Editing Raster graphics (section)

==Uses==
===Image storage===
{{see also |Bitmap}}
[[File:The use of a raster data structure to summarize a point pattern.gif|thumb|Using a raster to summarize a point pattern]]

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]].<ref name="MSDN_bitmapTypes" /><ref name="RasterVsVector">{{cite web |url=https://vector-conversions.com/vectorizing/raster_vs_vector.html |title=Raster vs Vector |publisher=Gomez Graphics Vector Conversions |access-date=1 January 2019 |quote=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. |archive-date=5 January 2019 |archive-url=https://web.archive.org/web/20190105151547/http://vector-conversions.com/vectorizing/raster_vs_vector.html |url-status=live }}</ref> A '''raster data''' structure is based on a (usually rectangular, square-based) [[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====
{{main | Image 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:
{{aligned table|cols=12|col1header=y|class=wikitable|leftright=on
| 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, 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===
{{main | Electronic television |Computer monitor}}

Early [[mechanical television]]s developed in the 1920s employed rasterization principles. [[Electronic television]] based on [[cathode-ray tube]] displays are [[raster scan]]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.<ref>{{cite web|url=http://foldoc.org/bitmap+display|title=bitmap display |publisher=FOLDOC |date=2002-05-15 |access-date=30 November 2014|archive-date=16 June 2018|archive-url=https://web.archive.org/web/20180616103833/http://foldoc.org/bitmap+display|url-status=live}}</ref> 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.<ref>Murray, Stephen. "[https://link-gale-com.libaccess.lib.mcmaster.ca/apps/doc/CX3401200218/GVRL?u=ocul_mcmaster&sid=GVRL&xid=acaf5d43 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.</ref>

An early scanned display with raster computer graphics was invented in the late 1960s by A. Michael Noll at [[Bell Labs]],<ref>{{cite journal |first=A. Michael |last=Noll |title=Scanned-Display Computer Graphics |journal=Communications of the ACM |volume=14 |issue=3 |pages=143–150 |date=March 1971 |doi=10.1145/362566.362567|s2cid=2210619 |doi-access=free }}</ref> 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.<ref>{{cite web|url=http://noll.uscannenberg.org/Patents.htm|title=Patents|publisher=Noll.uscannenberg.org|access-date=30 November 2014|archive-date=22 February 2014|archive-url=https://web.archive.org/web/20140222131415/http://noll.uscannenberg.org/Patents.htm|url-status=live}}</ref>

===Printing===
{{main | Laser printing | Inkjet 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===
{{Main|Voxel}}

Three-dimensional [[voxel]] raster graphics are employed in video games and are also used in medical imaging such as [[Magnetic resonance imaging|MRI scanners]].<ref>{{cite web|url=http://www.cis.rit.edu/htbooks/mri/chap-1/chap-1.htm|title=CHAPTER-1|publisher=Cis.rit.edu|access-date=30 November 2014|archive-date=16 December 2014|archive-url=https://web.archive.org/web/20141216104702/http://www.cis.rit.edu/htbooks/mri/chap-1/chap-1.htm|url-status=live}}</ref>

===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.<ref name="Bolstad">{{cite book |last1=Bolstad |first1=Paul |title=GIS Fundamentals: A First Text on Geographic Information Systems |date=2008 |publisher=Eider Press |page=42 |edition=3rd}}</ref> 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 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 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.