Chroma removal or blue / green screen keying is one of the major tasks of VFX post production pipeline. Check out Disney’s solution for keying through method of color unmixing.
Chroma shooting is one of the major shooting elements for most of the movies. The purpose is multi-fold: the background / props can be changed, minimize the budget, filming a dangerous shot, shooting in a controlled environment and many others. But, the blue / green screen keying can be expensive in post production if shooting is not proper. Pulling out difficult keys can be herculean task.
There can be various reasons why green / blue screen shooting is not useful. Some of them are as follows:
- Shooting in low light condition
- Color patches / uneven shades on chroma cloth
- Chroma color and character cloth are of same color
- Character is very close to the chroma screen. It creates spill issues on character edges.
- Motion blurred shooting
- Defocus shooting
- Width and height of chroma cloth is lesser than the acting area. In such situation, rotoscopy needs to combine with keying.
Generally, VFX artists uses various chroma removal tools like Primatte, KEYLIGHT, IBK Keyer, Ultra Keyer, Ultimatte or other third party plugins. Multi keyer technique can also be used for complex shots. Even if things don’t work out, manual or tracking roto with paint can be the solution.
On an advanced technical note, visual effects artist can use custom keying. Custom keying means to remove chroma without using any of the industry standard tools or third party plugins. Various mathematical operations are achieved on color channels for blue / green screen keying and spill suppression. Working on this fundamental theory, Disney Research team has derived high quality interactive chroma removal technique through Color Unmixing.
The method is developed by Yagiz Aksoy, Tunç Aydın, Aljoscha Smolic and Marc Pollefeys from ETH Zurich lab. The team suggested color unmixing algorithm for green screen keying. They also compared their output with above mentioned industry standard keying tools and proved that their output is better. It also consumes less amount of time in post production pipeline.
Check out video of Disney’s solution to green screen keying.