In raster scanning, the beam sweeps horizontally left-to-right at a steady rate, then blanks and rapidly moves back to the left, where it turns back on and sweeps out the next line. The beam position (sweeps) follow roughly a sawtooth wave. Thus, while the number of scan lines (vertical resolution) is unambiguously defined, the horizontal resolution is more approximate, according to how quickly the signal can change over the course of the scan line. Analog television has discrete scan lines (discrete vertical resolution), but does not have discrete pixels (horizontal resolution) – it instead varies the signal continuously over the scan line. This ordering of pixels by rows is known as raster order, or raster scan order. Each scan line can be transmitted in the form of an analog signal as it is read from the video source, as in television systems, or can be further divided into discrete pixels for processing in a computer system. In a raster scan, an image is subdivided into a sequence of (usually horizontal) strips known as " scan lines". These values are retrieved from the refresh buffer and painted onto the screen one row at a time. This memory area holds the values for each pixel on the screen. The data to be drawn is stored in an area of memory called the refresh buffer or frame buffer. Although often a great deal faster, it is similar in the most general sense to how one's gaze travels when one reads lines of text.
It is a systematic process of covering the area progressively, one line at a time. The pattern left by the lines of a rake, when drawn straight, resembles the parallel lines of a raster: this line-by-line scanning is what creates a raster. The word raster comes from the Latin word rastrum (a rake), which is derived from radere (to scrape) see also rastrum, an instrument for drawing musical staff lines. By analogy, the term is used for raster graphics, the pattern of image storage and transmission used in most computer bitmap image systems. Raster-scan display sample visible gaps between the horizontal scan lines divide each characterĪ raster scan, or raster scanning, is the rectangular pattern of image capture and reconstruction in television.
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