In computer graphics, a raster graphics or bitmap image is a dot matrixdata structure that represents a generally rectangular grid of pixels, viewable via a monitor, paper, or other display medium. Raster images are stored in image files with varying formats. A bitmap is a rectangular grid of pixels, with each pixel's color being specified by a number of bits. A bitmap might be created for storage in the display's video memory or as a device-independent bitmap file. A raster is technically characterized by the width and height of the image in pixels and by the number of bits per pixel. The printing and prepress industries know raster graphics as contones. The opposite to contones is "line work", usually implemented as vector graphics in digital systems. Vector images can be rasterized, and raster images vectorized, 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.
Etymology
The word "raster" has its origins in the Latin ', which is derived from '. It originates from the raster scan of cathode ray tubevideo 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.
Applications
Computer displays
Most modern computers have bitmapped displays, where each on-screen pixel directly corresponds to a small number of bits in memory. 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. An early scanned display with raster computer graphics was invented in the late 1960s by A. Michael Noll at Bell Labs, 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.
data is commonly stored in a raster format to encode geographic data as the pixel values. Georeferencing information can also be associated with pixels.
Resolution
Raster graphics are resolution dependent, meaning they cannot scale up to an arbitrary resolution without loss of apparent quality. This property contrasts with the capabilities of vector graphics, which easily scale up to the quality of the device 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, and some modern consumer printers can resolve 2400 dots per inch 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 printing. However, for printing technologies that perform color mixing through dithering rather than through overprinting, 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.
