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Comment by jabelk | Hacker News Reader

I have dabbled in a little bit of video production, and on the (rare) occasion that I catch a few minutes of televised football, that yellow line always blows me away. Mentioning how difficult it must be to whoever is actually watching the game results in weird stares as if it's the most natural thing in the world, but I'm glad to read that I'm not going crazy. And I had no idea how extensive the setup actually was.

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Yeah totally, having some experience with 3D motion tracking and green screens, I was thinking: wow, it's so smooth and so perfectly tracked, AND cuts between player/grass so well. I know how finicky it can be to key out elements, so it is quite amazing that whatever algorithm they have in play can do it so well (is there a jersey color that's close to grass color lol? Wonder what the edge cases are).

It's alluded to in the article that outdoor fields and green jerseys are the hardest. For outdoor fields the lighting is affected by the time of day, cloud coverage, players moving along the field, and plenty of other things I'm sure. It says they spent a lot of time creating a unique palette for the fields and that domed stadiums were preferred by the engineers on site. Indoor lighting probably reduced the shadows a lot.

I wonder if they use any tracking technology that looks at large groups of pixels over time. If you could track a player as a closed shape moving through space, you could mask out that player from the background even if pixels on the player's uniform were identical in color to the field's palette.

There are many teams that have very similar jersey colors to grass. Moreover, what is even more impressive, is there are some astroturf fields that aren't even green [1]! And the first down line works just fine on that as well.

[1]: http://upload.wikimedia.org/wikipedia/en/5/56/Bronco_Stadium...

I've had that same conversation with non-technical people. "Complicated? They're just drawing a yellow line on the screen!"

The first time I saw that line my first thought was along the lines of "holy shit, that technology is insane. How are they overlaying it like that?". I was seriously impressed with it.

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