Join Mark Christiansen for an in-depth discussion in this video Overview: What is color keying?, part of After Effects Compositing: 4 Color Keying.
- Before we jump straight into the nitty-gritty of keying green screen or blue screen footage, let's take a brief look at what color keying is and why it even exists? Or, maybe more to the point, why does it still exist? The color keying process was actually devised about 60 years ago. That's six oh. It was a clever solution to a difficult problem. How do you take an actor and maybe some setting around that actor and place them into a scene where the actor could not, or maybe should not for safety reasons, have appeared? There were also cases where it was just simpler or more economical to take a shot on a stage.
For example, here, for both stylistic and budget reasons, it's a lot simpler to shoot an actor in a car on a stage than do a complilcated car-to-car shoot which probably would involve permits and so on as well. So the solution was that color would stand in for transparency. In the earliest days, this was a blue screen. And the blue also contained a good deal of green. So blue and green were used to play against the red photo-chemically using an optical printer.
Later it was found that computers can start with basically the same principle and that green is actually digitally even more separate, in many cases, from red or blue. Although the third digital primary red is also theoretically a color that could be distinguished from the other two, for the most part, foreground subjects have flesh which has a lot of red in it. And so red, generally speaking, isn't a really great choice. So we still use green screens and blue screens today simply because no reliable method has yet been devised to distinguish foreground from background and retain fine edge detail.
It's the quality of the edges that makes color keying a better choice than alternatives such as luma keying, which uses only light levels and is generally much cruder, or rotoscoping which is generally a lot more work especially when it comes to fine detail. We'll get to all of these techniques. But right now, let's dive straight into how to create a feature film quality key using just what's built in to After Effects.
Beginning with a brief explanation of the keying process, Mark takes you through the steps involved in creating a perfect green-screen key: generating a rough matte, eliminating color spill and matte lines, and refining problematic edges. He shows how to work with Keylight and Primatte—two indispensable keying tools in After Effects—and explains when to use one over the other. And for times when green screen won't work, he shows how to generate high-contrast mattes, or luma keys, based on the luminance data in your footage. Last, learn about compression and how to prep a shot for keying.
- What is color keying?
- Using garbage mattes
- Getting started with Keylight
- Understanding the Screen Color, Clip Black, and Clip White adjustments
- Eliminating spill with Advanced Spill Suppressor
- Using Key Cleaner to refine edges automatically
- Dividing a matte with holdout mattes
- Breaking down a complex color key
- Creating a luma key with Extract
- Setting up sky replacement
- Using Refine Soft Matte to improve edge detail
- Feathering edges with Channel Blur
- Knowing when to avoid green screen
- Prepping a shot for keying