We’ve all seen the yellow/blue lines in football broadcasts that indicate the scrimmage and line to gain. Ever wonder how it works?
As you probably guessed, the line itself is superimposed via chromakey, so anything green- or dirt-colored gets replaced, as would any other solid color they choose. This is why player uniforms, shoes, field logos, etc. are unaffected. Simple enough. But how the lines are actually tracked in motion has left many scratching our heads.
A virtual map of the field is created by placing a laser in the center of the field and measuring the elevations from the middle crown to the edges. Then a handful of specific cameras are outfitted with units that sense and transmit the camera data to the truck, such as zoom distance, pan, and tilt degrees. At the truck, this geometry is then applied to the virtual map and superimposed over the camera’s image. Sort of the opposite of a motion track – now the camera’s movement is the variable instead of screen reference points.*
All the motion-tracking features of the broadcast are handled from this truck, including the Virtual Down and Distance (usually in the red zone), quarterback’s PassTrack, and Video-in-Perspective, where they superimpose network graphics/video over a fake jumbotron that appears to be part of the stadium, even as the camera moves.
Similar systems are now also used for every major sport. Check out the complete suite of services from Sportvision.
* Recording camera data is very common in the visual effects world, as it’s used both to replicate complex camera moves perfectly for multiple passes (to build composite elements), as well as to apply/sync them to animated 3D environments later.