Raster Scan display is a graphic monitor employing a CRT.
The electron beam moves across the screen, one row at a time from top to bottom.
To create a pattern of illuminated spots, the beam is turned on and off as it moves across the line.
Frame buffer or refresh buffer stores the picture definition.
This memory area holds the set of intensity values for all the screen points.
Stored intensity values are then retrieved from the refresh buffer and painted on the screen one row at a time.
Each row is called a scan line and each screen point is referred to as a pixel.
The capacity of a raster-scan system to store intensity information for each screen point makes it well suited for the realistic display of scenes containing subtle shading and color patterns such as TV screens.
Intensity range for a pixel position depends on the capability of the raster system.
In a simple black-and-white system, each screen point is either on or off, so only one bit per pixel is needed to control the intensity of screen position.
Additional bits are needed when color and intensity variations can be displayed.
On a black-and-white system with one bit per pixel, the frame buffer is commonly called a bitmap.
For systems with multiple bits per pixel, the frame buffer is often referred to as a pixmap.
Refreshing on a raster-scan displays is carried out at the rate of 60 to 80 frames per second(Hz).
At the end of each scan line, the electron beam returns to the left side of the screen to begin displaying the next scan line.
The return to the left of the screen, after refreshing each scan line, is called the horizontal retrace of the electron beam.
At the end of each frame, the electron beam returns to the top of the left corner of the screen to begin the next frame, this is called vertical retrace.