Keying

Keying, or chroma keying, is a simple visual effect that allows us to make parts of what's in our camera frame disappear or become transparent. Chroma keying is often used in conjunction with a green screen so some other background or footage can be layered into the shot or scene.

The part of the video we remove is based on colour. Today the colour used is almost always green, but in the past blue screens have also been used. So why do we use these colours? And are there are colours that can be used? You'll find out soon enough but first here is a list of what you will need to complete the activities in this lesson.

What you need:

  • Video recording device
  • Tripod or static stabilisation support
  • Video editing software
  • A chroma-key backdrop (green screen)

Some things to remember when keying:

  • Composition. Your frame will need to stay consistent throughout the scene, making sure the green screen takes up the whole frame
  • Lighting. It's best if your lighting stays consistent throughout the scene. This will make it easier to key out later
  • Spatial awareness. Remember to stay on the green screen to get the best result
  • Have fun. This is a creative and fun activity, let your imagination run wild
  • Location location location. Be creative with your new location. Think of going somewhere that you wouldn’t normally be able to go to, like outer space or an erupting volcano.

Activities

Exploration: Which colours key?

This simple activity will help you to understand which colours key well and why some colours are better for keying than others.

WHAT YOU NEED:

  • A range of objects. Try to get as many different colours as you can.
  • A stack of different coloured papers in a range of colours. Try to get red, blue, green, yellow, purple, and orange.
  • A digital camera or cameras.
  • Video editing software.
  • A background image.

WHAT YOU'LL DO

  • Work in small groups, each group having a different coloured piece of paper.
  • Photograph as many objects (one at a time) on the coloured paper as you can.
  • Photograph your hands on the paper.
  • Import the photos to the video editor. 
  • Bring each of your photos to the timeline. Stretch them out to three seconds.
  • Bring the background image to the timeline. Be sure to add it underneath your photos. Stretch it out so it underscores all of your photos.
  • Apply the keying effect to your photo.
  • Specify the keying colour to match the colour of the paper. Do this in the keying effect options, look for an eyedropper icon.
  • See which colours key best with the different objects and their hands.
  • Discuss with the class why you think the different colours work well or not so well.

Exercise 1: Location change

 This exercise will see your actors transported to a whole new world (cue Aladdin song) or location just like magic. Try to have a reason why your actors will be in this new setting, as it will seem strange if suddenly, the movie location changes to somewhere unusual. Depending on the size of green screen you have, you may only be able to film mid shots and closeups rather than long shots

PREPARATION

Use Google Image Search to find a location that is wild and wonderful, somewhere that your actors wouldn’t be able to go, but that fits with the movie script. Using the tools options of Google Image Search, set the usage rights to "Labelled for Reuse". This means you can use the image without infringing the image maker's copyright. You should also set the size to larger than 6MBs, this means your image won't pixelate when it's enlarged.

Find a space large enough in your classroom or school to hang your keying fabric and set your camera up on a tripod in front of it. Make sure your whole frame is taken up by the green of the fabric. If there are any areas not covered they won't be keyed and it could look odd in your film.

FILMING

  1. Compose your shot
  2. Position your actor(s) to begin recording
  3. Begin recording
  4. Call 'action'
  5. Act
  6. Call 'cut'
  7. Stop recording.

EDITING

  1. Import your footage and location image into your video editor
  2. Bring your clip onto the timeline
  3. Bring your location image onto the timeline, being careful to place it underneath your clip
  4. Find your keying effect in your video editing program
  5. Apply your keying effect to your clip (in most video editors, this is as simple as dragging the effect to the desired clip)

Exercise 2: Floating head

This exercise will allow you to create some fun and special monster or horror effects. By using the magic of the green screen and keying, not just to take you somewhere amazing, but to affect their own bodies in the movie. Your actors could lose an arm or a leg or even lose their heads. You will need some extra chroma key material to wrap around your limbs. We've demonstrated how you can achieve this effect in the video below.

PREPARATION

Use Google Image Search to find a location that fits with the movie script, remembering the tools options you used in exercise one.

Find a space large enough in your classroom to hang your keying fabric and set your camera up on a tripod in front of it, making sure your whole frame is taken up by the green of the fabric. If there are any areas not covered, then they won't be keyed and it could look funny in your movie.

Ensure your actor is wearing the keying clothes before the scene starts so that they don't lose their heads during the scene, or try combining this with a jump cut so that they do.

FILMING

  1. Compose your frame
  2. Wrap a part of yourself in your chroma key material
  3. Position your actor to begin recording
  4. Begin recording
  5. Call action
  6. Act
  7. Call cut
  8. Stop Recording

EDITING

  1. Import your footage and location image into your video editor
  2. Bring your clip onto the timeline
  3. Bring your location image onto the timeline, being careful to place it underneath your clip
  4. Find your keying effect in your video editing program (sometimes a quick Google search can help with this)
  5. Apply your keying  effect to your clip (in most video editors, this is as simple as dragging the effect to the clip you want to apply it to)

Extension

The Headless Wonder

This extension requires a little extra coordination and two actors to play the same character. One actor will be the 'body' and another actor will be the 'head' being carried by the body. The actor playing the body will need to wear a chroma key hood that covers their whole head, you can get thin ones that are see-through from the inside. The actor playing the head will need to wrap their entire body in chroma key cloth, or wear a full body chroma key outfit. During filming, the 'head' will follow and be held by the 'body'.

Well done you've mastered chroma keying. Next up, layering and masking.

Layering and masking

More info