Basic premise

To make use of a "Chroma" key effect is to make part of an image transparent based on the color value within that image. This is popularly known as a green screen technique.

You don't often hear the phrase "green screen" in professional settings because in actuality any color can be the target for removal from a still image or video footage. You've probably seen examples of both green and blue backdrops being used in the behind-the-scenes footage of hollywood movies or whather reporters standing in front of an animated map of the eath and either are perfectly viable.

The reasoning for which color is used "on set" is dependent on several things. It used to be that the use of digital or film as a medium would dictate the backdrop color (blue typically offers less contrast than green in digital recording while the opposite is true for film) but with digital cameras becoming the norm that is now less of a concern. For the most part the primary factor will be either cost (use what you have available for smaller projects) or the subject to be isolated. If any of the isolated element includes the color blue then it should be obvious a green backdrop is preferred so that the software can find the contrast between them. If the subject includes green then a blue backdrop is preferred.

Types of Key Isolation

The actual effects involved with Chroma Keying may differ between programs.

Filming conditions

Using expensive backdrops and the highest defintion of cameras will only serve to improve the quality of your footage. Whether or not the final product you create looks "believable" is another matter altogether.

While there are many plug-ins designed to handle chroma keying and there are several different plug-ins that are included with any given video editing package the one we will be using for these pages is the "Keylight" effect for Adobe After Effects.

The Keylight plug-in

Within Adobe After Effects you can find the "Keylight 1.2" plug-in by searching for it in the "Effects & Presets" search box or by expanding the "Keying" category within the same window.

Like all effects it can be added to the desired layer by simply dragging and dropping the plug-in onto the layer or double clicking the plug-in while the layer is selected. The plug-in can only be applied to one layer at a time.

The most basic use of the plug-in involves the following.

  1. Add the effect to the desired layer.
  2. While the layer to be keyed is visible in a monitor Left-Click the eye dropper icon.
  3. Left-Click a point within the monitor made up of the color you wish to remove from the video layer.

With source material of a high enough quality this single use tool alone will produce a good result. The chosen color should now appear as transparent and revealing any layer that appears below it. But that might not always be the case.

If you look closer at the resulting footage in the composition monitor you'll probably notice one of two, or both, possible artifacts. The first is a slight transparency to some parts of the subject that should otherwise be opaque, and the second might be artifacts around the subject where you were hoping for complete transparency.

We can help isolate the prevelance of these artifacts for our own judgement via two simple properties within the effect options windows. The properties to look for are "Clip Black" and "Clip White".

These two properties change the threshold at which point a color within the footage is close enough to the target chroma value to be considered either black or white. For instance if you've selected a solid green color for your target value (0,255,0) and the "Clip Black" value remains at the default value of "0.0" then anything less than that full green value will start to shift from dark grey to light. However, if we were to raise the "Clip Black" value then we can include more colors that are simply "near" complete green values to be included in being completely transparent.

The same goes for the "Clip White" property. A lower value for this property will include more of the image that *isn't* the target color as being completely opaque.

To better see the effect of these settings and allow for us to judge their effect with our own eyes it will be *very* helpful to see the combined matte of the image. This is a simple greyscale image available by choosing "Combined Matte" from the dropdown menu at the top of the effect settings. Think of it as a mask similar to a layer mask in Photoshop (because that's what it essentially is). Black areas represent transparent parts of the layer while white areas will represent opaque parts.

Just remember it is possible to adjust these values too far. Observe what happens when you place the "Clip Black" value at 50 and the "Clip White" value at 51. The difference between transparent and opaque will be complete or not at all and introduce aliasing that gives the image a jagged, ugly edge around the subject. The ideal levels will be dictated by your personal judgement.

This measurement is not 1:1. The Clip properties range from 0 to 100 so a value of "50" is more akin to a threshold color of 128 (just remember it's using the overall intensity).