<P> 1st & Ten is sometimes used to refer the class of systems capable of adding first down lines and similar visual elements, and not just the original Sportvision system . </P> <P> Over time, usage has evolved . Some television football broadcasts change the color of the line from yellow to red on 4th down, or show a second computer - generated line (usually blue in color) that marks the line of scrimmage . Other lines can also be projected to show other types of field position, including markings for the red zone and the optimum maximum distance for a placekicker's statistical field goal range . In extreme situations, such as snowstorm or blizzard conditions, an entire virtual field with yard and boundary markers can be projected onto the field in order to allow league officials, broadcasters and viewers some way to follow action when all field markings are obscured by snow, fog or mud . </P> <P> The system makes use of a combination of motion sensors mounted on the broadcast cameras to record what they are viewing, and / or the use of match moving computer graphics technology and an enhanced version of chroma key or "green screen" technology . </P> <P> (A competing system using different technology is called L - VIS for Live Video Insertion System .) </P>

How do they project images on football field