BBC’S 'FIGHTBOX' USES SAFIRE CHROMA KEYERS IN NEW DEPTH KEYING METHODS (July 2004)

‘FightBox’, the BBC’s groundbreaking computer game programme, is using eight Crystal Vision Safire chroma keyers to help the broadcaster to pioneer a new depth keying technique that moves away from the traditional green-screen method. The resulting output is very impressive and more realistic than anything else of its kind.

Traditionally, shows featuring computer games have failed to inspire with flat, obviously computer-generated images. The revolutionary format of ‘FightBox’ sees virtual ‘combatants’ fight for supremacy from within a real ‘Arena’. These PC generated fighters are in fact a product of human contestants, who downloaded ‘FightKit’ software with which to build them from the Internet. Following a series of web-based challenges, the best performing contestants and their corresponding warriors were invited to take place in the televised competition to find the ‘FightBox Champion’, where they controlled the combatants from pods suspended from the ceiling.

The show was developed in conjunction with a real computer game and the on-screen television graphics were taken straight from the PC version. The challenge was to combine these graphics with those of the presenters and the studio audience into a single realistic image. This all had to be done in real time as the programme was filmed in front of a live audience to create a more exciting atmosphere.

There were several issues that meant that a simple keying operation would not be possible. First of all when switching from one camera to another, the computer-generated images would need to match the camera angle. This problem was overcome by using eight Safire chroma keyers – one on each camera. This allowed pre-rendered graphics to be keyed onto the video before it was sent to the presentation mixer and also allowed the vision mixer to have a preview. This was a realistic solution because Safire offers superb quality at a very reasonable cost.

Another issue was that the action would sometimes need to seem to happen behind objects in the studio in order to appear real, whether those objects were part of the set or humans. Although a 3D model of the set could have been produced and rendered to produce a mask signal from the viewpoint of the camera, this would only work with static objects. The BBC therefore needed to devise other ways to generate a key signal from people in the Foreground, without using the traditional chroma key background.

Depth keying using infra-red illumination was used to generate over the shoulder shots of the contestants controlling their ‘Warriors’. An infra-red camera was mounted next to a main camera which illuminated the player. The signal from the infra-red camera was then used directly as an alpha channel by precise alignment with the main camera.

In situations where only one camera could be used, such as filming a presenter with a zoom lens, a different way of depth keying was necessary: pulsed illumination was used to light up the person in the Foreground and provide a key signal.

With so many different sources of video and varied processing techniques, it became a challenge to get the timing correct on these signals. In order to relieve this problem the BBC also purchased eight ViViD124s which allow a variable video delay from 0 up to two seconds. The ViViD124 and Safire products can be used in the same frame with any of Crystal Vision’s video or audio modules, fitting up to 12 modules in 2U.

Based at Whittlesford near Cambridge in the UK, Crystal Vision provides digital keyers and a full range of digital and analogue interface equipment including converters, decoders, encoders, distribution amplifiers and audio embedders to the professional broadcasting industry worldwide.