The lines numbered as 0,1,2,3,4,5..10 represent lines that the electrons beam is “painting” on the screen from left to right.
When the beam reaches the right side of the bottom line, it has to move back to the upper left corner of the screen.
Horizontal retrace is used to refer to the beam moving back to the left at the end of each line.
Vertical retrace refers to the movement from bottom to top.
VERTICAL RETRACE HORIZONTAL RETRACE
The screen is painted 60 times per second but only half of the lines are painted per frame.
For example : Very odd-numbered lines, alternating back for forth between even-numbered and odd-numbered lines on each pass.
The entire screen, in two passes, is painted 30 times every second.
The alternative to interlacing is called progressive scanning, which paints every line on the screen 60 times per second.
Most computer monitor use progressive scanning because it significantly reduce flicker.
In a Random scan system, also called vector, stoke writing, or calligraphic the electron beam directly draws the picture.
A B C D E F
Here the electron gun of a CRT illuminates points and / or straight lines in any order.
The display processor repeatedly reads a variable ‘display file’ defining a sequence of X,Y coordinates the electron gun the whole display area is updated many times a second from image data held in raster memory.
Advantages of random scan:
Very high resolution, limited only by monitor.
Easy animation, just draw at different position.
Requires little memory.
Requires “intelligent electron beam, i.e., processor controlled.
Limited screen density before have flicker, can’t draw a complex image.