digital imagingDigital imaging is the creation of digital images, typically froma physical scene. The term is often assumed to imply or includethe processing, compression, storage, printing, and display ofsuch images. The most usual method is by digital photographywith a digital camera but other methods are also employed.A digital photograph may be created directly from a physicalscene by a camera or similar device. It is used to pass somekind of information to the viewers. there are two types ofdigital images. one is raster images and the other is vectorimages.
vector imagesA vector image is a collection ofconnected lines and curves thatproduce objects. When creating avector image in a vector illustrationprogram, node or drawing points areinserted and lines and curves connectnotes together. This is the sameprinciple as "connect the dots". Eachnode, line and curve is deﬁned in thedrawing by the graphics software by amathematical description. Ever yaspect of a vector object is deﬁned bymath included node position, nodelocation, line length and on down theline.
advantages of vectorsData can be represented at its original resolutionand form without generalization. Graphic output isusually more aesthetically pleasing (traditionalcartographic representation); Since most data, e.g.hard copy maps, is in vector form no dataconversion is required. Accurate geographiclocation of data is maintained. Allows for efﬁcientencoding of topology, and as a result more efﬁcientoperations that require topological information, e.g.proximity, network analysis.
disadvantages of vectorsThe location of each vertex needs to be stored explicitly.For effective analysis, vector data must be convertedinto a topological structure. This is often processingintensive and usually requires extensive data cleaning.In order to view on a computer screen, they must beconverted in the monitor resolution. the computer mustrecalculate the number of pixels every time it refreshes.
raster imagesA raster image is a collection of dotscalled pixels. Each pixel is a tinycolored square. When an image isscanned, the image is converted to acollection of pixels called a rasterimage. Scanned graphics and webgraphics (JPEG and GIF ﬁles) are themost common forms of raster images.Web graphics, including JPEG andGIF ﬁles, are always low-resolutionraster images. For this reason, webgraphics are always a poor choice forimprinting and editing. Production-ready clip art is an essential tool forcreating high quality imprints.
advantages of rasters The geographic location of each cell is implied by itsposition in the cell matrix. Accordingly, other than anorigin point, e.g. bottom left corner, no geographiccoordinates are stored. Due to the nature of the datastorage technique data analysis is usually easy toprogram and quick to perform. The inherent nature of raster maps, e.g. one attributemaps, is ideally suited for mathematical modeling andquantitative analysis. Discrete data, e.g. forestrystands, is accommodated equally well as continuousdata, e.g. elevation data, and facilitates the integratingof the two data types. Grid-cell systems are verycompatible with raster-based output devices, e.g.electrostatic plotters, graphic terminals.
disadvantages of rastersThe cell size determines the resolution at which the data isrepresented.; It is especially difﬁcult to adequatelyrepresent linear features depending on the cell resolution.Accordingly, network linkages are difﬁcult to establish.Processing of associated attribute data may be cumbersomeif large amounts of data exists. Raster maps inherentlyreﬂect only one attribute or characteristic for an area. Sincemost input data is in vector form, data must undergo vector-to-raster conversion. Besides increased processingrequirements this may introduce data integrity concernsdue to generalization and choice of inappropriate cell size.Most output maps from grid-cell systems do not conform tohigh-quality cartographic needs.
GIF format The GIF name is a acronym for Graphics Interchange Format, and was ﬁrstintroduced by Compuserve in 1987. This is a popular format next to the JPEGformat and is still used on Web pages today. Digital cameras, however, don’t usethis format because it only supports 256 colours.There are currently two versions of GIF in use: GIF87a (the original version) andGIF89a. The GIF89a version supports multiple images in stream which allowsyou to create animated GIF images.Advantages – One advantage to a GIF is that it uses lossless compression to makean image smaller. This means that no image information is lost duringcompression. It also has the advantage of making one of the colours in the imagetransparent to allow the background to show through. As mentioned earlier, youcan create simple animations by combining multiple images into a single GIF89aﬁle.Disadvantages – The biggest disadvantage for the GIF ﬁle format is that it onlysupports 256 colours, which isn’t nearly enough for a digital photo. This meansthat GIF ﬁles are restricted to sharp-edged line art (such as logos) that have alimited number of colours.
JPEG formatThis is perhaps the most popular ﬁle format used for digital pictures, especially now thatall digital cameras store their pictures in this format. Similar to GIF, JPEG is also anacronym for Joint Photographic Experts Group.Unlike the GIF ﬁle format, the JPEG format can compress images that contain up to 16.7million colours. This is more than enough for a digital photo. Besides the number ofcolours, another difference between the GIF format and the JPEG format is the type ofcompression used. While a GIF uses a lossless compress, a JPEG ﬁle is compressed using alossy compression algorithm. This means that each time you save a JPEG ﬁle, it losesmore information.Advantages – The biggest advantage is the ability to save a digital photo in a very smallsize (in bytes). Another advantage is that it can save an image that includes up to 16.7million colours, which is more than enough for a digital picture. It is a very popularformat for those two reasons alone.Disadvantages – The biggest disadvantage is that the compression used in a JPEG imageis lossy. Each time you save a JPEG image, the more data you lose. Another disadvantageis that if you compress an image too much, you will get JPEG "artifacts". These look likedistorted sections of the images, usually looking like squares. You should minimize theamount of compression of the image, while at the same time managing the size of the ﬁle,to reduce these artifacts.
TIFF formatAs with many names in the computer world, the TIFF name is also an acronym.TIFF is short for Tagged Image File Format. This a very popular format forarchiving digital photos, mainly because it is widely supported by most imageediting programs across many different operating systems.Similar to the JPEG format, it supports 24-bit (16.7 million) colours. You caneven save up to 48-bits of colour information within the ﬁle. The trade off is theﬁle size. While JPEG ﬁles are compressed, TIFF ﬁles are generallyuncompressed, which results in very large ﬁle sizes.Advantages – Since the TIFF ﬁle format is supported by pretty much all imageeditors on many different operating systems, it is the ﬁrst choice for archivingdigital photos. Since TIFFs don’t use compression, at least by default, you cancontinuously save a TIFF ﬁle and never lose any information. When editingphotos, it is best to ﬁrst save the ﬁle in a TIFF format during editing, and thenJPEG once your are ﬁnished.Disadvantages – The biggest disadvantage for a TIFF ﬁle is the size. Comparedto JPEGs, TIFF ﬁles are enormous. Depending on the bit depth (24 or 48) youare talking 100MB+ for a single ﬁle. Although, with the size of hard drives andDVDs it will be worth it to save your photo archives in the TIFF format.
referenceshttp://bgis.sanbi.org/gis-primer/page_19.htmhttp://technicallyeasy.net/2007/09/overview-of-various-image-file-formats/http://wiki.arcadecontrols.com/wiki/Raster_versus_Vector_artworkhttp://www.signindustry.com/computers/a r t i c l e s / 2 0 0 4 - 1 1 - 3 0 -DASvector_v_raster.php3