Raster graphics

In computer graphics, a raster graphics or bitmap image is a dot matrix data structure that represents a generally rectangular grid of pixels (points of colour), viewable via a monitor, paper, or other display medium. Raster images are stored in image files with varying formats. In computer graphics, a raster graphics or bitmap image is a dot matrix data structure that represents a generally rectangular grid of pixels (points of colour), 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 (or color depth, which determines the number of colors it can represent). The printing and prepress industries know raster graphics as contones (from 'continuous tones'). The opposite to contones is 'line work', usually implemented as vector graphics in digital systems. Vector images can be rasterized (converted into pixels), and raster images vectorized (raster images converted into vector graphics), by software. In both cases some information is lost, although vectorizing can also restore some information back to machine readability, as happens in optical character recognition. The word 'raster' has its origins in the Latin rastrum (a rake), which is derived from radere (to scrape). It originates from the raster scan of cathode ray tube (CRT) video monitors, which paint the image line by line by magnetically 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. 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. Most computer images are stored in raster graphics formats or compressed variations, including GIF, JPEG, and PNG, which are popular on the World Wide Web. Three-dimensional voxel raster graphics are employed in video games and are also used in medical imaging such as MRI scanners. GIS data is commonly stored in a raster format to encode geographic data as the pixel values. Georeferencing information can also be associated with pixels. 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 (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.