When
film
is
exposed,
its
light-sensitive
emulsion
is
chemically
altered.
Suppose
half
of
a
frame
of
film
was
covered
and
protected
from
an
exposure
to
light
-
it
would
still
be
virgin
and
could
certainly
be
exposed
at
a
later
time.
That's
the
idea
behind
mattes
and
matting
work.
Protect
an
area
of
the
frame
during
an
exposure
so
it
can
be
exposed
later
to
combine
different
images
that
do
not
ordinarily
appear
at
the
same
time.
Suppose
you
are
an
only
child
and
wanted
a
shot
of
you
arguing
with
a
non-existing
evil
twin
sibling.
This
trick
can
be
accomplished
by
masking
half
of
the
film
with
a
"matte"
(an
opaque
black
mask)
in
front
of
the
camera's
lens
and
photographing
you
on
the
left
of
the
frame
first.
After
shooting
this
sequence,
one
side
of
the
film
has
been
exposed
while
the
other
side
is
still
virgin.
Now,
rewind
the
film
to
the
beginning
of
the
sequence.
Flip
the
matte
over
to
cover
and
protect
the
other
half
of
the
film
that
has
already
been
exposed.
Now
stand
on
the
right
side
of
the
frame
(as
the
evil
twin)
and
argue
back
(I
think
Captain
Kirk
did
this
in
a
Star
Trek
episode
once).
With
the
originally
exposed
side
now
protected,
the
virgin
side
is
now
being
burned-in.
With
both
sides
of
the
film
burned-in,
develop
the
film
and
put
it
on
the
projector.
You
will
no
longer
be
an
only
child
(on
film
at
least).
That's
one
of
the
simplest
forms
of
a
matte
know
to
man.
Mattes
are
used
to
composite
two
shots
together
and
can
be
used
to
composite
complex
moving
shots
if
your
travelling
mattes
are
rotoscoped
properly.
