CGI – Blue vs. Green Screen Chroma Keying

CGI – Blue vs. Green Screen

Chroma key compositing, or chroma keying, is a special effects / post-production technique for compositing (layering) two images or video streams together.

Almost all studios nowadays use green screens, however there was a time when blue chroma key screens were the main stay.

One of the things the general public are unaware of is whichever colour is used, has to be avoided in any of the set pieces, props and costumes/clothing of the person being filmed because the colour will pick up and show any background overlay (such as a mountain scene) – a big no-no.

Green chroma key screens became the ultimate universal ‘CGI canvas’ because with the blue chroma key there was more ‘bleeding’ around the actors and set items making the foreground images a little blurry with a blending effect with the superimposed background.

Processing a green backdropProcessing a green backdropYou generally can tell when you watch the older movies, the scenes where they used blue chroma key and another major contributing factor that creates problems is the casting of shadows upon the screen itself so proper cross lighting is important.

The following are excerpts from Wikipedia as I couldn’t explain it better myself… Thanks Wiki.

Processing a green backdrop

Virtual television studio with green-screen technique.

Virtual television studio with green-screen technique.

The high amount of contrast between different parts of the screen is not ideal (see even lighting). Green reflections on the desk would also make keying difficult.

Green is used as a backdrop more than any other color because image sensors in digital video cameras are most sensitive to green, due to the Bayer pattern allocating more pixels to the green channel, mimicking the human eye’s increased sensitivity to green light.

Therefore, the green camera channel contains the least “noise” and can produce the cleanest key, matte, or mask. Additionally, less light is needed to illuminate green, again because of the higher sensitivity to green in image sensors.

Bright green has also become favored since a blue background may match a subject’s eye color or common items of clothing such as jeans.

Processing a blue backdrop

Before digital chroma keying, blue-screening was accomplished using film. The camera color negative was printed onto high-contrast black and white film, using either a filter or the color sensitivity of the black and white film to limit it to the blue channel.

Film set for The Spiderwick Chronicles, where a special effects scene using bluescreen chroma key is in preparation

Film set for The Spiderwick Chronicles, where a special effects scene using bluescreen chroma key is in preparation

Assuming this film was a negative it produced clear where the blue-screen was, black elsewhere, except it also produced clear for any white objects (since they also contained blue). Removing these spots could be done by a suitable double-exposure with the color positive, and many other techniques.

The end result was a clear background with an opaque shape of the subject in the middle. This is called a female matte, similar to an alpha matte in digital keying. Copying this film onto another high-contrast negative produced the opposite ‘male matte’.

The background negative was then packed with the female matte and exposed onto a final strip of film, then the camera negative was packed with the male matte was double-printed onto this same film. These two images combined together creates the final effect.

One comment to CGI – Blue vs. Green Screen Chroma Keying

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