Luma Mattes

Luma Mattes

The honest truth is that I don’t use the above effects very
often; there are less immediately obvious but more powerful
ways to generate transparency from color data:
. Blending modes
. Blending modes with color adjustments
. Shift channels and levels
In the interest of full disclosure, I often use track mattes
(as detailed in Chapter 3) with a duplicate of the layer,

instead of luminance keys applied directly to the layer.
With a track matte, I can use Levels to work with color
ranges on all channels in order to refi ne transparency.
Other artists may consider this approach more cumbersome,
preferring to work with a single layer and an effect,
so the choice as always is yours.

Blue and Green Screen Keys

Keylight is useful in many keying situations, not just studiocreated
blue- or green-screen shots. For example, you can
use Keylight for removal of a murky blue sky (Figure 6.8).
You wouldn’t use Keylight to pull a luminance key, however,
or when you’re simply trying to isolate a certain color range
within the shot; its effectiveness decreases the further you
get from the three primary colors.
Keylight is most typically used on footage shot against a
uniform, saturated, primary color background, where preservation
of edge detail is of utmost importance.

The basic steps are
1. Garbage matte any areas of the background that can
easily be masked out. “Easily” means you do not have an
articulated matte (you don’t animate individual mask
points). As a rule of thumb, limit this to what you can
accomplish in about 20 minutes or less .

2. Attempt a fi rst pass quickly, keeping this matte on the
loose side (preserving as much edge detail as possible)
to be refi ned later.
3. Preview this at full resolution, in full motion, against
a bright primary color. In rare cases, you’re done, but
before you throw up your arms in victory, carefully
examine the alpha channel of the toughest frames
(usually the ones with the most motion, refl ection, or
semi-opaque detail). Note any obvious holes in the foreground
or areas of the background that have failed to
disappear, as well as any noise in the solid areas of the
matte (Figure 6.11).
If things look noisy and chaotic in the alpha channel or
the edges are clamped and chewy, you can
. Start over and try a new pass
. Apply noise reduction to the plate, then start over
(see “Noise Suppression” later in this chapter)
. Articulate or track garbage/holdout mattes to isolate
problem portions of the footage (see “Beyond
Keylights: Better Mattes,” later in this chapter)
4. If necessary, separate the plate for multiple passes. At
the very least, it’s often useful to create a solid core and
a completely transparent background so that you can
focus only on the edge (detailed in the next section).
You may also need to separate individual parts of a layer
for a separate keying pass, such as hair or a fast-moving,
motion blurred limb.
The shadows need to be
eliminated with a hard matte; they go
up the wall and off set. A hard matte
negatively influences fine detail such
as motion blur and hair, any of which
may require its own holdout matte.

5. Refi ne the edge. Zoom in on a challenging area of the
foreground edge (200% to 400%), and refi ne the key
to try to accommodate it, using strategies outlined in
the steps that follow. Challenging areas may include
. Fine detail such as hair
. Motion blurred foreground elements
. Cast shadows
You must also watch out for, and consider rotoscoping,
foreground features that can threaten an effective key,
such as
. Areas of the foreground that refl ect the background
color
. Edge areas whose color nearly matches the background
. Areas of poor contrast (typically underlit regions of
the shot)
6. Preview the shot in full motion. Again, note holes and
noise that crop up on individual frames, and use the
strategies outlined in the section “Beyond Keylights:
Better Mattes” to overcome these problems. Approaches
you may add at this stage are
. More holdout mattes (typically masks), either for
the purpose of keying elements individually or rotoscoping
them out of the shot
. Isolation of the matte edge, for the purpose of refi ning
or blurring it (see “Matte Problems” later in this
chapter)