Movie Making Manual/Effects/Chroma Key

The process of chroma keying a shot has two sections: shooting and post production. Chroma key compositing, or chroma keying, is a special effect for compositing two images together based on color hues. Chroma key is generally considered as "green screen" but can be any colour. Green and Blue are chosen because those colours are the farthest away from skin tone.

Basic Steps

1. First, you need to choose your background colour.
2. Set up your lights
3. Set up your camera set exposure
4. Take a few test shots with actor to confirm exposure
5. Shoot footage

Does it have to be Green?

No, you can use any colour screen you wish. Any compositing software worth its place in this world will be able to chroma-key any colour however, Blue and Green are often chosen because the human skin contains very little blue or green pigment. Green screens are often used for shoots outside as sunlight can register on a camera as "blue" causing problems when chroma keying. Also, blue screens can be harder to chroma-key due to the increase of digital noise primarily found in the blue channel of video files.

General tips

• Make sure the lens is clean and filters aren't usually a good idea. Diffusion can make a key very hard to pull.
• Avoid using the same colour in the actor's clothes as in the green screen.
• One way to check that your exposure is about correct is to place a bit of white card under the key light and then view the scene through a green filter. The white card should "disappear" against the green screen.
• The higher the bitrate, the more colour information, the easier the keying, all lossy compression formats will result in decreased edge detail.
• Aim to use the highest bit depth you can, 4:4:4 is ideal but usually unrealistic.

Lighting

• Light the background evenly and brightly, but not overexposed.
• Light you character adequately paying close attention to backlights to isolate character.

Spill

Spill is reflected light bouncing onto the actors, it can also wrap around the actors. To avoid spill:

• Stand as far back from the green screen as you can.
• Light you character adequately.

Standing away from your background can also avoid shadows.

Other

• Shoot with the lowest ISO you can to avoid grain which can make keying hard.
• Shoot with a higher shutter speed (>1/100th) to avoid motion blur.

Post-Production: Pulling a Key

1. Take your footage into your preferred VFX software.
2. Create a garbage matte to remove any objects.
3. Drag your Chroma key plugin onto your footage.
4. Select the pen tool and choose a colour near the edge of your character.
5. Adjust your settings to suit your footage.
6. Play through the footage to verify the key was successful.
7. Add a choker if the footage is looking sharp or jagged.

Tools

• Adobe (Keylight)
  • Premiere Pro
  • After Effects
• Sony Vegas
• Autodesk Combustion
• Final Cut
• Wax 2.0

External Links

Film Riot "How To GREEN SCREEN! - Special Effects Tutorial"

Losmarkoss "After Effects Green Screen Tutorial using Keylight"

Click Action Films "Cheap Green Screen Lighting"