Green Screen, also known as Blue Screen or Chroma Keying, is a technique in video whereby you film your subject in front of a green or blue background and remove the blue/green colour to reveal another image behind. Most people know it best from weather forecasts – the forecaster isn’t actually standing in front of a weather map at all. It is used heavily in films and television programs too – see this link for an amazing video showing the potential of the technique.
How we’ve used it:
Students found it very motivating to use the green screen, as they could transport themselves anywhere they needed, for example play football at Bramall Lane, or play the guitar on stage at Wembley. We have found it particularly good for the students from the special schools we work with, as they can act out certain behaviours in a safe setting – for example crossing a road for a Green Cross Code video.
At the CLC we use a green screen (we decided on green as less people wear green than blue – anything the same colour as the background will also become transparent!) made up of a large green curtain on a curtain rail. You get best results with a very bright, vibrant green or blue colour, for example:
We also bought a Lastolite portable green screen with frame, which packs up small enough to take into schools. However, you don’t have to use a piece of cloth, it could be a wall painted blue or green with enough room for filming in front.
The Lighting and Other Considerations
You get the best results from an evenly lit background, as any difference in shade of colour will cause problems in the editing stage. If possible light the background and the subject separately, as this helps with crisp edges (and reduces the green halo effect). Try and avoid the subject casting shadows on the screen. Finally it is difficult to get a good result when there is a lot of movement; static scenes (e.g. news reports) work best.
We use Adobe Premiere Pro to edit our green screen videos, but this is an expensive option for schools. There is a cheaper version, Adobe Premiere Elements, which costs around £60 for a single licence, and this still gives a good result:
- Drag your background image onto Video 1 track on timeline.
- Drag your green screen clip onto Video 2 track (i.e. above the background).
- Click on the Edit tab on the right and then Effects. Scroll down to find the Keying section and drag the Chroma Key effect to the green screen clip. (There is a specific Green Screen effect too, but the Chroma Key one is easier to tweak to get best result).
- Click on Edit Effect and open the Chroma Key effect (click on triangle).
- Click on the eyedropper icon then click on the preview screen to pick the most representative colour of the background (i.e. not too dark or light a shade).
- Use the Similarity and Blend sliders to get rid of any remaining background colour. I generally aim for Similarity between 15-30%, and Blend <20%.
- If there is a problem area in a corner (which never appears behind the subject), use the Eight-Point Garbage Matte effect. You can drag any of the 8 points to mask out areas you don’t want.
See below for a screen shot of Adobe Premiere Elements:
- If you have access to a Mac, then iMovie has a Green Screen function:
- Click on iMovie > Preferences in the top menu bar, and tick the box next to Show Advanced Tools.
- Drag your background image or video onto the project area first.
- Now drag your green screen clip on top of the background image. In the menu that appears, choose Green Screen.
- The software does all the tricky stuff (like choosing the colour to remove) and gives a surprisingly good result.
It is possible to chroma key video in Windows Movie Maker – see how to do it here. However the end result isn’t great unless you have a very well lit background, without too much variation in colour.