Chroma-Keying for Stop Motion Animation: Introduction

Posted by Nate Eckerson on

Wrap a blue or green screen around your animated actors and they will be magically transported into new worlds - or so you might think. A successful "key" can be difficult to do well, but it is possible to achieve great results using free software on both OS X and Windows!

Learn about the chroma-key effect, how to incorporate it into your productions, and our answer to the question: "Blue or green screen?"

Understanding Chroma Keying

Before digging into the nitty-gritty of the greenscreen / bluescreen / chroma-keying process, it is important to understand what chroma keying does and doesn't do.

When a chroma key effect is applied to footage, the software analyzes the pixel information and selects all the pixels of a certain color. In this example, we have footage that was shot against a blue background. The blue pixels are all selected, and then made transparent. This allows a background image (the yellow brick sky) to appear in their place.

Why are blue and green used as key colors? The technology was originally developed for live video productions. Human skin contains no blue or green tones, so people didn't fade away when the effect was applied. Technically, your key can be any color, but sticking to blue and green is best when following the techniques in this tutorial.

It's important to understand that chroma keying does not magically make green and blue things transparent. You always need a background image to replace whatever green or blue pixels are removed from your footage.

Don't expect this to happen unless you have captured an image of the background WITHOUT the blue pants!

My recommendation is that you use chroma keying only in instances where you have to replace the background or large areas of a long animation sequence. In many cases you will achieve faster, higher quality results if you follow a similar process to that described in our Paint.NET tutorial.

In the next post, learn about the many screen materials you can use.