PROJECTS
Free-d
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BBC R&D has developed a system which meets all of these requirements. The system uses a number of markers placed out-of-shot, for example on the ceiling (Fig. 1). The optimum marker diameter depends on the height of the studio, but is typically about 20 cm. The markers are viewed by a small auxiliary camera mounted on the side of each normal studio camera (Fig. 2). The markers are composed of a number of concentric white and black rings forming a type of barcode; each marker has a unique code number. To make the markers easily visible despite the presence of the studio lights, they are illuminated by a cluster of bright LEDs around the auxiliary camera, and the white rings are made out of a retro-reflective material.

The serial digital video signal from the auxiliary camera is analysed by a purpose-built hardware unit (Fig. 3) to calculate the precise position and orientation of the camera in real-time. The unit identifies the markers, measures their positions in the image to sub-pixel accuracy, and reads their barcodes (Fig. 4). Knowing the position of each marker in the studio, the position and orientation of the camera can be computed. The settings of the studio camera’s zoom and focus are measured using conventional mechanical sensors, and are passed to the hardware unit by multiplexing them with the signal from the auxiliary camera. The unit generates an RS422 signal which conveys the position, orientation, zoom and focus data to the virtual set system.

When the markers are installed in a studio, the position of each marker needs to be measured to an accuracy of about ±1mm. However, the system can perform these measurements itself. First, approximate marker positions (to an accuracy of about 10cm) are entered into the system. Then the camera is moved around the studio, and the system analyses the images it sees in order to refine the marker position measurements to the required accuracy. As long as the markers are mounted rigidly, this process need only be done once.

Markers should be mounted at a minimum of two different heights (differing by about 0.5-1m) for optimum performance. At least 4-5 markers must be visible at any time. For example, this might require markers to be mounted at intervals of about 1m in each direction on the ceiling of a studio 10m high, for a maximum working height of the camera of 3m. These figures allow for up to about half the markers being obscured by lights, microphone booms, and so on.

The camera tracking system was first demonstrated in public at IBC'97, and was subsequently licensed to Radamec Broadcast Systems who sell it under the name "free-d".

The system is now in daily use by many broadcasters worldwide. UK programmes using free-d include ITN News, BBC News, and BBC programmes such as BAMZOOKi, Fightbox, and the 2001 and 2005 Election Night shows.