In the context of display technology, such as Cathode Ray Tube (CRT) monitors and some older television systems, horizontal retracing and vertical retracing refer to the processes involved in repositioning the electron beam to start a new line or a new frame after completing the previous one.
Horizontal Retracing
Horizontal retracing is the process by which the electron beam moves back to the left side of the screen after completing a single horizontal scan line (from left to right). This retracing occurs during the horizontal blanking interval, a brief period when the beam is turned off to prevent it from displaying any unwanted lines as it moves back to the starting position of the next line.
Purpose: To prepare for drawing the next horizontal line.
Process: The beam finishes scanning one line, turns off, and moves back to the left edge.
Duration: Typically a very short period, occurring after each horizontal scan line.
Vertical Retracing
Vertical retracing is the process by which the electron beam moves from the bottom-right corner of the screen back to the top-left corner to start drawing a new frame. This retracing occurs during the vertical blanking interval, a longer period compared to horizontal retracing, during which the beam is turned off to avoid drawing lines as it moves back to the start position.
Purpose: To prepare for drawing the next frame.
Process: After completing all horizontal lines for a frame, the beam turns off and moves back to the top-left corner.
Duration: Longer than horizontal retracing, occurring after each complete frame.