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How to bring the curriculum alive through the use of chroma key technology

Introduction

Chroma keying, more commonly known as 'green screening', is a video effect that enables a coloured background to be removed from a subject in order to reveal an image or video stream - right (Fig. 1). Whilst frequently employed in the making of Hollywood blockbusters, green screening is actually well within the capabilities and budget of most schools.

The technique is highly effective and can be used for role-play and storytelling within different topics/subjects and is suitable for any phase of learning. The multimodal nature of green screening can be very motivating for all types of learners and is a great way to seamlessly integrate ICT across the curriculum.

Getting started

Whilst getting started is quite straightforward, there are a few essential items required for green screening:

-         A digital video camera

-         A tripod

-         A modern video editor (not Movie Maker or iMovie)

-         A large open space with good light

-         A coloured backdrop such as a wall or cloth

In practice any uniformly coloured background will work, although blues and greens tend to achieve best results. Whilst cloths are not essential, they do provide greater flexibility than fixed backgrounds and it helps to invest in more than one, so that a second can be used to place on the floor and drape over bodies or other objects.

Important tip: Remember to make sure that children's clothing and props avoid whatever colour has been chosen for the chroma key background.

There are a few tips and tricks for setting up a green screen cloth correctly (Fig. 2):

The cloth needs to be stretched as tightly as possible to avoid creases that can potentially cast shadows. Also try to eliminate any streaky light falling onto the cloth. It should be mounted high enough up the wall to cover the height of the tallest participant. The cloth, or a second cloth, must also be used to cover a substantial acting area on the floor.

Filming technique

For most projects, it is not really practical to record sound at the same time as filming. This is because the quality or recorded sound is generally poor without radio microphones or a number of fixed microphones. Moreover, there are usually constant interruptions in public areas which will necessitate frequent retakes. It is better to record an audio track as a narration at a later time.

It is generally important to keep the camera very still when filming and a tripod is almost essential for this purpose. Whilst it is possible to experiment with long shots as well as close-ups (Fig. 3), changing the zoom during a shot does not work well with this technique. The simplest shot from an editing point of view captures the whole body in the centre of the cloth so that none of the peripheral edges of the room can been seen. As a general rule it is better to lose legs in a shot than the head of an individual. Sometimes it is necessary to film a perspective that does break these rules, but many potential problems can be overcome in the editing process.

Collecting together media elements

It is important to source high quality media to replace the green background - this material can consist of still images, video footage or a combination. Background media can be full colour, colour tinted or even black and white for older-style films. During editing it is possible to add filters to the filmed footage to ensure it matches the background. It is also a good idea to collect ambient sounds and background music to enhance the recorded narration.

Background images, video footage, ambient sounds and music can all be sourced from DVDs/CDs as well as the Internet. It is a good idea to check copyright on digital media, particularly if the finished product is likely to have an audience beyond the school. Some repositories host material which has been checked for copyright or which has been specifically released under a Creative Commons licence.

Useful repositories include:

NEN Gallery - http://gallery.nen.gov.uk
Audio Network - http://www.audionetwork.nen.gov.uk

Another way to gather suitable digital media is via screen capture technology. Still images can be captured from whatever is showing on a computer screen by pressing the 'Prt Scr' key on a Windows PC or 'Command-Shift -3 or Command-Shift -4' in Mac OS X. Video capture can be achieved by using specialist software such as Fraps. Modern computer games and virtual worlds like Second Life can provide excellent source material for curriculum projects as the graphics tend to be high quality and photo-realistic.

Fraps - http://www.fraps.com

The editing process

Nearly all commercial video editors support chroma keying. Here are a few examples, although there are many others available for purchase:

Adobe Premiere Elements
Ulead Media Studio Pro
Pinnacle Studio
Sony Vegas
Final Cut Express

Tip: Be aware that Movie Maker and iMovie do not currently support chroma keying.

Whilst the specific details of how to chroma key will vary between video editors, each project is set up in a similar way. Video editors work along a timeline and support multiple video and audio tracks which can be layered on top of each other. In every project, the background image or video stream needs to be placed onto the lowest level track. The next video track should contain the green screen video footage. Images and video footage can even be placed on upper level tracks so that any filmed footage can appear between a foreground and a background (Figs. 4 and 5).

Video editor interface (Adobe Premier CS 5.5)

Once the media elements have been organised on the video tracks correctly, the next step is to apply a chroma key effect onto the green screen footage. These effects can work in different ways, but in the example shown the chroma key effect requires the user to select an area of the green screen colour and then alter the similarity in order to remove the green colour (Fig. 6).

Complex projects can sometimes have several video and image tracks all layered on top of each other and it is often necessary to use additional editing tools to manipulate the media into the correct size and position.

In the example (Fig. 7), a video clip (captured from a game using Fraps software) needs to be resized, rotated, cropped and moved into position (onto the pages of a book being read by the girl).

Here are some ideas for using green screen technology:

  • weather reports
  • stories set in fantastical locations
  • traditional tales
  • stories from other cultures
  • historical re-enactments

View some examples here:

Titanic - http://vimeo.com/38246663

Myst - http://vimeo.com/35743827

 

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