Editing Raster graphics

{{Short description|Image display as a 2D grid of pixels}}
{{more citations needed|date=November 2016}}

[[file:Rgb-raster-image.svg|right|thumb|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]], a '''raster graphic''', '''raster image''', or simply '''raster''' is a two-dimensional [[image]] or picture represented as a rectangular [[Matrix (mathematics)|matrix]] or grid of [[pixel]]s, viewable via a [[computer display]], [[paper]], or other display medium. A raster image is technically characterized by the width and height of the image in pixels and by the number of [[bits per pixel]].<ref>{{Cite web |title=Introduction to Computer Graphics, Section 1.1 -- Painting and Drawing |url=https://math.hws.edu/graphicsbook/c1/s1.html |access-date=2024-08-25 |website=math.hws.edu}}</ref> Raster images are stored in [[image file]]s with varying [[electronic publishing|dissemination]], [[raster graphics editor|production]], [[3D rendering|generation]], and [[raw image format|acquisition formats]].

The [[printing]] and [[prepress]] industries know raster graphics as '''contones''' (from "continuous [[tints and shades|tones]]"). In contrast, ''[[line art]]'' is usually implemented as [[vector graphics]] in digital systems.<ref>{{cite web|url=http://www.google.nl/patents/US6469805|title=Patent US6469805 – Post raster-image processing controls for digital color image printing|publisher=Google.nl|access-date=30 November 2014|archive-date=5 December 2014|archive-url=https://web.archive.org/web/20141205111425/http://www.google.nl/patents/US6469805|url-status=live}}</ref>

[[File:Matrix transpose.gif|thumb|[[transpose|Transposing]] an image to covert raster organization (a relatively costly operation for packed formats with less than a byte per pixel); [[function composition|composing]] an additional raster line [[reflection (mathematics)|reflection]] (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 of [[linear algebra]], where mathematical objects of [[matrix (mathematics)|matrix]] structure are of central concern.

==Etymology==
The word "raster" has its origins in the Latin ''[[wiktionary:rastrum|rastrum]]'' (a rake), which is derived from ''[[wiktionary:radere|radere]]'' (to scrape). It originates from the [[raster scan]] of [[cathode-ray tube]] (CRT) [[video monitor]]s, which draw the image line by line by magnetically or electrostatically steering a focused [[electron beam]].<ref>{{cite journal |first1=Michael |last1=Bach |first2=Thomas |last2=Meigen |first3=Hans |last3=Strasburger |year=1997 |title=Raster-scan cathode-ray tubes for vision research – limits of resolution in space, time and intensity, and some solutions  |journal=Spatial Vision |volume=10 |issue=4 |pages=403–14 |pmid=9176948 |doi=10.1163/156856897X00311}}</ref> 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==
[[File:Raster graphic fish 20x23squares sdtv-example.png|thumb|A simple raster graphic]]

The fundamental strategy underlying the raster data model is the [[tessellation]] of a plane, into a two-dimensional array of squares, each called a ''cell'' or ''[[pixel]]'' (from "picture element"). In [[digital photography]], the plane is the [[visual field]] as projected onto the [[image sensor]]; in [[computer art]], the plane is a virtual canvas; in [[geographic information systems]], the plane is a [[Map projection|projection]] of the Earth's surface. The size of each square pixel, known as the ''resolution'' or ''support'', is constant across the grid.
Raster or ''gridded data'' may be the result of a [[gridding]] 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 specified ''pixel format'', the data type for each number. Common pixel formats are [[binary image|binary]], [[gray-scale]], [[palette (computing)|palettized]], and [[RGB color model|full-color]], where [[color depth]]<ref name="MSDN_bitmapTypes">{{cite web 
|url=https://docs.microsoft.com/en-us/dotnet/framework/winforms/advanced/types-of-bitmaps?view=netframework-4.7.2 
|title=Types of Bitmaps 
|date=29 March 2017 
|website=Microsoft Docs 
|publisher=Microsoft 
|access-date=1 January 2019 
|quote=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). 
|archive-date=2 January 2019 
|archive-url=https://web.archive.org/web/20190102143109/https://docs.microsoft.com/en-us/dotnet/framework/winforms/advanced/types-of-bitmaps?view=netframework-4.7.2 
|url-status=live 
}}</ref> determines the fidelity of the colors represented, and [[color space]] determines the range of color coverage (which is often less than the full range of human [[color vision]]). Most modern color raster formats represent color using 24&nbsp;bits (over 16&nbsp;million distinct colors), with 8&nbsp;bits (values 0–255) for each [[color channel]] (red, green, and blue). The digital sensors used for [[remote sensing]] and [[astronomy]] are often able to detect and store wavelengths beyond the [[visible spectrum]]; the large [[charge-coupled device|CCD]] bitmapped sensor at the [[Vera C.&nbsp;Rubin Observatory]] captures 3.2&nbsp;gigapixels in a single image (6.4&nbsp;GB raw<!-- from 1.28 PB/year / 200,000 images/year -->), over six [[channel (digital image)|color channels]] which exceed the [[electromagnetic spectrum|spectral]] range of human color vision.

==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.

==Resolution==
{{further|Image resolution}}
{{more citations needed section|date=November 2016|talk=}}
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]], 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 [[dither]]ing ([[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]], 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.<ref>{{Cite web
  | last = Fulton
  | first = Wayne
  | title = Color Printer Resolution
  | work = A few scanning tips
  | date = April 10, 2010
  | url = http://www.scantips.com/basics3b.html
  | access-date = August 21, 2011
  | archive-date = August 5, 2011
  | archive-url = https://web.archive.org/web/20110805001736/http://www.scantips.com/basics3b.html
  | url-status = live
  }}</ref>

==Raster-based image editors==
Raster-based image editors, such as [[PaintShop Pro]], [[Corel Painter]], [[Adobe Photoshop]], [[Paint.NET]], [[Microsoft Paint]], [[Krita]], 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.<ref name="RasterVsVector" /> Most<ref>{{Cite web |last=Tooker |first=Logan |date=2022-02-02 |title=Photoshop vs. CorelDRAW: Which Is Better for Graphic Editors? |url=https://www.makeuseof.com/photoshop-vs-coreldraw/ |access-date=2024-07-13 |website=MUO |language=en}}</ref> 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]].<ref>{{cite web |url=https://store.hp.com/app/tech-takes/print-basics-rgb-vs-cmyk |title=Print Basics: RGB Versus CMYK |date=12 June 2018 |website=HP Tech Takes |publisher=HP |access-date=1 January 2019 |quote=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.) |archive-date=2 January 2019 |archive-url=https://web.archive.org/web/20190102143055/https://store.hp.com/app/tech-takes/print-basics-rgb-vs-cmyk |url-status=live }}</ref>

==See also==<!-- New links in alphabetical order please -->
*[[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==
{{Reflist}}

{{Graphics file formats}}

{{DEFAULTSORT:Raster Graphics}}
[[Category:Raster graphics| ]]
[[Category:Computer graphics data structures]]
[[Category:Graphics file formats]]
[[Category:Digital geometry]]