A raster scan is a method of constructing an image through the use of horizontal lines. The lines can be analog representations of the image, or they can be a sequence of pixels in which each dot represents a tiny rectangular area of the image. One of the primary applications of raster scaning technology has been in traditional display devices such as televisions or computer monitors. Some computerized printers also use similar methods to construct images on paper. Most digital images files also are stored and reconstructed using raster scan techniques.
In a television or computer monitor, an image is constructed using raster scan technology by starting in the upper left-hand corner of the screen and drawing a horizontal line that ends on the right edge of the screen. The line returns to the left side, dropping a tiny amount downward, and draws the next line of the image. When the beam that is drawing the image reaches the bottom right corner of the screen, effectively indicating the whole image was rendered, it moves back to the upper left corner to start again, an action known as a vertical retrace. This process occurs dozens of times every second to create a smooth moving image.
Even though a raster scan is capable of producing a very realistic image, the actual process creates a minute amount of nearly unnoticeable distortion in the image. At the end of each horizontal line being drawn, the beam must return to the left-hand side of the screen, which is called blanking or horizontal retrace. This is accomplished most efficiently by actually drawing each visible line with a slight downward slope toward the bottom right of the screen. In this way, during the horizontal retrace, the beam moves back in an almost straight horizontal line. Although it is the fastest way to draw the image, it actually means a monitor using raster scan technology is drawing the image skewed at a very miniscule angle.
Computer software that saves images digitally uses a similar technique to encode information and subsequently to decompress it. A raster scan of the image starts in the upper left corner of the image and progresses in the same way toward the bottom right. Instead of saving an entire line of analog information, however, the image is converted into small rectangles called pixels that can be set to a single color. The collection of the pixels in horizontal lines forms the image not only in the file encoding, but also in computer memory when the image is displayed.