Thursday, March 4, 2010

Chroma Key

by Nathan Stout


Doing green-screen effects used to be a rare thing. I was very proud when I did my first one back in 2002. Now eyerone and heir grandmother can do it.


Sometimes you see green-screen that looks fantastic and many times you see crappy green-screen. I can say that my green-screen effects are just so-so.


I know what would make it look better but it's basically a matter of resources. I will take you though how to do some green-screen effects yourself using Adobe products.


You will need Adobe Premier (5 or eariler) or Adobe AfterEffects. Premier 5 or eariler used to allow you to do green-screen effects within the program but after 5 they removed it so you would have to do it third party (aka buy their AfterEffects product).


I will show you the AfterEffects way (which is more common and you can play with the settings more). First off you will need to capture your video infront of a bright green type material. The brighter the better. Basically this color should be different than any other color in the shot. This means you shouldn't have the person or object wear the same color as your background. This also means you really don't have to have a green material for your green-screen. The term is actually 'keying' or 'chroma key'. People use green because it works really well. For this we will just refer to it as green-screen. Ok, back to the items you want to get for your background. Make it Florescent for best effect:
  • Fabric
  • Painted background
  • Posterboard

The best green screen material I have used (within my budget) was florscent green posterboard. It is REALLY bright. The next item you will need is lighting. You can ofcourse use regualr indoor lighting but it is gonna look crappy. The brighter the light the better it looks. I noticed that florscent lighting works the best (on a budget).

The real trick to making the green-screen effect look good is the position of the subject from the camera and green screen. This is the magic formula; have the green screen as far away in the background as possible. Have your person in the middle (far from the green screen) and have the camera far from the person. Have the person something like ten feet from the screen. Here is a little diagram:

Screen ------------------> Person ------------> Camera

You need to make sure that BOTH the green-screen and the person are well lit (especially the green-screen. I suggest you have some lights on the green-screen and then some lights on the person.

Film your subject and make sure they stay in the 'green area'. Then import that footage to your PC and into Adobe AfterEffects.

1.) Select the footage and then go to 'effects' then 'keying' then 'color range'

In this area use the + eyedropper to select every area of green you can find. When you select it, that area will vanish. The point is for you to tell AfterEffects where all the green is that you want gone.

2.) Select the footage then go to 'effects' then 'color correction' then 'hue/saturation'

3.) In this area you click on the 'color channel' button and select 'greens'

4.) Then you move the 'green saturation' and 'green lightness' bars all the way to zero.

This take what green is still in the image and basically suck all the color out of it, eliminating all green from the image.

5.) Next you import your background footage (the footage that goes behind the subject) and drop it in behind (or under) the green screen footage and you should be good to go!

This is an example of my green screen work (following the instrucitons I just showed you). Now it could be SO MUCH better if I had the proper room for filming.

This is an example of my very first green screen work I did with Abobe Premier 5 back in the day.

Either way, good or bad green screen is fun to do some crazy stuff with.

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