- This article is about the modern notion of the visual effect, largely as it pertains to the BBC Wales production of Doctor Who. If you are looking for people who were credited with the title "Visual Effects" during the 1963 version of Doctor Who, please see visual effects designers.
Visual effects — often abbreviated as VFX — are those elements of a shot which cannot be achieved exclusively by practical means during principal photography. They can be accomplished with the varying techniques of CGI, model work and other techniques. Even practical effects can be used as an element in a "visual effects shot", as when specially filmed flame or lava effects are composited into a scene.
For the most part, the starting "canvass" for a VFX is a blue or green screen placed behind actors during principal photography. This solid patch of colour can then be removed and replaced with another image. During much of the original series, this was achieved through a process known as colour separation overlay, whereby a blue screen would be fed images during the live recording of the shot. Since the TV movie, most of the VFX seen on Doctor Who have been achieved through the use of CGI, a more nuanced approach which allows elements rendered on a computer to be composited into a shot captured during principal photography. As contrasted with CSO and other optical/analogue effects, CGI work is completed exclusively in post-production. For this reason, many no longer consider CSO to be a true visual effect, but rather a "special effect".