Difference Mattes

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.