnostics. As well as origination of digital images, digitization of analog images allowed the enhancement and restoration of archaeological
Archaeology
Archaeology, or archeology , is the study of human society, primarily through the recovery and analysis of the material culture and environmental data that they have left behind, which includes artifacts, architecture, biofacts and cultural landscapes...
artefacts and began to be used in fields as diverse as nuclear medicine
Nuclear medicine
In nuclear medicine procedures, elemental radionuclides are combined with other elements to form chemical compounds, or else combined with existing pharmaceutical compounds, to form radiopharmaceuticals. These radiopharmaceuticals, once administered to the patient, can localize to specific organs...
, astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
, law enforcement
Law enforcement agency
In North American English, a law enforcement agency is a government agency responsible for the enforcement of the laws.Outside North America, such organizations are called police services. In North America, some of these services are called police while others have other names In North American...
, defence and industry
Industry
Industry refers to the production of an economic good or service within an economy.-Industrial sectors:There are four key industrial economic sectors: the primary sector, largely raw material extraction industries such as mining and farming; the secondary sector, involving refining, construction,...
.
Advances in microprocessor technology paved the way for the development and marketing of 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...
s (CCDs) for use in a wide range of image capture
Image Capture
Image Capture is an application program from Apple that enables users to upload pictures from digital cameras or scanners which are either connected directly to the computer or the network...
devices and gradually displaced the use of analog film
Photographic film
Photographic film is a sheet of plastic coated with an emulsion containing light-sensitive silver halide salts with variable crystal sizes that determine the sensitivity, contrast and resolution of the film...
and tape
Videotape
A videotape is a recording of images and sounds on to magnetic tape as opposed to film stock or random access digital media. Videotapes are also used for storing scientific or medical data, such as the data produced by an electrocardiogram...
in photography and videography towards the end of the 20th century. The computing power necessary to process digital image capture also allowed computer-generated digital images to achieve a level of refinement close to photorealism
Photorealism
Photorealism is the genre of painting based on using the camera and photographs to gather information and then from this information creating a painting that appears photographic...
.
See also
- Computer printerComputer printerIn computing, a printer is a peripheral which produces a text or graphics of documents stored in electronic form, usually on physical print media such as paper or transparencies. Many printers are primarily used as local peripherals, and are attached by a printer cable or, in most new printers, a...
- Digital image editing
- Digital imagingDigital imagingDigital 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...
- Digital geometryDigital geometryDigital geometry deals with discrete sets considered to be digitized models or images of objects of the 2D or 3D Euclidean space.Simply put, digitizing is replacing an object by a discrete set of its points...
- Digital image processingDigital image processingDigital image processing is the use of computer algorithms to perform image processing on digital images. As a subcategory or field of digital signal processing, digital image processing has many advantages over analog image processing...
- Digital photographyDigital photographyDigital photography is a form of photography that uses an array of light sensitive sensors to capture the image focused by the lens, as opposed to an exposure on light sensitive film...
- Geocoded photoGeocoded photoA geotagged photograph is a photograph which is associated with a geographical location by geotagging. Usually this is done by assigning at least a latitude and longitude to the image, and optionally altitude, compass bearing and other fields may also be included.In theory, every part of a picture...
- Image compressionImage compressionThe objective of image compression is to reduce irrelevance and redundancy of the image data in order to be able to store or transmit data in an efficient form.- Lossy and lossless compression :...
- Image file formats
- Image scannerImage scannerIn computing, an image scanner—often abbreviated to just scanner—is a device that optically scans images, printed text, handwriting, or an object, and converts it to a digital image. Common examples found in offices are variations of the desktop scanner where the document is placed on a glass...
- Optical character recognitionOptical character recognitionOptical character recognition, usually abbreviated to OCR, is the mechanical or electronic translation of scanned images of handwritten, typewritten or printed text into machine-encoded text. It is widely used to convert books and documents into electronic files, to computerize a record-keeping...
- Personal storage devicePersonal storage deviceA portable storage device is a small hard drive designed to hold any kind of digital data. This is slightly different from a portable media player, which stores and plays music and movies.Some are fixed size hard drives of 256GB, 320GB, etc...
(PSD) - ScreenshotScreenshotA screenshot , screen capture , screen dump, screengrab , or print screen is an image taken by a computer to record the visible items displayed on the monitor, television, or another visual output device...
- Signal processingSignal processingSignal processing is an area of systems engineering, electrical engineering and applied mathematics that deals with operations on or analysis of signals, in either discrete or continuous time...
- Digital image correlationDigital image correlationDigital Image Correlation and Tracking is an optical method that employs tracking & image registration techniques for accurate 2D and 3D measurements of changes in images. This is often used to measure deformation , displacement, and strain, but it is widely applied in many areas of science and...