Chroma Key Pioneer Petro Vlahos Dies


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The man who made one of the most popular TV and movie technologies work has died at the age of 96. Petro Vlahos did not invent “bluescreening” but was responsible for making it work smoothly.

The technique, known colloquially as chroma key compositing, is now widely used for special effects, both in a genuine attempt to create the illusion of footage being “real” and for graphics such as in weather forecasting. Although it’s mainly used with blue or green screens, the technique isn’t reliant on a specific color: blue and green are simply the least likely to clash with skin tones.

Chroma key in the movies actually dates back to before the second world war, with the 1940 release The Thief of Baghdad earning praise for its use of the technique. Vlahos refined the technique, using it for Ben Hur and Mary Poppins. He received a special award from the Academy of Motion Picture Arts and Science in 1964 for his work on the system.

Vlahos developed chroma key with a technique which he called color-different travelling matte. Rather than simply use a transparent/opaque matte to let the artificial background “show through” in the right places, Vlahos used a technique that broke down each frame into its blue, green and red components.

This removed the problem of an unnatural glow around the “real” objects filmed and merged with the artificial background. It also meant the system worked much better with tricky visual images such as smoke, glass and moving hair.

The BBC explains that Vlahos later developed a further refinement that used a white background lit by sodium lamps, with a resulting yellow glow. A prism split the image by filtering the yellow glow and the camera made two recordings: one which recorded the yellow glow in monochrome, which was used to create the matte, and one which recorded the action in color without the glow. Combining the resulting matte and footage created a much cleaner combination.

This refinement, used in Bedknobs and Broomsticks and The Birds among others, later fell from favor as traditional chromakey recording improved.

The Academy notes that as Vlahos’ various patents ran out, studios developed and modified his techniques but ultimately “every green screen or blue screen shot in innumerable films (including every blockbuster fantasy film of recent times) employs variants of the original Vlahos techniques.”

(Image credit: Hollyckuhno via Creative Commons license)





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