The most common type of graphics monitor employing a CRT is the Raster-scan displays, based on television technology JPG images are raster based. Light occurs when an electron beam stimulates a phosphor. In Raster scan, the electron beam from electron gun is swept horizontally across the phosphor one row at time from top to bottom. The electron beam sweeps back and forth from left to right across the screen. The beam is on, while it moves from left to right. The beam is off, when it moves back from right to left. This phenomenon is called the horizontal retrace. As soon as the beam reaches the bottom of the screen, it is turned off and is rapidly retraced back to the top to start again. This is called the vertical retrace. Raster scan displays maintain the steady image on the screen by repeating scanning of the same image. This process is known as refreshing of screen.
A Raster-Scan System Displays an Object as a Set of Discrete Points Across each Scan Line
Typically, a graphics display consist of three components: frame buffer, video controller or display controller, and a TV screen or Picture definition is stored in a memory area called the refresh buffer or frame buffer. This memory area holds the set of intensity values for all the screen points. The stored intensity values are then retrieved from frame buffer and painted on the screenone row at a time. Each screen point is referred as Pixel orpel. Each pixel on the screen can be specified by it row and column number. Intensity range for pixel position depends on capability of the raster system. In black and white system, the point on screen is either on or off. Only one bit is needed to control the intensity of the screen. In case of color systems, 2 bits are requiredOne to represent ON (1), another one is OFF (0). Refreshing on raster scan is carried out at the rate of 60 to 80 frames per seconds. The video or display controller has direct access to memory locations in the frame buffer. It is responsible for retrieving data from the frame buffer and passing it to the display device. It reads bytes of data from frame buffer and converts 0’s and 1’s in one line into its corresponding video signals and this is called a scan line. If the intensity is one (1) then controller sends a signal to display a dot in the corresponding position on the screen. If the intensity is zero (0) then no dot is displayed. monitor.