Thesmiley 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.
Transposing an image to covert raster organization (a relatively costly operation for packed formats with less than a byte per pixel);composing an additional raster linereflection (almost free), either before or afterwards, amounts to a 90° image rotation in one direction or the other.
Many raster manipulations map directly onto the mathematical formalisms oflinear algebra, where mathematical objects ofmatrix structure are of central concern.
The word "raster" has its origins in the Latinrastrum (a rake), which is derived fromradere (to scrape). It originates from theraster scan ofcathode-ray tube (CRT)video monitors, which draw the image line by line by magnetically or electrostatically steering a focusedelectron beam.[3] By association, it can also refer to a rectangular grid of pixels. The wordrastrum is now used to refer to a device for drawing musical staff lines.
The fundamental strategy underlying the raster data model is thetessellation of a plane, into a two-dimensional array of squares, each called acell orpixel (from "picture element"). Indigital photography, the plane is thevisual field as projected onto theimage sensor; incomputer art, the plane is a virtual canvas; ingeographic information systems, the plane is aprojection of the Earth's surface. The size of each square pixel, known as theresolution orsupport, is constant across the grid.Raster orgridded data may be the result of agridding procedure.
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 specifiedpixel format, the data type for each number. Common pixel formats arebinary,gray-scale,palettized, andfull-color, wherecolor depth[4] determines the fidelity of the colors represented, andcolor space determines the range of color coverage (which is often less than the full range of humancolor vision). Most modern color raster formats represent color using 24 bits (over 16 million distinct colors), with 8 bits (values 0–255) for eachcolor channel (red, green, and blue). The digital sensors used forremote sensing andastronomy are often able to detect and store wavelengths beyond thevisible spectrum; the largeCCD bitmapped sensor at theVera C. Rubin Observatory captures 3.2 gigapixels in a single image (6.4 GB raw), over sixcolor channels which exceed thespectral range of human color vision.
Most computer images are stored inraster graphics formats or compressed variations, includingGIF,JPEG, andPNG, which are popular on theWorld Wide Web.[4][5] Araster data structure is based on a (usually rectangular, square-based)tessellation of the 2Dplane 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 arow-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 alookup table has been used to color each of the cells in an image D. Here are the numbers as a serial row-major array:
To reconstruct the two-dimensional grid, the file must include aheader 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 othermetadata tags, such as those specified in theExif standard.
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 includerun-length encoding (RLE),JPEG,LZ (the basis forPNG andZIP),Lempel–Ziv–Welch (LZW) (the basis forGIF), 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:
values
1
3
0
1
12
8
0
1
4
3
...
lengths
1
1
2
1
1
1
1
1
1
2
...
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 from their neighbors, the RLE file would be up to twice the size of the original.
Some compression algorithms, such as RLE and LZW, arelossless, where the original pixel values can be perfectly regenerated from the compressed data. Other algorithms, such as JPEG, arelossy, because the parameterized patterns are only an approximation of the original pixel values, so the latter can only be estimated from the compressed data.
Vector images (line work) can berasterized (converted into pixels), and raster imagesvectorized (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 ofoptical character recognition.
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.[6] 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 agraphics 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.[7]
An early scanned display with raster computer graphics was invented in the late 1960s by A. Michael Noll atBell Labs,[8] 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.[9]
During the 1970s and 1980s,pen plotters, usingVector 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 bothlaser andinkjet printers. When the source information is vector, rendering specifications and software such asPostScript are used to create the raster image.
Geographic phenomena are commonly represented in a raster format inGIS. The raster grid isgeoreferenced, so that each pixel (commonly called acell in GIS because the "picture" part of "pixel" is not relevant) represents a square region of geographic space.[11] The value of each cell then represents some measurable (qualitative or quantitative) property of that region, typically conceptualized as afield. Examples of fields commonly represented in rasters include: temperature, population density, soil moisture, land cover, surface elevation, etc. Two sampling models are used to derive cell values from the field: in alattice, the value is measured at the center point of each cell; in agrid, the value is a summary (usually a mean or mode) of the value over the entire cell.
Raster graphics are resolution dependent, meaning they cannot scale up to an arbitrary resolution withoutloss of apparent quality. This property contrasts with the capabilities ofvector graphics, which easily scale up to the quality of the devicerendering them. Raster graphics deal more practically than vector graphics with photographs and photo-realistic images, while vector graphics often serve better fortypesetting or forgraphic design. Modern computer-monitors typically display about 72 to 130pixels per inch (PPI), and some modern consumer printers can resolve 2400dots 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 throughdithering (halftone) rather than throughoverprinting (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 increasecolor depth, 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.[12]
^Bach, Michael; Meigen, Thomas; Strasburger, Hans (1997). "Raster-scan cathode-ray tubes for vision research – limits of resolution in space, time and intensity, and some solutions".Spatial Vision.10 (4):403–14.doi:10.1163/156856897X00311.PMID9176948.
^ab"Types of Bitmaps".Microsoft Docs. Microsoft. 29 March 2017.Archived from the original on 2 January 2019. Retrieved1 January 2019.The number of bits devoted to an individual pixel determines the number of colors that can be assigned to that pixel. For example, if each pixel is represented by 4 bits, then a given pixel can be assigned one of 16 different colors (2^4 = 16).
^ab"Raster vs Vector". Gomez Graphics Vector Conversions.Archived from the original on 5 January 2019. Retrieved1 January 2019.Raster images are created with pixel-based programs or captured with a camera or scanner. They are more common in general such as jpg, gif, png, and are widely used on the web.
^"bitmap display". FOLDOC. 2002-05-15.Archived from the original on 16 June 2018. Retrieved30 November 2014.
^Murray, Stephen. "Graphic Devices".Computer Sciences, edited by Roger R. Flynn, vol. 2: Software and Hardware, Macmillan Reference USA, 2002, pp. 81–83.Gale eBooks. Accessed 3 Aug. 2020.
^"Print Basics: RGB Versus CMYK".HP Tech Takes. HP. 12 June 2018.Archived from the original on 2 January 2019. Retrieved1 January 2019.If people are going to see it on a computer monitor, choose RGB. If you're printing it, use CMYK. (Tip: In Adobe® Photoshop®, you can choose between RGB and CMYK color channels by going to the Image menu and selecting Mode.)
