It's only a yellow line. How hard can it be?
January 6, 2009 9:50 PM Subscribe
posted by SpacemanStix (52 comments total)
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The computer generated first-down line in American football is something we take for granted these days. However, the logistics required to make this work is pretty complex. At the very least, have you considered this: if it's computer generated on a moving image, how do they draw it under the people running around on the field, and not over them? And it gets a bit more complicated than this. "Here are some of the problems that have to be solved in order for this system to work
* The system has to know the orientation of the field with respect to the camera so that it can paint the first down line with the correct perspective from that camera's point of view.
* The system has to know, in that same perspective framework, exactly where every yard line is.
* Given that the cameraperson can move the camera, the system has to be able to sense the camera's movement (tilt, pan, zoom, focus) and understand the perspective change resulting from the movement.
* Given that the camera can pan while viewing the field, the system has to be able to recalculate the perspective at a rate of 30 frames per second as the camera moves.
* A football field is not flat -- it crests very gently in the middle to help rainwater run off. So the line calculated by the system has to appropriately follow the curve of the field.
* A football game is shot by multiple cameras at different places in the stadium, so the system has to do all of this work for several cameras.
* The system has to be able to sense when players, referees or the ball cross over the first down line so it does not paint the line on top of them.
* The system has to be aware of superimposed graphics that the network might overlay on the scene."
Here's a detailed explanation regarding how it's done. And here's a cool video showing it in action