why does a green screen have to be green
If you attend a Super Bowl party on Sunday, youÁll probably hear at least one casual football viewer ask, ÁHow do they get that yellow first-down line on the field? Á While ÁmagicÁ is a fine answer in its own right, the real explanation is a bit more technologically intense. LetÁs have a look at the background and mechanics behind every football fanÁs shining beacon: the yellow first-down line. According to Allen St. JohnÁs 2009 book
The Billion Dollar Game: Behind the Scenes of the Greatest Day in American Sport - Super Bowl Sunday, the first-down line actually emerged from the ashes of one of sports broadcastingÁs bigger debacles: the FoxTrax system for hockey, which was designed by a company called Sportvision. Áwhich hockey fans no doubt remember as the much-maligned ÁtechnopuckÁ that debuted in 1996Áemployed a system of cameras and sensors around a hockey rink to place a little blue halo around the puck. FoxTrax wasn't a great fit for NHL broadcasts: Hockey purists hated the intrusion into their game, and casual fans didnÁt flock to hockey just because the puck was suddenly easier to follow. However, the system inspired producers to think of new ways to insert computerized images into live sports broadcasts. The idea of using a line to mark the first down in football was a natural extension, and Sportvision debuted its during ESPNÁs broadcast of a Bengals-Ravens tilt on September 27, 1998. A couple of months later, rival company Princeton Video Image unveiled its Yellow Down Line system during a Steelers-Lions broadcast on CBS. (Sportvision is still kicking, and ESPN acquired all of PVIÁs intellectual property in December 2010. ) BUT HOW DOES IT WORK? It takes lots of computers, sensors, and smart technicians to make this little yellow line happen.
Long before the game begins, technicians make a digital 3D model of the field, including all of the yard lines. While a football field may look flat to the naked eye, itÁs actually subtly curved with a crown in the middle to help rainwater flow away. Each field has its own unique contours, so before the season begins, broadcasters need to get a 3D model of each stadiumÁs field. These models of the field help sidestep the rest of the technological challenges inherent to putting a line on the field. On game day, each camera used in the broadcast contains sensors that record its location, tilt, pan, and zoom and transmit this data to the networkÁs graphics truck in the stadiumÁs parking lot. These readings allow the computers in the truck to process exactly where each camera is within the 3D model and the perspective of each camera. (According to, the computers recalculate the perspective 30 times per second as the camera moves. ) After they get their hands on all of this information, the folks in the graphics truck know where to put the first-down line, but thatÁs only part of the task. When you watch a football game on television, youÁll notice that the first-down line appears to actually be painted on the field; if a player or official crosses the line, he doesnÁt turn yellow.
Instead, it looks like the playerÁs cleat is positioned on top of an actual painted line. This effect is fairly straightforward, but itÁs difficult to achieve. To integrate the line onto the field of play, the technicians and their computers put together two separate color palettes before each game. One palette contains the colorsÁusually greens and brownsÁthat naturally occur on the fieldÁs turf. These colors will automatically be converted into yellow when the line is drawn on to the field. All of the other colors that could show up on the fieldÁthings like uniforms, shoes, footballs, and penalty flagsÁgo into a separate palette. Colors that appear on this second palette are never converted into yellow when the first-down line is drawn. Thus, if a playerÁs foot is situated ÁonÁ the line, everything around his cleat will turn yellow, but the cleat itself will remain black. According to How Stuff Works, this drawing/colorizing process refreshes 60 times per second. All this technologyÁand the people needed to run itÁwasnÁt cheap at first. It could cost broadcasters anywhere from $25,000 to $30,000 per game to put the yellow line on the field. Sportvision had to deploy a truck and a four-man crew with five racks of equipment. The cost has come down since then, and the process is now less labor-intensive. One technician using one or two computers can run the system, according to Sportvision, and some games can even be done without anyone actually at the venue. Now you can explain it to everyone at your Super Bowl party during one of the less-exciting.
Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at. This post originally appeared in 2011. It does not have to be green. The technique can be done using any color, even colors which are outside of the spectrum of human vision (as long as you develop a camera that can pick it up). Green is used often because it's easiest for most modern cameras to pick up, giving the editor the cleanest possible image. It's also a less common clothing color, meaning it's less likely to cause any embarrassing disappearing outfits. Blue was once common, but it's common in wardrobes and is a common eye color. Still, you can avoid this, and it's a good color to try because it shouldn't interfere with skin color. White is a bad color because it's common in wardrobes and eyes. Same for black, only add to that hair. Both of these play a strong role in lighting and shadowing. Avoid these. Red is a bad color because it's seen in your lips and mouth and your skin tone. To select a good color, pick one that's not going to be found on anything else in the scene. Almost neon colors work well because they're bright and strong. For materials, anything matte and flat is good. Shine, reflections, and heavy texture is bad. How does it work? With digital processing, it gives the editor a solid background that's easily removed by software. Speaking of which, most consumer video software with chromakey capability (Adobe Premiere for example) will allow you an option to select any color to be removed.
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