The BBC's Virtual Studio technology was used to enable real time composition of the 3D rendered graphics with live camera feeds. Each studio camera has a dedicated render PC to render the virtual scene from that camera's perspective. To know what a studio camera's perspective is, each camera is fitted with a second 'Free-D' camera which points towards the ceiling. On the ceiling are reflective, circular bar codes. The 3-D camera data is fed to a computer system that identifies the targets on the ceiling and calculates that camera's position and orientation, 50 times a second. Series 4 adopted vinten tracking peds instead of FREE-D as an alternative approach.