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Why does the average green screen, such as one on the local news station, or some video on youtube, leave an outline and is obviously fake, but in some movies these effects look totally real. I was just watching behind the scenes clips of The Amazing Spiderman, and the green (or blue) screen effects looked so realistic when I saw the movie that I wouldn't have ever thought it was filmed in a studio.

Is it the quality of the camera that produces this effect, lighting, editing software, or a combination?

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2 Answers 2

  • A movie production can do another take if it doesn't look good.

  • Typically, a quality TV series or movie spends $100,000s per finished minute in post production cleaning up green screen shots. The commentary in the updated Battlestar Galactica mentioned how one can tell there was money left over on an episode when there is more than a few seconds of the hangar bay background in the shot.

  • The weather is usually aired live with little chance to adapt to last minute changes in wardrobe, lighting, and camera angle. Ultimately, they don't deeply care if there is fringing, bleeding, or an unintended region affected by chromakey.

  • The most common mistake of green screen use is overlighting the green screen. Its reflection on the back of the subject causes artifacts at the fringe which are hard to fix even in post production. (I know this because I once did such cleanup on an art house production.) The green screen should be lit with pure green light only to improve saturation, not brightness. The effect seen in person is a very dark looking green. (I saw the set up in Untraceable, it it is flawless—a scene where Diane Lane comes out of an elevator into the FBI lobby, seen from over her shoulder.)

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@wallyk's answer is excellent, and I would add just one thing: the greenscreen work you see done by local news and live TV is done in real time, unlike a movie where computers can render for hours, days, weeks etc. There's only so much polishing that even a powerful computer can do at 30 frames per second, while also pumping the rendered video stream back out to be broadcast. When the source video is recorded footage rather than a live stream, and the computer or network of computers can take more than one-thirtieth of a second to render each frame, the quality of the output is considerably higher. Entirely different software is used for not-live greenscreen work, and it produces much better results (though not as quickly).

Building off this point, starting with already-shot footage means that compositors can do preliminary work on it even before the green gets keyed out—color-correcting the image so that the green pops more, for example.

For an example of the very best live greenscreen replacement that money can buy, look at NFL games and the computer-generated line of scrimmage/first-down line that sometimes goes underneath players; or some MLB games have ads behind the batters that are added digitally, keying out a greenscreen "ad" in a stadium.

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I have worked at Grass Valley, makers of video broadcast equipment such as is used by NFL video productions. Their high end equipment uses FPGAs instead of brute computer power to do the effects. –  wallyk Apr 19 at 5:06

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