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 9.2). 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 9.3).

Figure 9.2 The image to be composited had fine details with a lot of color spill in the hair.
Chroma Spill

Figure 9.3

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

Figure 9.3

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. 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. I also increased the Clip Black and decreased the Clip White Screen Matte settings to get the cleanest edges. Finally, I moved the edge Color Saturation down to 0.0, eliminating the majority of the color bleed in the hair (Figure 9.4).

Figure 9.4 Increasing the Despill Bias and decreasing the Edge Color Saturation 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 9.5). The fighter pilot was to be composited over the background with the two trailing jets in flight against the sky.

Figure 9.5 This was a more complicated composite, where the chroma key image had a very dark background with which to deal.

Figure 9.5 This was a more complicated composite, where the chroma key image had a very dark background with which to deal.

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 9.6). The resulting effect was more of a double exposure than a keyed matte.

Figure 9.6 The initial key provided only a very crude beginning to our composite matte, requiring some major adjustments to the matte.

Figure 9.6 The initial key provided only a very crude beginning to our composite matte, requiring some major adjustments to the 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 9.7). Once I evened out the matte's density and edges, I returned to the Final Result view and adjusted the Despill Bias down to eliminate any bleed or "holes" in the subject.

Figure 9.7

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

Figure 9.7

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

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