Chapter 8: Matte and Keying Plug-ins

If you have the Standard version of After Effects, your blue-screen keying choices are pretty limited. You can manage to get a decent outline of your subjects, but forget about fine details such as hair, transparency, or shadows. The Professional Bundle version of After Effects 6 now ships with a very powerful professional chroma keying plug-in called Keylight. Other professional keying Plug-ins have been available from third-party developers for several years , as well as specialized Plug-ins specifically for DV.

Keylight

This is the plug-in that currently ships with the After Effects Professional (Production on v.5.5) Bundle. This new version of Keylight is derived from the version used on high-end software-editing systems for the professional film industry and is simple and easy to use. If you have this plug-in installed, you probably won't need anything else for your general keying needs. It does an overall great job and retains the texture and subtle nuances of the keyed material's highlights and shadows.

In most cases, a single click of the Eyedropper from within the plug-in will give you great results with either green-screen or blue-screen shots. With the background layer and blue-screen or green-screen layer (also known as the chroma key layer) placed on top in the Comp window, select the Keylight plug-in from the Effects menu (Effects > Keying > Keylight). Use the Eyedropper tool in the Effects window and select the blue or green chroma background (Figure 8.1). If you have a clean shot of the chroma key layer, then even details such as fine hair will be nicely composited with the background layer at this point.

Figure 8.1: One simple click with the Eyedropper tool in the Keylight effect plug-in will give you good results most of the time without the need for further adjustments.

Removing Chroma Spill from Details

When you have a chroma key shot that has a lot of background color bleed or "spill" into the details and edges of the subject, the shot will require a bit more than just the simple click to remove it. Keylight can remove spill from fine hair or separate reflections from glass with only a few adjustments.

In this first example, the figure was lit very lightly against a dark blue-screen background, which was composited against a scenic background (Figure 8.2). The problem was that her fine, wavy blonde hair was picking up too much of the surrounding chroma hue, so when the initial key was made, there was still a great deal of blue- magenta color in her hair that needed to be removed (Figure 8.3).

Figure 8.2: The image to be composited had fine details with a lot of color spill in the hair.

Figure 8.3: The initial keying still left a lot of residual color in the hair details.

For this example, just increasing the Despill Bias from 0.0 to 25.0 seemed to eliminate most of the unwanted color spill, without taking away the natural colors of the figure's skin and hair color (Figure 8.4). The default plug-in setting has the Lock Biases Together check box selected, which changes the Alpha Bias along with the Despill Bias when adjusted.

Figure 8.4: Increasing the Despill Bias helped to eliminate the color spill in the hair details.

This next example had a very dark, poorly lit, green-screen background in the chroma key image (Figure 8.5). The fighter pilot was to be composited over the background with the two trailing jets in flight against the sky.

Figure 8.5: This was a more complicated composite, where the chroma key image had a very dark background to deal with.

When the initial key was applied by using the Eyedropper tool in the most neutral green background area, most of the image was adversely affected by not providing a clean matte (Figure 8.6). The resulting effect was more of a double exposure than a keyed matte.

Figure 8.6: The initial key provided only a very crude beginning to our composite matte.

At this point, adjusting the Screen Strength and Screen Balance sliders helped bring back the matte details. By changing the view selector to Screen Matte, I could preview the density and edges of the matte being generated (Figure 8.7). Once I evened out the matte's density and edges, I then returned to the Final Result view and adjusted the Despill Bias down to eliminate any bleed or "holes" in the subject.

Figure 8.7: Making a few adjustments to the density of the matte provided the desired results.