Chroma Key – How to make Superman fly in the sky
In the previous Friday Fundas section I mentioned Cinema is a magical medium and how optical illusion can create perceived reality with techniques like Forced Perspective. One of the readers had asked an interesting question on how scenes similar to Kumbakarna and the tiny soldiers, Gulliver and Lilliputs are made. While some of the scenes may be forced perspective but the scenes which involve the Lilliputs walk over the giant body of Gulliver is covered by what is known as the Chroma Key Compositing or Chroma Keying Technique. This also popularly called as the Green Screen effect or the Blue screen effect.
The basic of Chroma Key is overlapping two images and make them merge with each other. Let us consider the flying action of Superman and the image of sky behind him. As images are built of pixels you would want to replace the background from the image of superman in flying action with that of the image of the sky. In order to achieve it you would want to make any pixels around Superman transparent. Well Chroma Keying is the technique to do that. The technique involves choosing certain color range of pixels and making it transparent.
In the initial days this was done using a blue screen at the background and then blue color was made transparent to merge with another frame to make it look like they are happening in the same space. It should be noted that no other character or object in the first scene should have blue color. As Superman has a blue costume instead of a blue screen a green screen could be used. In the modern days a green screen is used prevalently as the image sensors in digital video cameras are more sensitive to green there by allocating more pixels to green channel.
Prior to the digital era Chroma Key was also performed on the films. It was popularly referred to as Matte technique. The technique is as old as 1930s. The scene comprises of two scenes one the foreground shot and the other the background shot. The background of the foreground shot when taken would be a blue screen. Now the camera color negative of this shot was printed onto a high contrast black and white film using either a filter or the black and white film to limit the blue color. The resultant film produced transparent color wherever it found blue. The end result is the foreground object with a transparent background. This is called the “Female matte”. This was now copied onto the high contrast negative of the background scene which is called the “Male matte”. The end result is the negative of the film with both these scenes merged with each other.
One of the key challenges in this technique was camera movement. The camera movement used when shooting the foreground should be used exactly the same way while shooting the background scene. Later techniques were developed to automate this process to move the camera exactly the same way.
Well next time you see Superman fly or Batman jump across buildings you would remember there is always a Chroma Key behind it.