Chroma Key Fundamentals

There are endless possibilities with blue screen/green screen chroma key effects. From big Hollywood studio lots to your local news station, blue screen and green screen backgrounds are commonly used to key out specific areas of a video clip.

note

The main reason why these somewhat-fluorescent blue and green colors are used for chroma keying is because these color values are least apparent in human skin. Therefore, you can completely key out these values without altering the appearance of most human beings onscreen.


Generally, your local weatherperson does not actually stand in front of a giant computer screen that displays maps and weather conditions. In reality, that person is standing in front of a wall that is painted with bright "chroma" blue or green paint. Then a graphic image is keyed into the shot where the blue or green portion of the original image once existed (see Figure 7.11).

Figure 7.11. News weather forecasters stand in front of a blue or green wall while the weather maps are digitally combined in the control room.

graphics/07fig11.gif

The process works like this:

  1. The weatherperson stands in front of a blue or green wall. Balanced lighting is always important for getting clean keys.

  2. The camera shoots the person so that all you see is the person over a blue or green background (completely filling up the remaining visible area). All other images should be kept out of frame.

  3. In the control room, the live camera feed is combined with the computer-generated graphics such as weather maps. A chroma key effect is applied to this combined image.

  4. The result is a single image of the weatherperson displayed over the computer-generated maps.

This technology is used for many Hollywood-style effects found in many feature films. A good example is when you want an actor to be in a scene where it is too dangerous or virtually impossible to shoot a scene live. Think of any movie you've seen where an actor appears in a "virtual" world where amazing 3D effects occur around him. If you created a scene consisting of a computer-generated room, and you wanted to show your actor interacting with this scene as if he were really there, you would need to shoot and edit this using blue screen or green screen technology:

  1. Carefully plan, storyboard, and coordinate the scene before you shoot any footage or create any graphics.

  2. Set up a room (preferably a studio) where you can paint the entire shooting area chroma blue or green. Be aware of lighting to reduce the number of shadows and various shades of your key colors. The greater the variances in shades of key colors, the more difficult it will be to accurately separate the actor from the background. Theoretically, all you need is enough clean blue screen around the actor and his movements. Any areas that need to be cropped or masked can be done as a separate step. This technique is commonly called creating a garbage matte.

  3. Shoot the actor doing his routine as if he were really in the virtual scene.

  4. Create the graphics and animations of the virtual scene.

  5. Digitize the video footage of the actor's scene into Premiere.

  6. Import the virtual animation scene into Premiere.

  7. Edit the animation footage onto Video Track 1.

  8. Edit the actor's footage onto Video Track 2.

  9. Apply the chroma key effect onto the actor's footage, keying out the blue/green background to display the animation scene behind him.

tip

You might want to consider trying the non-red key. This function allows you to perform key effects similar to the remove green- or blue-screen effects, but it allows you to blend the two colors.




Premiere 6. 5 Fundamentals
Premiere 6.5 Fundamentals
ISBN: B000H2MVO4
EAN: N/A
Year: 2006
Pages: 219

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