Game Development Reference
The Color Difference Key Effect
If the concept of keying intimidates you, then you may want
to skip ahead and come back later to the Color Difference Key
effect. This effect works really well, and can even key out tough
jobs (like smoke or wispy hair) better than most. But it can be a
little daunting at fi rst.
To follow along with me, open up the Color Difference Key.aep
project from the Chapter 10 folder of the exercise fi les. For this
project, we'll be using a photo rather than a video. This effect does
much better with lightly-compressed or noncompressed video.
And since those are more challenging to distribute, and we're only
using one frame anyway, we're using a photo. Apply the Color
Difference Key effect to the Chad Key layer in the Chad Key comp.
I realize that these parameters look somewhat intimidating at
fi rst glance. They're certainly more intimidating than the param-
eters of the other keying effects. But they make a lot more sense
when you realize what the effect is doing.
Using Fig. 10.5 as a guide, you'll see that there are actually three
main sets of properties here: Partial A, Partial B, and Matte. The
Color Difference Key effect creates two mattes, and then com-
bines them together to create the fi nal alpha. Partial A is the fi rst
matte. Partial B is the second matte. And the Matte controls affect
the fi nal result (the alpha channel). Essentially, we have controls
similar to those seen in the Levels effect for each of the three
groups of properties.
To use this effect with this example, we fi rst need to change the
Key Color value, which is set to a default blue color. Click the eye-
dropper next to the Key Color swatch and then click on the green
background of the photo. From my experience with this effect,
the initial results are always terrible. But don't worry about that.
We'll get this key near perfect in just a moment (Fig. 10.6).
The next step to perfect our results is to adjust each of the two
partial mattes individually. In this effect, you can do this in two
ways. You can use the matte selector and eyedropper tools at the
top of the effect in the Effect Controls panel, or you can use the
regular controls at the bottom of the effect. We need practice at
both, so we'll use the controls at the top of the effect to adjust
Partial A, and then the controls at the bottom to adjust Partial B.
First, select the A matte by clicking the A button underneath
the layer thumbnail in the upper right-hand corner of the effect.
Now we're going to adjust this matte by using the eyedroppers.
The top eyedropper is the equivalent of the Key Color property,
which we've already adjusted. The second (black) eyedropper is
used to click on the areas of the matte that should be black (com-
pletely transparent). Partial A's background looks pretty good, but
just to be safe, you can select the black eyedropper and click in
the area of the image that has the green screen.
Tweaking the Color
Difference Key Effect
Here are a couple
tips when working
with this effect.
First, use the View drop
down to see the results
of individual mattes or
the fi nal matte. Second,
I usually get the best (and
quickest) results from the
Color Difference Key effect
by getting a rough key
using the thumbnails and
eyedroppers at the top,
and then by fi ne tuning
my results with the regular
parameters at the bottom.