The Technology Behind the Magic Yellow Line

CurtMonash writes "Fandome offers a fascinating video explaining how the first-down line on football broadcasts actually works. Evidently, theres a lot of processing both to calculate the exact location being photographed on the field — including optical sensors and two steps of encoding — and to draw a line in exactly the right place onscreen. For those who don't want to watch the whole video, highlights are here."

Re:Weathermen have been doing this for ???

[iluvcapra]

Yeah - it is one guy and as long as he doesn't put on an annoyingly green tie - it all just works.

There are several substantive differences:

• The green or blue weather map is a straight chromakey, the matted background is opaque and the removed background is monochromatic. The first-down line/overlays have to be added to a surface of varying (but reasonably predictable) colors, and it's laid over the action, with objects "in front" (not grass) matted out of the overlay. This is very complicated.
• The camera is in motion, panning and tilting while the overlay is happening. The weatherman always does his schtick in front of a camera on "lockdown," because if the camera moved, the weatherman would move (w/r/t the frame) on a different plane from his chromakeyed background.

I'd read the article if it weren't slashdotted, it appears very interesting...

Youtube Mirror for the video

[iammani]

http://www.youtube.com/watch?v=Vh9af_gXxlM&fmt=18 [youtube.com]

Re:Amazing...

[cobraR478]

The yellow line is for television broadcasts. It has no impact on the actual game. It does not exist in the NFL rulebook. However, your idea wouldn't work anyways even if implemented as a system of determining the position of the ball. You would have to have a mechanism to determine where the ball is when a player is tackled or goes out of bounds. Basically, you would have to determine the position of the ball when any part of the person carrying the ball touches the ground, except their hands or feet. You would also have to determine the position of the ball when any part of the body of the person carrying the ball touches the white out of bounds lines. So if I have the ball and get tackled but then stretch my arm out with the ball to get more distance, the ball does not move forward. Good luck developing a system to handle that. Maybe its possible, but i doubt it would be cheap enough to be worth it in the near future.

Re:Amazing...

[ITFromHome]

"completely arbitrary position"
The referees of the NFL (which I am not) would tend to disagree. I'm not trying to start a sports discussion off-topic but remember that the yellow line is only for home viewers. The measurments of the first down are very exact once the referee makes the initial spot of the ball. The NFL will NEVER take away that power from on-field persons...but back to the yellow line. TV viewers have a perspective which selects only the players at the start of the play that eventually narrows in on only the ball carrier. Before the yellow line TV viewers could not see the sidelines during a play, which is where the "chain gang" remain. Everyone in the stadium can reference the sidelines. Now TV viewers can reference the yellow line.

Re:Watch the video

[Reality Master 101]

The way we used to say this in the Hospital game is, "different-good is as bad as different-bad."

Re:Weathermen have been doing this for ???

[Thelasko]

The green or blue weather map is a straight chromakey, the matted background is opaque and the removed background is monochromatic. The first-down line/overlays have to be added to a surface of varying (but reasonably predictable) colors, and it's laid over the action, with objects "in front" (not grass) matted out of the overlay. This is very complicated.

What's more interesting is that it works in Green Bay, [wikipedia.org] where the field may randomly switch from green to white in a matter of minutes, and the player's on the field are wearing green.

It's not just the yellow line

[Animats]

Not being a sports fan, I don't see much of this stuff, but I once visited the company in Silicon Valley that makes the gear. [sportvision.com] The "yellow line" is one of the easier applications. It's basically a camera with encoders driving a fairly simple video processor. Calibration is manual; there's a setup display that shows the normal lines of a football field, and someone aligns the corners to match the real image from the camera. When the generated image matches the real one, the system is in alignment.

That's 1998 technology. The newer systems have gone way beyond that. Ads on billboards are sometimes replaced using the same system. Ads you see on the air may not be what people in the stadium are seeing. There's player tracking, ball tracking, the "virtual strike zone" for baseball, GPS-based tracking for NASCAR, and virtual billboard insertion into everything.