Linear Keyers and Hi-Con Mattes

Linear Keyers and Hi-Con Mattes

There are cases in which edge detail is less of a factor
because the matte is used to adjust, not layer, an element;
for example, you could hold out the highlight areas of an
image for adjustment using a high-contrast (hi-con) matte.
You might create this matte with a linear keyer.
Linear keyers are relatively simple and defi ne a selection
range based on a single channel only. This could be it red,
green, or blue, or just overall luminance. They’re useful
in a wide variety of cases outside the scope of blue- and
green-screen shots, although similar principles apply with
Keylight. (Keylight is covered later in this chapter.)
The most useful linear keyers are
. Extract
. Linear Color Key
The keyers to avoid are
. Luma Key
. Color Key
because each is limited to a bitmap (black and white) selection.
Only by choking and blurring the result with Edge
Thin and Edge Feather controls can you add threshold
adjustment.
The Extract and Linear Color Key
Extract is useful for luminance (luma) keying, because it uses
the black and white points of an image or any of its individual
channels. Linear Color Key is a more appropriate tool
to use to isolate a particular color (or color range).
Extract
The Extract key includes a histogram to help you isolate
thresholds of black and white; these are then graded with
black and white softness settings. You can work with averaged
RGB luminance, or you can access histogram controls
for each of the color channels.

One of the three color channels nearly always has better
defi ned contrast than overall luminance, which is merely
an average of the three. Either green or red is typically the
brightest and most contrasty channel, while blue generally
has higher noise.
Extract is interactive and easy to use and is a cousin to Levels.
Its histogram shows the likely white or black thresholds
on each channel. You bring in the White Point or Black
Point (the upper of the two small square controls below
the histogram) and then threshold (soften) that adjustment
with the White Softness or Black Softness controls
(the lower of the two small squares).