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How the NFL’s magic yellow line works


The virtual yellow line in NFL broadcasts
is great. It tells viewers how far the offense needs
to advance for a first down. It looks really simple and elegant but creating
that line was a massive engineering challenge. It started in the mid 90s when the Fox Sports
network tried to make hockey easier to watch. “Scientists at Fox Sports laboratories
are working on new technology.” “You won’t believe your eyes.” They embedded infrared transmitters inside
the puck and placed sensors around the rink, So that live tv viewers saw a blue glow around
the puck at all times and a red comet tail if it traveled over 70 miles per hour.
Hockey fans didn’t really embrace “glow puck” as it came to be known.
So the technology was retired when the broadcasting rights for hockey switched to ABC a few years
later. But the team of engineers they had assembled
for the project was just getting started. They left Fox Sports to create a new company
called SportVision. and in 1998, they debuted the “First and
Ten” line on ESPN. “Until now, this marker was
the only reference fans in the stadium and at home had for the first down.”
The key challenge in making the yellow line is that the scene is constantly changing,
which means the yellow line has to constantly change.
Not only are there 3 different cameras used for the wide shots of the field, each camera
pans, tilts and zooms to follow the action. So the first thing Sportvision does before
the game is create a 3D mathematical model of each football field using laser surveying
tools. And during the game they gather data from
the cameras about their pan, tilt, and zoom positions for every single frame.
So when the operator specifies that the first down is at the 43rd yard line, for example,
the computers combine the camera data with their own model of the field to draw the yellow
line in the proper perspective ..and to redraw it, for every frame being
broadcast to viewers. The final step is what makes the line kind of magical
— removing any part of the line obstructed by players, refs or the ball so that the line
looks like it’s underneath them, almost painted on the field.
The way the computers know which pixels to remove is by sampling the colors – think of
the field as a giant green screen. But anyone who has worked with green or blue
screens knows that you need a really uniform and evenly lit background for it to work well.
So Sportvision identifies in advance which shades of green and brown are in the field
given the lighting conditions — those are the colors to be covered by the yellow line.
And they identify which colors are in the players uniforms and should never be covered
by yellow. It works amazingly well. Here’s the Packers,
wearing green, in the rain. No problem. It only fails in the most extreme weather,
like this 2013 game in Philly. The line ends up all over players, but on the other hand
the system was helpfully used to insert the yardage numbers that had been covered up with
snow. The whole yellow line process delays the live
broadcast by less than a second. And not surprisingly, it was an immediate
success. Sportvision won an Emmy for it, and went on to make virtual visual aids for
NASCAR, baseball, sailing and the Olympics. And football broadcasts have since added more
graphics, like the line of scrimmage and perhaps unnecessary large arrows showing the same
information that’s in the scorebox. But if that’s annoying consider this:
This type of technology is being used insert ads into stadiums and onto fields for a lot
of sports broadcasts. But the NFL doesn’t allow it. In the grand
tradition of the yellow line, the graphics on the field are not there to sell you things,
but to help you follow the game.

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