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 electronic photodetectors interfaced to an analog-to-digital converter (ADC) to produce images focused by a lens, as opposed to an exposure on photographic film. The digitized image i ...

, a raster graphic represents a two-dimensional picture as a rectangular matrix or grid of square

pixel In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a raster image, or the smallest point in an all points addressable display device. In most digital display devices, pixels are the s ...

s, viewable via a computer display,

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 e ...

, 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. Raster images are stored in image files with varying dissemination, production,

generation A generation refers to all of the people born and living at about the same time, regarded collectively. It can also be described as, "the average period, generally considered to be about 20–⁠30 years, during which children are born and gro ...

, and acquisition formats. 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 Cylinder and the Cylinders of Nabonidus. The ...

and

prepress Prepress is the term used in the Printing and Publishing industries for the processes and procedures that occur between the creation of a print layout and the final printing. The prepress process includes the preparation of artwork for press, media ...

industries know raster graphics as contones (from ''continuous tones''). In contrast,

line art Line art or line drawing is any image that consists of distinct straight lines or curves placed against a background (usually plain), without gradations in shade (darkness) or hue (color) to represent two-dimensional or three-dimensional obje ...

is usually implemented as

vector graphics Vector graphics is a form of computer graphics in which visual images are created directly from geometric shapes defined on a Cartesian plane, such as points, lines, curves and polygons. The associated mechanisms may include vector display ...

in digital systems. Matrix transpose

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 matric ...

, where mathematical objects of

matrix Matrix most commonly refers to: * ''The Matrix'' (franchise), an American media franchise ** '' The Matrix'', a 1999 science-fiction action film ** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchi ...

structure are of central concern.

Etymology

The word "raster" has its origins in the Latin '' rastrum'' (a rake), which is derived from '' radere'' (to scrape). It originates from the raster scan of cathode ray tube (CRT) video monitors, which paint the image line by line by magnetically or electrostatically steering a focused electron beam. By association, it can also refer to a rectangular grid of pixels. The word rastrum is now used to refer to a device for drawing musical staff lines.

Data model

Raster graphic fish 20x23squares sdtv-example

The fundamental strategy underlying the raster model is the

tessellation A tessellation or tiling is the covering of a surface, often a plane, using one or more geometric shapes, called ''tiles'', with no overlaps and no gaps. In mathematics, tessellation can be generalized to higher dimensions and a variety of ge ...

of a plane, into a two-dimensional array of squares, each called a ''cell'' or ''

'' (from "picture element"). In

, 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

image sensor An image sensor or imager is a sensor that detects and conveys information used to make an image. It does so by converting the variable attenuation of light waves (as they pass through or reflect off objects) into signals, small bursts of c ...

; in

computer art Computer art is any art in which computers play a role in production or display of the artwork. Such art can be an image, sound, animation, video, CD-ROM, DVD-ROM, video game, website, algorithm, performance or gallery installation. Many tr ...

, the plane is a virtual canvas; in geographic information systems, 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, gray-scale, palettized, and full-color, where color depth 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 colorwhether such representation entails an analog or a digital representa ...

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 ...

). Most modern color raster formats represent color using 24 bits (over 16 million distinct colors), with 8 bits (values 0–255) for each color channel (red, green, and blue). The digital sensors used for

remote sensing 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 especially to acquiring information about Ear ...

and

astronomy Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objects of interest ...

are often able to detect and store wavelengths beyond the

visible spectrum The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called ''visible light'' or simply light. A typical human eye will respond to wav ...

; 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 spectral range of human color vision.

Applications

Image storage

The use of a raster data structure to summarize a point pattern

Most computer images are stored in raster graphics formats or compressed variations, including GIF,

JPEG JPEG ( ) is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography. The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and im ...

, and PNG, which are popular on the

World Wide Web The World Wide Web (WWW), commonly known as the Web, is an information system enabling documents and other web resources to be accessed over the Internet. Documents and downloadable media are made available to the network through web se ...

. A raster data structure is based on a (usually rectangular, square-based)

of the 2D 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 In computer science, a lookup table (LUT) is an array that replaces runtime computation with a simpler array indexing operation. The process is termed as "direct addressing" and LUTs differ from hash tables in a way that, to retrieve a value v w ...

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 Georeferencing means that the internal coordinate system of a map or aerial photo image can be related to a geographic coordinate system. The relevant coordinate transforms are typically stored within the image file (GeoPDF and GeoTIFF are examples ...

parameters for geographic data, or other metadata tags, such as those specified in the

Exif Exchangeable image file format (officially Exif, according to JEIDA/JEITA/CIPA specifications) is a standard that specifies formats for images, sound, and ancillary tags used by digital cameras (including smartphones), scanners and other syste ...

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