Adigital image is animage composed ofpicture elements, also known aspixels, each withfinite,discrete quantities of numeric representation for itsintensity orgray level that is an output from itstwo-dimensional functions fed as input by itsspatial coordinates denoted withx,y on the x-axis and y-axis, respectively.[1] Depending on whether theimage resolution is fixed, it may be ofvector orraster type.By itself, the term "digital image" usually refers toraster images orbitmapped images (as opposed tovector images).[citation needed]
Raster images have a finite set ofdigital values, calledpicture elements orpixels. 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 araster image or raster map, a two-dimensional array of small integers. These values are often transmitted or stored in acompressed form.
Raster images can becreated by a variety of input devices and techniques, such asdigital cameras,scanners, 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 ofcomputer graphics. The field ofdigital image processing is the study of algorithms for their transformation.
Most users come into contact with raster images through digital cameras, which use any of severalimage file formats.
Somedigital cameras give access to almost all the data captured by the camera, using araw 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 asOpenRAW to influence manufacturers to release these records publicly. An alternative may beDigital Negative (DNG), a proprietary Adobe product described as "the public, archival format for digital camera raw data".[2] 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.[3][4][5][6][7][8][9][10]
Vector images resulted from mathematical geometry (vector). In mathematical terms, avector consists of both a magnitude, or length, and a direction.
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).
Example of vector file types areEPS,PDF, andAI.
Image viewer software displayed on images.Web browsers can display standard internet images formats includingJPEG,GIF andPNG. Some can showSVG format which is a standardW3C format. In the past, when the Internet was still slow, it was common to provide "preview" images that would load and appear on the website before being replaced by the main image (to give at preliminary impression). Now Internet is fast enough and this preview image is seldom used.
Some scientific images can be very large (for instance, the 46 gigapixel size image of theMilky Way, about 194 Gb in size).[11] Such images are difficult to download and are usually browsed online through more complexweb interfaces.
Some viewers offer aslideshow utility to display a sequence of images.
Earlydigital fax machines such as theBartlane cable picture transmission system 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 (SEAC) atNIST.[12] The advancement of digital imagery continued in the early 1960s, alongside development of thespace program and inmedical research. Projects at theJet Propulsion Laboratory,MIT,Bell Labs and theUniversity of Maryland, among others, used digital images to advancesatellite imagery, wirephoto standards conversion,medical imaging,videophone technology,character recognition, and photo enhancement.[13]
Rapid advances indigital imaging began with the introduction ofMOS integrated circuits in the 1960s andmicroprocessors in the early 1970s, alongside progress in relatedcomputer memory storage,display technologies, anddata compression algorithms.
The invention of computerized axial tomography (CAT scanning), usingx-rays 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 ofarchaeological artifacts and began to be used in fields as diverse asnuclear medicine,astronomy,law enforcement,defence andindustry.[14]
Advances in microprocessor technology paved the way for the development and marketing ofcharge-coupled devices (CCDs) for use in a wide range ofimage capture devices and gradually displaced the use of analogfilm andtape in photography and videography towards the end of the 20th century. The computing power necessary to process digital image capture also allowedcomputer-generated digital images to achieve a level of refinement close tophotorealism.[15]
The first semiconductor image sensor was the CCD, developed byWillard S. Boyle andGeorge E. Smith at Bell Labs in 1969.[16] While researching MOS technology, they realized that an electric charge was the analogy of the magnetic bubble and that it could be stored on a tinyMOS capacitor. As it was fairly straightforward tofabricate a series of MOS capacitors in a row, they connected a suitable voltage to them so that the charge could be stepped along from one to the next.[17] The CCD is a semiconductor circuit that was later used in the firstdigital video cameras fortelevision broadcasting.[18]
Early CCD sensors suffered fromshutter lag. This was largely resolved with the invention of thepinned photodiode (PPD).[19] It was invented byNobukazu Teranishi, Hiromitsu Shiraki and Yasuo Ishihara atNEC in 1980.[19][20] It was aphotodetector structure with low lag, lownoise, highquantum efficiency and lowdark current.[19] In 1987, the PPD began to be incorporated into most CCD devices, becoming a fixture inconsumer electronicvideo cameras and thendigital still cameras. Since then, the PPD has been used in nearly all CCD sensors and then CMOS sensors.[19]
TheNMOSactive-pixel sensor (APS) was invented byOlympus in Japan during the mid-1980s. This was enabled by advances in MOSsemiconductor device fabrication, withMOSFET scaling reaching smallermicron and then sub-micron levels.[21][22] The NMOS APS was fabricated by Tsutomu Nakamura's team at Olympus in 1985.[23] TheCMOS active-pixel sensor (CMOS sensor) was later developed byEric Fossum's team at theNASAJet Propulsion Laboratory in 1993.[19] By 2007, sales of CMOS sensors had surpassed CCD sensors.[24]
An important development in digitalimage compression technology was thediscrete cosine transform (DCT), alossy compression technique first proposed byNasir Ahmed in 1972.[25] DCT compression is used inJPEG, which was introduced by theJoint Photographic Experts Group in 1992.[26] JPEG compresses images down to much smaller file sizes, and has become the most widely used image file format on theInternet.[27]
In digital imaging, amosaic is a combination of non-overlapping images, arranged in sometessellation.Gigapixel images are an example of such digital image mosaics.Satellite imagery are often mosaicked to cover Earth regions.
Interactive viewing is provided byvirtual-reality photography.
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