CSO

CSO is the common name for a special effects procedure championed by producer Barry Letts during the Jon Pertwee era. It was a somewhat primitive forerunner to CGI, wherein two different live shots can be mixed together to imply that the two elements are actually in the same shot. As the name implies it involves separating the color of the background from one shot then overlaying that now-"background=less" shot onto another one. What distinguishes CSO from other, similar special effects techniques is that the two shots are mixed together live.

Outside the BBC the process is generally known as chroma key.

The procedure fell out of favor in dramatic presentations largely due to its inability to depict and appropriate perspective between the two shots. The limitations of scale inherent in the process can clearly be seen in serials such as Invasion of the Dinosaurs and The Mutants. For this reason, CSO is now used mainly only in shots where scale is unimportant. A good example of such a usage is in the broadcasting of weather forecasts. A map will be placed in the same shot as a weather forecaster via CSO, largely because viewers aren't expecting that a weather broadcaster's hand will have the same scale as a map.

It has only been used in very incidental ways in the 2005 series of Doctor Who, such as when the "ghost forecast" was given in Army of Ghosts.