How the NFL's magic yellow line works

The clever engineering behind the virtual first-down line.


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Since the late 1990s, the virtual yellow line has been quietly enhancing football broadcasts by giving viewers a live, intuitive guide to the state of play. The graphic is engineered to appear painted on the field, rather than simply plopped on top of the players, so it doesn't distract from the game at all.

The line debuted during a September 27, 1998, game between the Baltimore Ravens and the Cincinnati Bengals. It was developed by a company called Sportvision Inc. and operated by six people in a 48-foot semi-truck parked outside the stadium.

ESPN was the only network that immediately agreed to pay the steep price of $25,000 per game. Before long, other companies began offering the yellow line to the other networks, and now you won't see a football game without it.


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Closed Caption:

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.