Top SFX shots No.6: The Birds
An extraordinarily complex piece of compositing (shown in the clip with inserts removed) which demonstrates Hitchcock's continuing urge to push the lackadaisical state of the art. The flapping of the birds' wings caused too much fringing for conventional blue-screen work to be utilised, and Hitchcock was forced to turn to the 'yellow screen' or 'sodium vapour process'. Only Walt Disney studios have ever been equipped for this process, and indeed only one camera has ever been rigged for it. SVP involves filming the subject against a screen lit with powerful sodium vapour lights utilising a very narrow spectrum of light. Unlike most compositing processes, SVP actually shoots two separate elements of the footage simultaneously using a beam-splitter; one reel exposed is regular photographic stock and the other an emulsion sensitive only to the sodium vapour wavelength. Very precise mattes are obtained from the latter, allowing the subject to be pulled out of the background and combined with any other in a later run through an optical printer. The fringing or 'matte line' effects are negligible compared to blue-screen work, but the very precise conditions under which the footage must be shot mitigated against its wide usage. Disney, to whom many shots in The Birds was farmed out, used the process in many films including Mary Poppins (1964), Freaky Friday (1976)and The Black Hole (1979).