Difference Mattes

Linear Color Key

The Linear Color Key offers direct selection of a key color
using an eyedropper tool. The default 10% Matching

Softness setting is arbitrary and defi nes a rather loose
range. I often end up with settings closer to 1%.
Note that there are, in fact, three eyedropper tools in the
Linear Color Key effect. The top one defi nes Key Color,
and the other two add and subtract Matching Tolerance. I
tend not to use these because they don’t work in the Comp
viewer; the main Key Color eyedropper and the Matching
sliders work for me.

There’s a hidden trick to getting better results with Linear
Color Key. Because it is linear, it will pick up hues that
seem unrelated. To reduce the effect of these, you can
add a second instance of Linear Color Key. Under Key

Operation, changing the default Key Colors setting to
Keep Colors does nothing if it’s the fi rst instance except
annul the effect. On the second instance, Keep Colors is
unaffected by the fi rst instance and can bring back hues
that were already keyed. The one-two punch will often
deliver the best result.

Difference Mattes

A difference matte is simple in principle: Frame two shots
identically, the fi rst containing the foreground subject, the
other without it (commonly called a clean plate). Compare
the two images and remove everything that matches identically,
leaving only the foreground subject. It sounds like
the type of thing a computer was built to do.
In practice, of course, there are all sorts of criteria that
preclude this from actually working very well, specifi cally
. Both shots must be locked off or motion stabilized to
match, and even then, any offset—even by a fraction of
a pixel—can kill a clean key.
. The foreground element may be rarely entirely
unique from the background; low luminance areas, in

particular, tend to be hard for the Difference Matte
effect to discern.
. Grain, slight changes of lighting, and other real-world
variables can cause a mismatch between two otherwise
identical shots. Raising the Blur Before Difference
setting helps correct for this, but only by introducing
inaccuracy.
To try this for yourself, begin with a locked-off shot containing
foreground action, ideally one in which a character
enters the frame. Duplicate the layer, and lock off
an empty frame of the background using Layer > Time >
Freeze Frame. Apply Difference Matte to the top layer.
Adjust Tolerance and Softness; if the result is noisy, try raising
the Blur Before Difference value.