How does green screening work with green objects?

Question

A question that has been on my mind for so long is: How exactly does green screening work with an object to vanish? Short example: Normal green screens as a background are replaced with an image the viewer sees, like a news show where they show the weather. The weather-man stands infront of the greenscreen and presents the weather - while the greenscreen is replaced with actual information, kinda simple. But how does it work when a real scene is filmed and only an object should be replaced with the actual background staying intact? I saw a still-image from a Harry Potter film where Hermione got a book in her hand which is being picked up by a green hand, so the hand could be removed later so the book looks like it is floating. How does the computer 'know' the background behind the hand? Its kinda hard to explain and I cant draw or show you what I mean since i'm posting via phone. Imagine the camera shot as a 2D Image - Hermione with the book in her hand and the green hand receiving and grabbing the book. As you film this, the hand is opaquing the background it is infront from. Since the hand is removed later, the background will be shown since there is no more hand to see (removed because of the effect for the movie) How is it possible to see the background? Is the scene once filmed without the hand and then again with the Hand to forge two images together in the end? I hope I explained it right so you can help me with that, it's really interesting and bugging me at the same time.

Answer 1

All the above are correct, Blue was used and sometimes often because back in the old DV days the algorithm for DV was slightly better. Problem is most everyone wears blue including me. Well I do have baby blue eyes. Ok Green Sure. Go to hobby lobby get you a nice green sheet and you are ready to go. Nottttttt..

I teach advanced compositing and greenscreen Visual effects.

First You have to light it right or you will facing a disaster. The best way to know your lighting is good is using a waveform monitor to give you the best coverage and evenly.. Most do not know this but to do it right you have to light the background separate. Forground is a controlled lighting depending on the scene lighting which to make it believable must be matched.

The best software to use. After Effects. Not much better the KeyLight. Ands they keep making it even better.

Sure premier has Ulta and Never liked it much . There are a lot of tricks to making a believable key so it is not easy..I can see a bad key in an instant and most out there are well very very bad.. It is an art to make it believable but a lot of fun. Then there is a combination of Motion tracking and greenscreen which is what you see often in the movies. It take a few years to learn it right however the future of most movies is going that direction.. Nearly all the major motion pictures have great percentage of Chroma work every narrative films.

Good luck.

Answer 2

Screens were originally blue but evolved into the Green-Screens we see today. The color green that is (usually) used has a very short wavelength (yes, physics comes into play) meaning that there is less chance of clothes and props fading into and out of the edited piece.

So in effect what we are telling our editing software (usually FinalCut or Adobe Premier) is that, "everything that has this specific wavelength does not exist." And so the camera just doesn't "look" at anything with that wavelength. Then you "layer" videos over video until you achieve the desired effect.

Green-screen is a lot of fun but even today with the huge advances in digital video production, filming in front of a real background is preferable. But I do love me some green-screen!

Answer 3

Not well! would be the short answer. But green screens need not be "green"! Depending on your editing software, other colors can be used as BG for the same purpose. Find out which basic colors are configurable on your editor, and find a BG in one of those colors that won't mask an active color in your shoot.