EDM Studio Inc. - Ring of Fire

Shooting a subject against a blue/green screen is an often-used technique for generating a "matte", a black-and-white image silouette that isolates the foreground target. This process is known as chromakeying. The matte can be used, for example, as a "cookie-cutter" mask to composite the subject into a new background scene. This can be done in post-production, or in the case of the television weatherperson backed by an animated view of tomorrow's forecast, in real-time during the nightly news.

There are three secrets to generating a good matte. First, ensure the backing screen is evenly lit, ideally a single shade and brightness of blue or green. This greatly eases the computer's task of differentiating foreground from background. Second, minimize so-called "colour spill", the blue or green tint that edges of the subject pick up from the diffuse glow of the colour screen. Spill pollutes foreground pixels with the colour "key" that you use to identify the background. Third, shave your subject's head - matte quality is literally defined by the resolution of the edge - and wispy hair causes all sorts of problems.

Production chromakey systems can be quite expensive. They use large swathes of specialty blue/green fabrics and numerous (hot) lights to create an even coloured background. Spill is often minimized by placing the screen far behind the actors - this works, but at the cost of increasing the size of the screen needed to cover the camera's view frustum.

Our "need" for a chroma-key solution arose out of the Access Grid, a next-generation multi-site video-conferencing system. In the AG, it is relatively common to have a dozen participants from twelve different locations, each putting out multiple video feeds. Background substitution offers a number of attractive possibilities:

Reduce visual clutter by filtering out room backgrounds. This leaves us with raw information in the form of isolated "talking heads".
Use contextual backgrounds: replace "dull" room interiors with iconic representation of person's locale, easily recognizable "tourist" video, or just information-rich data such as time-zone, country, etc.
Perhaps most exciting, transmit the matte and allow end viewer to synthesize a group montage.

However, as you'd be hard-pressed to get all Access Grid sites to paint their rooms green, let alone purchase real chromakey material, we needed a simple solution. Enter the "Ring of Fire" (RoF).

Prototyped by Darran Edmundson and then professionally engineered by ANU Electronics Technician Dennis Gibson, this annulus of colour-matched blue LEDs encircles the camera lense bathing the entire scene in an imperceptible blue light. By itself, RoF has no effect on the camera image. However, if we place a screen of retroreflective material behind the subject (3M Scotchlite, US $35/meter), LED light reaching the screen is preferentially directed back at the camera. The result, at least from the camera's perspective, is a beautiful blue background - ready for real-time matting and compositing. The Ring of Fire was used in a peer-reviewed showcase presentation to the AG stream of the world's premier supercomputer conference, SC Global 2003. The presentation was a demonstration of simple contextual background substitution in Access Grid video.

It's worth noting that this idea isn't new or novel. Light rings are sold for machine vision applications and a commercial implementation of the above is sold under the trade name "Chromatte".