Digital image

A digital image is an image composed of picture elements, namley pixels, each with finite, discrete quantities of numeric representation for its intensity or gray level that is an output from its two-dimensional functions fed as input by its spatial coordinates denoted with x, y on the x-axis and y-axis, respectively.[1] Depending on whether the image resolution is fixed, it may be of vector or raster type. By itself, the term "digital image" usually refers to raster images or bitmapped images (as opposed to vector images).


Raster images have a finite set of digital values, called picture elements or pixels. The digital image contains a fixed number of rows and columns of pixels. Pixels are the smallest individual element in an image, holding antiquated 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 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 of computer graphics. The field of digital image processing is the study of algorithms for their transformation.

Raster file formats

Most users come into contact with raster images through digital cameras, which use any of several image file formats.

Some digital cameras 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".[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, a vector 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).

Image viewing

Image viewer software displays images. Web browsers can display standard internet image formats including JPEG, GIF and PNG. Some can show SVG format which is a standard W3C format. In the past, when Internet was still slow, it was common to provide "preview" image that would load and appear on the web site 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 the Milky Way, about 194 Gb in size).[11] Such images are difficult to download and are usually browsed online through more complex web interfaces.

Some viewers offer a slideshow utility to display a sequence of images.


Early digital fax machines such as the Bartlane 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) at NIST.[12] 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.[13]

Rapid advances in digital imaging began with the introduction of MOS integrated circuits in the 1960s and microprocessors in the early 1970s, alongside progress in related computer memory storage, display technologies, and data compression algorithms.

The invention of computerized axial tomography (CAT scanning), using x-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 of archaeological artifacts and began to be used in fields as diverse as nuclear medicine, astronomy, law enforcement, defence and industry.[14]

Advances in microprocessor technology paved the way for the development and marketing of charge-coupled devices (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.[15]

Digital image sensors

The basis for digital image sensors is metal-oxide-semiconductor (MOS) technology,[16] which originates from the invention of the MOSFET (MOS field-effect transistor) by Mohamed M. Atalla and Dawon Kahng at Bell Labs in 1959.[17] This led to the development of digital semiconductor image sensors, including the charge-coupled device (CCD) and later the CMOS sensor.[16]

The first semiconductor image sensor was the CCD, developed by Willard S. Boyle and George E. Smith at Bell Labs in 1969.[18] 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 tiny MOS capacitor. As it was fairly straighforward to fabricate 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.[16] The CCD is a semiconductor circuit that was later used in the first digital video cameras for television broadcasting.[19]

Early CCD sensors suffered from shutter lag. This was largely resolved with the invention of the pinned photodiode (PPD).[20] It was invented by Nobukazu Teranishi, Hiromitsu Shiraki and Yasuo Ishihara at NEC in 1980.[20][21] It was a photodetector structure with low lag, low noise, high quantum efficiency and low dark current.[20] In 1987, the PPD began to be incorporated into most CCD devices, becoming a fixture in consumer electronic video cameras and then digital still cameras. Since then, the PPD has been used in nearly all CCD sensors and then CMOS sensors.[20]

The NMOS active-pixel sensor (APS) was invented by Olympus in Japan during the mid-1980s. This was enabled by advances in MOS semiconductor device fabrication, with MOSFET scaling reaching smaller micron and then sub-micron levels.[22][23] The NMOS APS was fabricated by Tsutomu Nakamura's team at Olympus in 1985.[24] The CMOS active-pixel sensor (CMOS sensor) was later developed by Eric Fossum's team at the NASA Jet Propulsion Laboratory in 1993.[20] By 2007, sales of CMOS sensors had surpassed CCD sensors.[25]

Digital image compression

An important development in digital image compression technology was the discrete cosine transform (DCT), a lossy compression technique first proposed by Nasir Ahmed in 1972.[26] DCT compression became the basis for JPEG, which was introduced by the Joint Photographic Experts Group in 1992.[27] JPEG compresses images down to much smaller file sizes, and has become the most widely used image file format on the Internet.[28] Its highly efficient DCT compression algorithm was largely responsible for the wide proliferation of digital images and digital photos,[29] with several billion JPEG images produced every day as of 2015.[30]

See also


  1. Gonzalez, Rafael (2018). Digital image processing. New York, NY: Pearson. ISBN 978-0-13-335672-4. OCLC 966609831.
  2. Digital Negative (DNG) Specification. San Jose: Adobe, 2005. Vers. p. 9. Accessed on October 10, 2007.
  3. universal photographic digital imaging guidelines (UPDIG): File formats - the raw file issue
  4. Archaeology Data Service / Digital Antiquity: Guides to Good Practice - Section 3 Archiving Raster Images - File Formats
  5. University of Connecticut: "Raw as Archival Still Image Format: A Consideration" by Michael J. Bennett and F. Barry Wheeler
  6. Inter-University Consortium for Political and Social Research: Obsolescence - File Formats and Software
  7. JISC Digital Media - Still Images: Choosing a File Format for Digital Still Images - File formats for master archive
  8. The J. Paul Getty Museum - Department of Photographs: Rapid Capture Backlog Project - Presentation Archived 2012-06-10 at the Wayback Machine
  9. American Institute for Conservation - Electronic Media Group: Digital Image File Formats
  10. Archives Association of British Columbia: Born Digital Photographs: Acquisition and Preservation Strategies (Rosaleen Hill)
  11. "This 46-Gigapixel photo of the Milky Way will blow your mind". Retrieved 5 July 2018.
  12. Fiftieth Anniversary of First Digital Image.
  13. Azriel Rosenfeld, Picture Processing by Computer, New York: Academic Press, 1969
  14. Gonzalez, Rafael, C; Woods, Richard E (2008). Digital Image Processing, 3rd Edition. Pearson Prentice Hall. p. 577. ISBN 978-0-13-168728-8.
  15. Jähne, Bernd (1993). Spatio-temporal image processing, Theory and Scientific Applications. Springer Verlag. p. 208. ISBN 3-540-57418-2.
  16. Williams, J. B. (2017). The Electronics Revolution: Inventing the Future. Springer. pp. 245–8. ISBN 9783319490885.
  17. "1960: Metal Oxide Semiconductor (MOS) Transistor Demonstrated". The Silicon Engine. Computer History Museum. Retrieved August 31, 2019.
  18. James R. Janesick (2001). Scientific charge-coupled devices. SPIE Press. pp. 3–4. ISBN 978-0-8194-3698-6.
  19. Boyle, William S; Smith, George E. (1970). "Charge Coupled Semiconductor Devices". Bell Syst. Tech. J. 49 (4): 587–593. doi:10.1002/j.1538-7305.1970.tb01790.x.
  20. Fossum, Eric R.; Hondongwa, D. B. (2014). "A Review of the Pinned Photodiode for CCD and CMOS Image Sensors". IEEE Journal of the Electron Devices Society. 2 (3): 33–43. doi:10.1109/JEDS.2014.2306412.
  21. U.S. Patent 4,484,210: Solid-state imaging device having a reduced image lag
  22. Fossum, Eric R. (12 July 1993). Blouke, Morley M. (ed.). "Active pixel sensors: are CCDs dinosaurs?". SPIE Proceedings Vol. 1900: Charge-Coupled Devices and Solid State Optical Sensors III. International Society for Optics and Photonics: 2–14. CiteSeerX doi:10.1117/12.148585.
  23. Fossum, Eric R. (2007). "Active Pixel Sensors" (PDF). Semantic Scholar. Retrieved 8 October 2019.
  24. Matsumoto, Kazuya; et al. (1985). "A new MOS phototransistor operating in a non-destructive readout mode". Japanese Journal of Applied Physics. 24 (5A): L323. Bibcode:1985JaJAP..24L.323M. doi:10.1143/JJAP.24.L323.
  25. "CMOS Image Sensor Sales Stay on Record-Breaking Pace". IC Insights. May 8, 2018. Retrieved 6 October 2019.
  26. Ahmed, Nasir (January 1991). "How I Came Up With the Discrete Cosine Transform". Digital Signal Processing. 1 (1): 4–5. doi:10.1016/1051-2004(91)90086-Z.
  28. "The JPEG image format explained". BT Group. 31 May 2018. Retrieved 5 August 2019.
  29. "What Is a JPEG? The Invisible Object You See Every Day". The Atlantic. 24 September 2013. Retrieved 13 September 2019.
  30. Baraniuk, Chris (15 October 2015). "Copy protections could come to JPEGs". BBC News. BBC. Retrieved 13 September 2019.
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