Animated Backgrounds and Chroma Keying
Chroma keying is used specifically for blue screening. It is quite common and easy to use. One such excellent example is the weather broadcast during the news. The weather maps are seen in the background behind the weatherman, when he is actually standing in front of a blue screen. This is perfect for those interested in inserting an animated background behind actors.
How It Works
This is a type of composite in which a specific color background, in most cases blue, is removed or made transparent and then it reveals another image. Any blue colors will be removed, so if the actor is wearing blue, then this also gets replaced. Green screen works in the same fashion. The colors green and blue were chosen because they are most different from skin colors. Compositing the 2 images requires computer editing software.
A blue color is frequently used because it is very complimentary to skin and provides minimal grain and very good details. The film is the best with blue, but green is typically used more often because green channels will retain more details and the green channels will be sampled more than blue channels. Ultimately your choice of color will completely depend on what you favor. Many times green works much better for outdoor shoots because of the sky.
One of the most difficult parts of using chroma keying is lighting and trying to avoid shadows. Any shadow will cause the color shade to alter and that shadowed spot may not be replaced. This is commonly seen with live broadcasts and low budget movies. When choosing the correct shade, it is best to make sure that the background color is at least 2 stops higher than the subject being filmed. In this way, the background color is easily differentiated from everything else.
The effect of melding the 2 pictures is used with editing software. Ultimately, this is achieved by using different layers. The blue color will have its own layer and then everything else will be in another layer. The blue screen can be completely replaced with a new image, in this case, animation, or it can be set as completely transparent. Therefore, when the blue screen footage is placed on top of stock footage or the animation, then this new footage will be seen through all the areas that have blue.
Most editing software will have some type of chroma keying available, including some free programs. While the software can completely remove the background if the original footage is not very good, there is only so much that software can do. Ensure that the lighting is good and color choice is appropriate. Think about the costumes and types of shots before actually shooting.
A few ideas for using chroma key include the ability to add backdrops for animations, or to show a different location when it is not possible to shoot in that location due to the budget. With animations, props can also be used. Good actors and directors will know how to use props to the best effect, particularly when they interact with animations. It is also possible to cover the actor with the blue or green screen to digitally insert in an odd body part or different costume.