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Definition: raster scan

Displaying or capturing a video image line by line. Computer monitors and TVs use this method whereby electrons are beamed (scanned) onto the phosphor coating on the screen a line at a time from left to right starting at the top-left corner. At the end of the line, the beam is turned off and moved back to the left and down one line, which is known as the "horizontal retrace." When the bottom-right corner is reached, the gun is returned to the top-left corner, known as the "vertical retrace." For TV signals, these "flyback" periods in which the electron beam is moved to a different line are also called the "horizontal" and "vertical blanking intervals."

Video Is the Reverse
Capturing video images uses the same raster scan sequence as the display, but in reverse. Instead of sending electrons to a material that creates light, light is directed to a material that holds a charge, and the charge is turned into an electronic signal. The first video cameras used a vacuum tube with a light-sensitive plate at one end. Subsequently, CCD and CMOS chips replaced the tube. See CRT, CCD sensor, CMOS sensor and rasterize.

Raster Scan Tracing
Starting at the top-left of the screen and going to the bottom-right, the electron beam is turned on a line at a time (1), then turned off to go back to the next line (2), then off once again to go back up to the top (3).