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If you’ve watch American football on television, you may have wondered how they make that yellow first down line look like it’s actually down on the field.

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It’s football season -- American football, that is.

That means that the nation’s most popular sport will be streaming into millions of homes and onto millions of screens around the country. But people watching at home don’t just see carbon-copies of the live game: They get graphics -- everything from the score to the subtle yellow lines marking the next first down.

And whether you think graphics add to the game or distract from it, there’s a lot of cool, behind-the-scenes technology that goes into making them possible. The simplest graphics in a football broadcast are the opaque ones, like the score and information boxes that cover up whatever’s on screen behind them. Making those is pretty easy.

A computer just replaces pixels from the camera with pixels of the graphic, so that you see the graphic instead of the original camera footage. It’s pretty simple technology, and there’s a good chance the video editing software built into your computer can do something like this. But other graphics, like the yellow first-down lines you see stretched across the field, are more complex.

If you’re not familiar with American football, the first-down line is pretty straightforward. The team on offense gets four plays, called downs, to move forward at least ten yards from where they started. If they can do it, the counter gets reset to the first down, and they have four more tries to go another ten yards.

If they can’t, the other team gets the ball, and a lot of people probably scream at their television. If you watch football on TV, you’ll be used to a helpful yellow stripe showing the next first-down line, which they project ten yards ahead of where the offensive team started. It’s easy to be fooled into thinking that the first-down line is actually down there on the field, instead of added by computers, because it doesn’t look or act like the other graphics.

It runs perfectly parallel to the real lines that are painted on the field, and players can even cross over the first-down line without turning yellow. That line is so much more convincing because a lot more work goes into making it fit in. The process actually starts long before the game does.

Before the season, companies make full, 3D measurements of every field in the league, capturing every nook and cranny in a computer model. Because fields might look flat, but they’re all slightly higher in the center for easier water drainage. And just like any big, grassy area, every field also has its own unique little bumps and ditches, like the famous bump near the old 10-yard line of Giants Stadium.

All that information gets fed into a model of the field, and it’s that that determines exactly where the first-down line graphics go -- and the exact shape they take. Computers near the stadium have that model locked and loaded on gameday, ready to work out where the next line on the field should be. But knowing the location and shape of the line is only half the battle, because the computer also has to figure out how that line should look on your screen, depending on where the cameras are pointing.

Each TV camera has sensors that report where it is and what direction it’s facing, so the computers can work out how the line would look to each camera if the line were actually down there on the field. Then, the computers project the yellow line on top of the camera images. But instead of just replacing any pixels with yellow, like they would’ve for simpler graphics, they only replace certain colors.

Generally, colors like green and brown get replaced, since they’re the kinds of colors you’ll find on a grassy field. But other colors get left alone. That way, the grass and dirt get replaced with a yellow stripe, but the players themselves don’t -- even if they walk across the line.

To avoid drawing over things like green uniforms, the set of colors to be replaced has to be specific to each field and each game. Which takes a lot of work and careful programming. Some versions of the algorithm can also avoid covering up players by not drawing the first-down line over any areas that have significantly changed since the last line was drawn -- like where a player is now standing.

No matter which method they use, all that modeling and computation goes into each and every one of the thirty frames of football streaming from the stadium to your home... every second. So whether you’re a football fan or a digital modeling fan or a clever graphics fan, a football broadcast probably has something for you -- even if you didn’t realize it. Thanks for watching this episode of SciShow!

You’ve probably heard all about athletes with lucky traditions or tokens they use before the big game -- and it turns out they might actually help. If you’d like to learn more about the self-fulfilling prophecy of luck, you can check out our episode all about it over at SciShow Psych.