Digital image

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.

Raster

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 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 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 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 to the number and nature of those samples:

• binary
• grayscale
• color
• false-color
• multi-spectral
• thematic
• picture function

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.

Raster file formats

Most users come into contact with raster images through digital cameras.

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”.^[1] 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.^{[2][3][4][5][6][7][8][9][10]}

Vector

Vector images resulted from mathematical geometry (vector). 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 viewer software displays images. Web browsers 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.

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.

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.

History

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 at NIST.^[11] 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.^[12]

Rapid advances in digital imaging began with the introduction of microprocessors in the early 1970s, alongside progress in related storage and display technologies. The invention of computerised 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 artefacts and began to be used in fields as diverse as nuclear medicine, astronomy, law enforcement, defence and industry.^[13]

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.^[14]

See also

Computer graphics portal

• Computer printer
• Digital image editing
• Digital imaging
• Digital geometry
• Digital image processing
• Digital photography
• Geocoded photo
• Image compression
• Image file formats
• Image scanner
• Optical character recognition
• Personal storage device (PSD)
• Screenshot
• Signal processing
• Digital image correlation

References

1. ^ Digital Negative (DNG) Specification. San Jose: Adobe, 2005. Vers. 1.1.0.0. p. 9. Accessed on October 10, 2007.
2. ^ universal photographic digital imaging guidelines (UPDIG): File formats - the raw file issue
3. ^ Archaeology Data Service / Digital Antiquity: Guides to Good Practice - Section 3 Archiving Raster Images - File Formats
4. ^ University of Connecticut: "Raw as Archival Still Image Format: A Consideration" by Michael J. Bennett and F. Barry Wheeler
5. ^ Inter-University Consortium for Political and Social Research: Obsolescence - File Formats and Software
6. ^ JISC Digital Media - Still Images: Choosing a File Format for Digital Still Images - File formats for master archive
7. ^ International Digital Enterprise Alliance, Digital Image Submission Criteria (DISC) Guidelines & Specifications 2007 (PDF)
8. ^ The J. Paul Getty Museum - Department of Photographs: Rapid Capture Backlog Project - Presentation
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. ^ Fiftieth Anniversary of First Digital Image.
12. ^ Azriel Rosenfeld, Picture Processing by Computer, New York: Academic Press, 1969
13. ^ Gonzalez, Rafael, C; Woods, Richard E (2008). Digital Image Processing, 3rd Edition. Pearson Prentice Hall. pp. 577. ISBN 013168728. http://books.google.com/books?id=8uGOnjRGEzoC&printsec=frontcover&dq=%22digital+image+processing%22+gonzalez#v=onepage&q=&f=false. 
14. ^ Jähne, Bernd (1993). Spatio-temporal image processing, Theory and Scientific Applications. Springer Verlag. pp. 208. ISBN 3540574182. http://books.google.com/books?id=gO6V5gh4IXsC&dq=Spatio-temporal+image+processing.