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Why Are Green Screens Green?

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There’s a scene in the Netflix sitcom Master of None where Aziz Ansari’s hapless commercial and B-movie actor Dev stands in front of a vast green backdrop in a film studio. “Honestly, most of this is going to be done in post [production],” the bored director of a hackneyed sci-fi film tells him. “It’s kind of meaningless. We really don’t even need you here. It’s going to be 99 percent CGI.”

The satire may have hit close to home for actors involved in Game of Thrones, the Marvel universe movies, or the countless other productions, both lavish and low-budget, that rely on a technique called chroma key for visual effects and fantastic scenery. Some actors spend hours in front of green backdrops, imagining the alien hoard, billowing fireball, or towering castle that special effects pros will later add digitally. “You kind of forget the plot a little,” said Idris Elba of working in front of a green screen on Thor, adding, “It’s a bit of, ‘Wait, what is this again? Oh, right, Frost Giant. Okay.'"

Out of all the colors on the spectrum, why green? Why does that particular hue work best for leaving a blank into which special effects can be painted?

The truth is, it doesn’t. Chroma-keying is the process of isolating a single color or brightness value in an electronic image and then making that value transparent in post-production, allowing another image or footage to be placed “beneath” the color that’s been blanked out. Although green is used so often that “green screen” and “chroma key” have become almost interchangeable, any color will work. Green is the go-to because it doesn’t match any natural skin tone or hair color, meaning no part of an actor will be edited out through chroma key.

When a green costume or prop is essential, a blue screen is often substituted. This caused a complication for the 2002 Spider-Man movie. Filmmakers had to use a blue screen for effects shots of the Green Goblin. But blue is an integral part of the web-slinger’s color scheme, so they switched to green screens for shots of the hero. Additional tricks and special effects were used to get the two characters in the same effects shot. Makers of the Justice League movie, which will inevitably feature Superman and Green Lantern side by side, take note.

While the technology might seem complex, Hollywood has been using chroma key since 1940’s The Thief of Baghdad, and local newscasts utilize it to put a meteorologist in front of a graphics outlining the forecast. This is one reason why weathermen (and weatherwomen) need to be mindful of what tie or dress they wear.

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Why Is the American Flag Displayed Backwards on Military Uniforms?
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In 1968, famed activist Abbie Hoffman decided to crash a meeting of the House Un-American Activities Committee in Washington by showing up in a shirt depicting the American flag. Hoffman was quickly surrounded by police, who ripped his shirt off and arrested him for desecration of the Red, White, and Blue.

Hoffman’s arrest is notable today because, while it might be unpatriotic to some, wearing the American flag, burning it, or otherwise disrespecting it is not a violation of any federal law. In 1989, the Supreme Court ruled that it would be unconstitutional to prosecute any such action. Still, Americans have very fervent and strict attitudes toward displaying the flag, a longstanding symbol of our country’s freedom. According to the U.S. Flag Code, which was first published in 1923, you shouldn’t let the flag touch the ground or hang it upside-down. While there’s no express prohibition about reversing the image, it’s probably a safe bet you shouldn’t do that, either.

Yet branches of the U.S. military are often spotted with a seeming mirror reflection of the flag on their right shoulder. If you look at a member in profile, the canton—the rectangle with the stars—is on the right. Isn’t that backwards? Shouldn’t it look like the flag on the left shoulder?

The American flag appears on a military uniform
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Not really. The flag is actually facing forward, and it’s not an optical illusion.

When a service member marches or walks forward, they assume the position of a flagpole, with the flag sewn on their uniform meant to resemble a flag flapping in the breeze. With the canton on the right, the flag would be fluttering behind them. If it were depicted with the canton on the left, the flag would be flying backward—as though it had been hung by the stripes instead of the stars nearest to the pole. The position of the flag is noted in Army Regulation 670-1, mandating the star field should face forward. The official term for this depiction is “reverse side flag.”

As for Hoffman: His conviction was overturned on appeal. In 1970, while at a flag-themed art show in New York, he was invited to get up and speak. He wore a flag shirt for the occasion.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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Big Questions
What Causes Sinkholes?
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Mark Ralston/AFP/Getty Images

This week, a sinkhole opened up on the White House lawn—likely the result of excess rainfall on the "legitimate swamp" surrounding the storied building, a geologist told The New York Times. While the event had some suggesting we call for Buffy's help, sinkholes are pretty common. In the past few days alone, cavernous maws in the earth have appeared in Maryland, North Carolina, Tennessee, and of course Florida, home to more sinkholes than any other state.

Sinkholes have gulped down suburban homes, cars, and entire fields in the past. How does the ground just open up like that?

Sinkholes are a simple matter of cause and effect. Urban sinkholes may be directly traced to underground water main breaks or collapsed sewer pipelines, into which city sidewalks crumple in the absence of any structural support. In more rural areas, such catastrophes might be attributed to abandoned mine shafts or salt caverns that can't take the weight anymore. These types of sinkholes are heavily influenced by human action, but most sinkholes are unpredictable, inevitable natural occurrences.

Florida is so prone to sinkholes because it has the misfortune of being built upon a foundation of limestone—solid rock, but the kind that is easily dissolved by acidic rain or groundwater. The karst process, in which the mildly acidic water wears away at fractures in the limestone, leaves empty space where there used to be stone, and even the residue is washed away. Any loose soil, grass, or—for example—luxury condominiums perched atop the hole in the ground aren't left with much support. Just as a house built on a weak foundation is more likely to collapse, the same is true of the ground itself. Gravity eventually takes its toll, aided by natural erosion, and so the hole begins to sink.

About 10 percent of the world's landscape is composed of karst regions. Despite being common, sinkholes' unforeseeable nature serves as proof that the ground beneath our feet may not be as solid as we think.

A version of this story originally ran in 2014.

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