Noise Suppression

Noise Suppression

For seriously sizzling mattes, Keylight includes a Screen
Pre-blur option that I would reserve for footage with a
clearly evident noise problem, such as heavy compression.
Blurring source footage before keying adds inaccuracy and
is something of a desperation move. The footage itself does
not appear blurred, but the matte does.
A better alternative for a fundamentally sound matte is
Screen Softness, under the Screen Matte controls. This
control blurs the screen matte itself, so it has a much better
chance of retaining detail than a pre-blur approach. As
was shown in Chapter 3, edges in nature are slightly soft,
and a modest amount of softness is appropriate even with a
perfectly healthy matte.


The Despot cleanup tools are meant to fi ll matte holes, but
at suffi ciently high levels they add blobbiness, so they are
rarely useful. An alternative approach, particularly with DV
formats (which, by the way, are guaranteed to add compression
noise and are not recommended for blue-screen
and green-screen work), is to
1. Convert the footage to YUV using Channel Combiner
(the From pop-up menu). This will make the clip look
very strange, because your monitor displays images as
RGB. Do not be alarmed.


2. Apply Channel Blur to the green and blue channels
only, at modest amounts (to gauge this, examine each
channel as you work—press Alt+2/Option+2 or Alt+3/
Option+3 while zoomed in on a noisy area). Make sure
Repeat Edge Pixels is checked.
3. Round-trip back from YUV to RGB, using a second
instance of Channel Combiner.
4. Apply Keylight.
Fringing and Choking
Fringing (excess edge opacity) and choking (lost edge
detail) are the ultimate tests of an otherwise problem-free
matte.
Screen Grow/Shrink deals with this issue directly; ideally
this control would be used for Garbage and Core passes
(described earlier) but not the Edge itself; otherwise it may
be a symptom of a more fundamental problem, an indication
that it’s time to start over.
It’s also not the last resort for choking and spreading a
matte; alternatives follow in “Beyond Keylights: Better
Mattes.”

Spill Suppression

Keylight suppresses color spill (foreground pixels contaminated
by refl ected color from the background) as part
of the keying operation. Thus spill-kill can be practically
automatic if you pull a good initial key.
There are a surprising number of cases in which Keylight’s
spill suppression is not what you want, for the following
reasons:
. Dramatic hue shifts occur to items whose colors are
anywhere near green (for example, cyan) or opposite
green (for example, magenta). It’s challenging enough
to keep green off of a green set, let alone its neighboring
and opposite hues.
. These hue shifts can also add graininess, even to footage
that was shot uncompressed and has little or no
source grain.


Should Keylight’s spill suppression become unwieldy or
otherwise useless for the preceding reasons, there is an
easy out: ordinarily View is set to Final Result, but set it to
Intermediate Result for the matte applied to the Alpha
without any change to RGB. The CC Composite effect does
the same thing, eliminating all RGB changes from preceding
effects but keeping the alpha.
Keylight itself also includes spill suppression tools, under
Edge Colour Correction, that infl uence only the edge
pixels. Enable its checkbox and adjust the controls below,
softening or growing the edge as needed to increase the
area of infl uence. Sometimes adjusting Luminance or Saturation
of edges is a quick fi x.
There are better ways than Keylight to kill spill, detailed in
the next section, which moves beyond this tool.