Re: electronic stripes

[John Denker <jsd@MONMOUTH.COM>]

At 10:13 AM 10/13/99 -0400, David Bone wrote:
> Does anyone know how the network puts the electronic first down stripe on
> the field during the sunday football broadcast?  Clearly it isn't painted
> on the grass, yet it seems very much like it is.

I've never seen it, but I've got a pretty good guess what you're talking
about and how it could be done.

1) It is easy to synthesize the image of a straight line.  Heck, they can
synthesize dinosaurs _ab initio_.  Straight lines are easy.

2) It is easy to superimpose one image on another.

3a) The first slightly tricky part is to make the stripe move "with the
grass" when the camera pans.  To do this, you need to know where the camera
is, and you need high-resolution shaft-encoders on the camera mounts.  Then
it's just trigonometry and three or four adjustable parameters.

3b) Because shaft encoders are imperfect, for zoomed-in shots you probably
need to get rid of high-frequency wiggles by using image-stabilization
technology -- the same stuff that is in your SteadyShot camcorder -- and
coupling its output to the line-generator.

4) The remaining slightly part is to make the stripe appear above the grass
but below the players.  This is presumably done with a chroma key, which is
the same trick they use for putting the actor into the scene with the
dinosaur, and for putting the weatherbroadcasterperson in front of the
weather map.  (You didn't think they really had a 8' by 8' display on the
wall of the weather studio, did you?)  Basically you look for grass-colored
pixels and allow them to be replaced by the stripe.  Other pixels are not
replaced.

______________________________________________________________
copyright (C) 1999 John S. Denker             jsd@monmouth.com