|MadSci Network: Engineering|
That yellow thing is generated by computer projection. Of all periodicals to cite, the December 2000 Issue of Playboy (p56) has one of the best descriptions of the method:
To create the line, three game cameras are equipped with encoders that record their precise focus, zoom, and angle. A spotter on the field marks the position of the first-down mark. Another computer contains a 3D model of the field and a palette of colors for the playing surface, the players and the referee. Yet another computer figures out which camera is on-air. Thirty times per second, this data is collected and sent to a central processor that changes the tiny squares that make up the line to yellow, but only if they match the palette of the playing surface. Last year the company that creates the marker for CBS introduced a "sponsored first-down line" (an advertising logo is superimposed on the field) for the telecast of the NFL Europe World Bowl Game.
You may have also seem similar technology used in the olympics during track and swimming events. The lanes were indicated by country flag, country abbreviation, and lane number.
Allow me to add a little bit more explanation of the mechanism.
The position of cameras used are not always fixed. The recording of focus, zoom, and angle in combination with the 3D model of field provides the information the computer requires to project the first-down line onto the field with the correct slant and perspective. It really is like a computer game in which the lines of a football field are slanted to generate a 3D perspective for a variety of imaginary camera positions.
The color matching is very important. Particular uniform colors present trouble, in particular the Green Bay Packers, Philadelphia Eagles, and Cleveland Browns when they are in their dark jerseys (green or dark brown). These colors can be fairly close to the colors of the field under normal circumstances. In addition, white jerseys during snowy conditions or just muddy jerseys on a muddy field can be problematic. The computer has to be very good at distinguishing green uniform from green field or white field lines from white pants of referee or players. Only rarely will you see the line wash out part of a shoe or something else. The resolution (number of pixels) that determines the line and its edges will determine the "choppiness" of the line. The more pixels the less choppy and the less invasive the line is to the viewer at home.
Tom "Bottom Line" Cull
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