Blue and Green Screen Keys

Blue and Green Screen Keys

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.


. 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.


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 (Figure 6.12)
. Isolation of the matte edge, for the purpose of refi ning
or blurring it (see “Matte Problems” later in this
chapter)