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In computer graphics and digital photography, a raster graphic, raster image, or simply raster is a digital image made up of a rectangular grid of tiny colored (usually square) so-called . Unlike vector graphics which use mathematical formulas to describe shapes and lines, raster images store the exact color of each pixel, making them ideal for photographs and images with complex colors and details. Raster images are characterized by their dimensions (width and height in pixels) and color depth (the number of bits per pixel). They can be displayed on computer display, printed on paper, or viewed on other media, and are stored in various image file formats.
The printing and prepress industries know raster graphics as contones (from "continuous tones"). In contrast, line art is usually implemented as vector graphics in digital systems.
Many raster manipulations map directly onto the mathematical formalisms of linear algebra, where mathematical objects of matrix structure are of central concern.
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, grayscale, palettized, and full-color, where color depth 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 bits (over 16 million distinct colors), with 8 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 CCD bitmapped sensor at the Vera C. Rubin Observatory captures 3.2 gigapixels in a single image (6.4 GB raw), over six color channels which exceed the spectral range of human color vision.
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.
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:
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.
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. 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.Murray, Stephen. " 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.
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.
However, for printing technologies that perform color mixing through (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.
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