Raster scanning is a method of constructing images using horizontal lines or pixels. It’s used in traditional display devices and some printers. The process involves drawing horizontal lines from the top left corner to the right edge of the screen, then dropping down and repeating until the entire image is rendered. This creates imperceptible distortion due to horizontal retrace. Digital images are also saved using raster scanning techniques with pixels forming the image in both file encoding and computer memory.
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 where each dot represents a tiny rectangular area of the image. One major application of raster scanning technology has been in traditional display devices such as televisions or computer monitors. Some computer printers also use similar methods to construct images on paper. Most digital image files are also archived and reconstructed using raster scanning techniques.
In a television or computer monitor, an image is constructed using raster scanning technology by starting at the top left corner of the screen and drawing a horizontal line ending at the right edge of the screen. The line goes back to the left side, dropping a small amount down and draw the next line of the picture. When the ray that is drawing the image reaches the lower right corner of the screen, effectively indicating that the entire image has been rendered, it returns to the upper left corner to start over, an action known as vertical retrace. This process occurs dozens of times per second to create a smooth moving image.
While a raster scan can produce a very realistic image, the actual process creates a tiny amount of almost imperceptible distortion in the image. At the end of each horizontal line drawn, the beam must return to the left side of the screen, called blanking or horizontal retrace. This is most efficiently accomplished by actually drawing every visible line with a slight downward slope towards the bottom right of the screen. Thus, during the horizontal retracement, the beam retracts in an almost straight horizontal line. While it’s the fastest way to draw the image, it actually means that a monitor using raster scanning technology is drawing the image at an angle at a very tiny angle.
Computer software that saves images digitally uses a similar technique to encode the information and then later decompress it. A raster scan of the image begins in the upper left corner of the image and proceeds similarly down to the 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 pixels in horizontal lines forms the image not only in the file encoding, but also in the computer’s memory when the image is displayed.
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