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upright=1, The Smiley, smiley face in the top left corner is a raster image. When enlarged, individual pixels appear as squares. Enlarging further, each pixel can be analyzed, with their colors constructed through combination of the values for red, green and blue. In computer graphics and
digital photography Digital photography uses cameras containing arrays of electronics, electronic photodetectors to produce images focused by a lens (optics), lens, as opposed to an exposure on photographic film. The captured images are digitization, digitized and ...
, a raster graphic is a mechanism that represents a two-dimensional image as a rectangular matrix or grid of square
pixel In digital imaging, a pixel, pel, or picture element is a smallest addressable element in a raster image, or the smallest addressable element in an all points addressable display device s, LED display and Vacuum fluorescent display, VF displa ...

pixel
s, viewable via a
computer display A computer monitor is an output device that displays information in pictorial or text form. A monitor usually comprises a electronic visual display, visual display, electronic circuit, some circuitry, a casing, and a power supply. The display dev ...
,
paper Paper is a thin sheet material produced by mechanically or chemically processing cellulose fibres derived from wood, Textile, rags, poaceae, grasses or other vegetable sources in water, draining the water through fine mesh leaving the fibre ev ...

paper
, or other display medium. A raster is technically characterized by the width and height of the image in pixels and by the number of
bits per pixel Color depth or colour depth (see spelling differences), also known as bit depth, is either the number of bits used to indicate Indication may refer to: * A synonym for sign * Human interface Highlighting the single object pointed to as a cur ...
. Raster images are stored in
image file File:TEIDE.JPG, An Synthetic aperture radar, SAR radar imaging, radar image acquired by the SIR-C/X-SAR radar on board the Space Shuttle Endeavour shows the Teide volcano. The city of Santa Cruz de Tenerife is visible as the purple and white a ...
s with varying
dissemination To disseminate (from lat. ''disseminare'' "scattering seeds"), in the field of communication Communication (from Latin ''communicare'', meaning "to share") is the act of developing Semantics, meaning among Subject (philosophy), entities or Org ...
,
production Production may be: Economics and business * Production (economics) * Production, the act of manufacturing goods * Production, in the outline of industrial organization, the act of making products (goods and services) * Production as a statistic, g ...
,
generation A generation is "all of the people born and living Living or The Living may refer to: Common meanings *Life, a condition that distinguishes organisms from inorganic objects and dead organisms ** extant taxon, Living species, one that is not ex ...
, and acquisition formats. The
printing Printing is a process for mass reproducing text and images An Synthetic aperture radar, SAR radar imaging, radar image acquired by the SIR-C/X-SAR radar on board the Space Shuttle Endeavour shows the Teide volcano. The city of Santa Cruz ...

printing
and
prepressPrepress is the term used in the printing Printing is a process for mass reproducing text and images using a master form or template. The earliest non-paper products involving printing include cylinder seals and objects such as the Cyrus Cylin ...
industries know raster graphics as contones (from ''continuous tones''). In contrast,
line art Line art or line drawing is any image File:TEIDE.JPG, An Synthetic aperture radar, SAR radar imaging, radar image acquired by the SIR-C/X-SAR radar on board the Space Shuttle Endeavour shows the Teide volcano. The city of Santa Cruz de Tener ...
is usually implemented as
vector graphics Vector graphics, as a form of computer graphics, is the set of mechanisms for creating visual images directly from geometric shapes defined on a Cartesian plane A Cartesian coordinate system (, ) in a plane is a coordinate system that ...
in digital systems. Many raster manipulations map directly onto the mathematical formalisms of
linear algebra Linear algebra is the branch of mathematics concerning linear equations such as: :a_1x_1+\cdots +a_nx_n=b, linear maps such as: :(x_1, \ldots, x_n) \mapsto a_1x_1+\cdots +a_nx_n, and their representations in vector spaces and through matrix (math ...
, where mathematical objects of
matrix Matrix or MATRIX may refer to: Science and mathematics * Matrix (mathematics), a rectangular array of numbers, symbols, or expressions * Matrix (logic), part of a formula in prenex normal form * Matrix (biology), the material in between a eukaryoti ...
structure are of central concern.


Etymology

The word "raster" has its origins in the Latin ''
rastrum A rastrum (or raster) is a five-pointed writing implement A writing implement or writing instrument is an object used to produce writing Writing is a medium of human communication that involves the representation of a language with writt ...
'' (a rake), which is derived from '' radere'' (to scrape). It originates from the
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 file:Rgb-raster-image.svg, upright=1, The Smiley, smiley face in the top left cor ...
of
cathode ray tube File:CRT monochrome.png, 250px, Cutaway rendering of a monochrome CRT: 1. Deflection coils2. Electron beam3. Focusing coil4. Phosphor layer on the inner side of the screen; emits light when struck by the electron beam5.&nbs ...

cathode ray tube
(CRT)
video monitor s, LED display and VF display, top to bottom. A display device is an output deviceAn output device is any piece of computer hardware Computer hardware includes the physical parts of a computer A computer is a machine that can be program ...
s, which paint the image line by line by magnetically or electrostatically steering a focused
electron beam Cathode rays (electron beam or e-beam) are streams of electrons observed in discharge tubes. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, glass behind the positive electrode is observed to glow, due to ...
. By association, it can also refer to a rectangular grid of pixels. The word
rastrum A rastrum (or raster) is a five-pointed writing implement A writing implement or writing instrument is an object used to produce writing Writing is a medium of human communication that involves the representation of a language with writt ...
is now used to refer to a device for drawing musical staff lines.


Data Model

The fundamental strategy underlying the raster model is the
tessellation A tessellation or tiling is the covering of a surface File:Water droplet lying on a damask.jpg, Water droplet lying on a damask. Surface tension is high enough to prevent floating below the textile. A surface, as the term is most generally ...

tessellation
of a plane, into a two dimensional array of squares, each called a ''cell'' or ''
pixel In digital imaging, a pixel, pel, or picture element is a smallest addressable element in a raster image, or the smallest addressable element in an all points addressable display device s, LED display and Vacuum fluorescent display, VF displa ...

pixel
'' (from "picture element"). In
digital photography Digital photography uses cameras containing arrays of electronics, electronic photodetectors to produce images focused by a lens (optics), lens, as opposed to an exposure on photographic film. The captured images are digitization, digitized and ...
, the plane is the
Visual field The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments". Or simply, visual field can be defined as the entire area that can be seen when an eye is fixed straight at a point. ...
as projected onto the CCD sensor; in
computer art Computer art is any art Art is a diverse range of (products of) human activities involving creative imagination to express technical proficiency, beauty, emotional power, or conceptual ideas. There is no generally agreed definition of wh ...

computer art
, the plane is a virtual canvas; in
Geographic information systems A geographic information system (GIS) is a conceptualized framework that provides the ability to capture and analyze spatial and geographic data. GIS applications (or GIS apps) are computer-based tools that allow the user to create interactive ...
, the plane is a projection of the Earth's surface. The size of each square pixel, known as the ''resolution'' or ''support'', is constant across the grid. A single numeric value is then stored for each pixel. For most images, this value is a visible color, but other measurements are possible, even numeric codes for qualitative categories. Each raster grid has a specified ''pixel format'', the data type for each number. Common pixel formats are
binary Binary may refer to: Science and technology Mathematics * Binary number In mathematics and digital electronics, a binary number is a number A number is a mathematical object used to counting, count, measurement, measure, and nominal ...
,
gray scale In digital photography Digital photography uses cameras containing arrays of electronics, electronic photodetectors to produce images focused by a lens (optics), lens, as opposed to an exposure on photographic film. The captured images a ...

gray scale
, palettized, and
full color
full color
, where
color depth Color depth or colour depth (see spelling differences Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in English orthography ...
determines the fidelity of the colors represented and
color space A color space is a specific organization of colors. In combination with color profiling supported by various physical devices, it supports reproducible representations of color -- whether such representation entails an analog signal, analog or a ...
determines the range of color coverage (which is often less than the full range of human
color vision Color vision, a feature of visual perception, is an ability to perceive differences between light composed of different wavelengths (i.e., different spectral power distributions) independently of light intensity. Color perception is a part of th ...

color vision
). Most modern color raster formats represent color using 24 bits (over 16 million distinct colors), 8 each (0-255) for red, green, and blue. The digital sensors used for
remote sensing image of Death Valley colored using polarimetry. Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied e ...

remote sensing
and
astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses mathematics, physi ...
are often able to detect and store wavelengths beyond the
visible spectrum File:Light Amplification by Stimulated Emission of Radiation.jpg, Laser beams with visible spectrum The visible spectrum is the portion of the electromagnetic spectrum that is visual perception, visible to the human eye. Electromagnetic radiation ...
; the large CCD bitmapped sensor at the Vera C. Rubin Observatory captures 3.2 gigapixels in a single image (6.4 GB raw), over six color channels which exceed the electromagnetic spectrum, spectral range of human color vision.


Applications


Image storage

Most computer images are stored in Image file formats#Raster formats, raster graphics formats or compressed variations, including GIF, JPEG, and Portable Network Graphics, PNG, which are popular on the World Wide Web. A raster data structure is based on a (usually rectangular, square-based)
tessellation A tessellation or tiling is the covering of a surface File:Water droplet lying on a damask.jpg, Water droplet lying on a damask. Surface tension is high enough to prevent floating below the textile. A surface, as the term is most generally ...

tessellation
of the 2D plane (geometry), plane into cells, each containing a single value. To store the data in a file, the two-dimensional array must be serialized. The most common way to do this is a ''row-major'' format, in which the cells along the first (usually top) row are listed left to right, followed immediately by those of the second row, and so on. In the example at right, the cells of tessellation A are overlaid on the point pattern B resulting in an array C of quadrant counts representing the number of points in each cell. For purposes of visualization a lookup table has been used to color each of the cells in an image D. Here are the numbers as a serial row-major array: 1 3 0 0 1 12 8 0 1 4 3 3 0 2 0 2 1 7 4 1 5 4 2 2 0 3 1 2 2 2 2 3 0 5 1 9 3 3 3 4 5 0 8 0 2 4 3 2 8 4 3 2 2 7 2 3 2 10 1 5 2 1 3 7 To reconstruct the two-dimensional grid, the file must include a ''header'' section at the beginning that contains at least the number of columns, and the pixel datatype (especially the number of bits or bytes per value) so the reader knows where each value ends to start reading the next one. Headers may also include the number of rows, georeferencing parameters for geographic data, or other metadata tags, such as those specified in the Exif standard.


Compression

High-resolution raster grids contain a large number of pixels, and thus consume a large amount of memory. This has led to multiple approaches to compressing the data volume into smaller files. The most common strategy is to look for patterns or trends in the pixel values, then store a parameterized form of the pattern instead of the original data. Common raster compression algorithms include run-length encoding (RLE), JPEG, LZ77 and LZ78, LZ (the basis for Portable Network Graphics, PNG and Zip (file format), ZIP), Lempel–Ziv–Welch , LZW (the basis for GIF), and others. For example, Run length encoding looks for repeated values in the array, and replaces them with the value and the number of times it appears. Thus, the raster above would be represented as: This technique is very efficient when there are large areas of identical values, such as a line drawing, but in a photograph where pixels are usually slightly different than their neighbors, the RLE file would be up to twice the size of the original. Some compression algorithms, such as RLE and LZW, are ''lossless'', where the original pixel values can be perfectly regenerated from the compressed data. Other algorithms, such as JPEG, are ''lossy'', because the parameterized patterns are only an approximation of the original pixel values, so the latter can only be estimated from the compressed data.


Raster/Vector Conversion

Vector images (line work) can be rasterisation, rasterized (converted into pixels), and raster images image tracing, vectorized (raster images converted into vector graphics), by software. In both cases some information is lost, although certain vectorization operations can recreate salient information, as in the case of optical character recognition.


Displays

Early mechanical televisions developed in the 1920s employed rasterization principles. Electronic television based on cathode-ray tube displays are
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 file:Rgb-raster-image.svg, upright=1, The Smiley, smiley face in the top left cor ...
ned with horizontal rasters painted left to right, and the raster lines painted top to bottom. Modern flat-panel displays such as LED monitors still use a raster approach. Each on-screen pixel directly corresponds to a small number of bits in memory. The screen is refreshed simply by scanning through pixels and coloring them according to each set of bits. The refresh procedure, being speed critical, is often implemented by dedicated circuitry, often as a part of a graphics processing unit. Using this approach, the computer contains an area of memory that holds all the data that are to be displayed. The central processor writes data into this region of memory and the video controller collects them from there. The bits of data stored in this block of memory are related to the eventual pattern of pixels that will be used to construct an image on the display. An early scanned display with raster computer graphics was invented in the late 1960s by A. Michael Noll at Bell Labs, but its patent application filed February 5, 1970 was abandoned at the Supreme Court in 1977 over the issue of the patentability of computer software.


Printing

During the 1970s and 1980s, Plotter , pen plotters, using Vector graphics, were common for creating precise drawings, especially on large format paper. However, since then almost all printers create the printed image as a raster grid, including both Laser printing , laser and Inkjet printing , inkjet printers. When the source information is vector, rendering specifications and software such as PostScript are used to create the raster image.


Three-dimensional rasters

Three-dimensional voxel raster graphics are employed in video games and are also used in medical imaging such as Magnetic resonance imaging, MRI scanners.


Geographic information systems

Geographic phenomena are commonly represented in a raster format in Geographic information system, GIS. The raster grid is ''Georeferencing , georeferenced'', so that each pixel (commonly called a ''cell'' in GIS because the "picture" part of "pixel" is not relevant) represents a square region of geographic space. The value of each cell then represents some measurable (Level of measurement, qualitative or quantitative) property of that region, typically conceptualized as a Field (geography) , field. Examples of fields commonly represented in raster include: temperature, population density, soil moisture, land cover, surface elevation, etc. Two sampling models are used do derive cell values from the field: in a ''lattice'', the value is measured at the center point of each cell; in a ''grid'', the value is a summary (usually a mean or mode) of the value over the entire cell.


Resolution

Raster graphics are resolution dependent, meaning they cannot scale up to an arbitrary resolution without pixellation, loss of apparent quality. This property contrasts with the capabilities of
vector graphics Vector graphics, as a form of computer graphics, is the set of mechanisms for creating visual images directly from geometric shapes defined on a Cartesian plane A Cartesian coordinate system (, ) in a plane is a coordinate system that ...
, which easily scale up to the quality of the device Rendering (computer graphics), rendering them. Raster graphics deal more practically than vector graphics with photographs and photo-realistic images, while vector graphics often serve better for typesetting or for graphic design. Modern computer-monitors typically display about 72 to 130 pixels per inch (PPI), and some modern consumer printers can resolve 2400 dots per inch (DPI) or more; determining the most appropriate image resolution for a given printer-resolution can pose difficulties, since printed output may have a greater level of detail than a viewer can discern on a monitor. Typically, a resolution of 150 to 300 PPI works well for 4-color process (CMYK) printing. However, for printing technologies that perform color mixing through dithering (halftone) rather than through overprinting (virtually all home/office inkjet and laser printers), printer DPI and image PPI have a very different meaning, and this can be misleading. Because, through the dithering process, the printer builds a single image pixel out of several printer dots to increase
color depth Color depth or colour depth (see spelling differences Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in English orthography ...
, the printer's DPI setting must be set far higher than the desired PPI to ensure sufficient color depth without sacrificing image resolution. Thus, for instance, printing an image at 250 PPI may actually require a printer setting of 1200 DPI.


Raster-based image editors

Raster-based image editors, such as PaintShop Pro, Corel Painter, Adobe Photoshop, Paint.NET, Microsoft Paint, and GIMP, revolve around editing pixels, unlike vector-based image editors, such as Xfig, CorelDRAW, Adobe Illustrator, or Inkscape, which revolve around editing lines and shapes (vector graphics, vectors). When an image is rendered in a raster-based image editor, the image is composed of millions of pixels. At its core, a raster image editor works by manipulating each individual pixel. Most pixel-based image editors work using the RGB color model#Digital representations, RGB color model, but some also allow the use of other color models such as the CMYK color model.


See also

*Comparison of raster graphics editors *Dither *Halftone *Pixel-art scaling algorithms *Raster graphics editor *Raster graphics file formats *Raster image processor *Raster scan *Rasterisation *Text semigraphics *Texture atlas *Vector graphics - a contrasting graphics method


References

{{DEFAULTSORT:Raster Graphics Raster graphics, Computer graphics data structures Graphics file formats Digital geometry