Introduction to chroma key: Working & crucial elements

Chroma key compositing or keying refers to applying special effects or post production techniques for merging two images or video streams. These are based on various colour schemes or chroma range. The trick is to replace original background with real-time images or scene; a common practice in movies and video game production. However today, it’s widely used to create interactive web-based videos for marketing purpose.

How it works?

For starters, it’s better to know a few things first about chroma keying such as foreground footage; the actual subject and coloured screen. Background footage is also necessary to replace this coloured screen with particular scenery.

Green, blue or sometimes red screen must be evenly lit to match exact colours all across. Shadows can seriously ruin the chroma key process but understanding foreground subject lighting would definitely help. Having two separate lights is the ideal situation whereas the subject must be far from screen where foreground illumination won’t interfere with screen lighting at the back.

Once the sample infuses properly with light and background colour keyed, it’s time to add a background scene. When the two are conjoined, subject can be transported anywhere in the world be it Mount Everest, the Roman Colosseum or top of Burj Khalifa. The only limit is your imagination and the type of film you’ve at your disposal.

Critical elements

  • Space

Space at the site helps to determine if chroma key shot is possible or not. While shooting against green, blue or red screen; always push subject far away from the screen to avoid shadow and minimise reflection.

  • Screen shade

Screen colour also makes a big difference, therefore always predetermine whether it’s green, blue or any other per say. Blue is a traditional colour used in production but for digital keying, green is preferred. Since digital cameras use Bayer pattern of Red Green Blue photo-sites, they’re much sensitive to green lighting. Meanwhile, green screen also requires less lighting than blue.

  • Camera

With cameras, you must be exact to get desirable results but sometimes may give you funky artefacts due to lighting, aspect ratio and other technicalities. With HD cameras now used widely, getting an acceptable key is much easier as pixels are smaller.

Conclusion

With digital age, chroma key techniques are gaining worldwide recognition especially with green screen. The finest example would be of upcoming Hollywood blockbuster “Star Wars Episode 7” where production team worked with chroma key in Dubai for desert scenes.