Color Keying
to combine live-action foreground footage and backgrounds
that could come from virtually anywhere. What
was once a fragile and expensive proposition is now fully
mainstream; whole fi lms such as 300, now rely on this
technique, while the Colbert Report invites anyone with a
computer to try the “Green Screen Challenge” (and runs
entries from none less than John Knoll).
The process goes by many names: color keying, blue
screening, green screening, pulling a matte, color differencing,
and even chroma keying—a term from analog
color television, the medium defi ned by chroma and heavily
populated with weather forecasters.
The purpose of this chapter is to help you not only with
color keying of blue- and green-screen footage but with all
cases in which pixel values (hue, saturation, and/or brightness)
stand in for transparency, allowing compositors to
effectively separate the foreground from the background
based on color data.
All of these methods extract luminance information that
is then applied to the alpha channel of a layer (or layers).
The black areas become transparent, the white areas
opaque, and the gray areas gradations of semi-opacity; it’s
the gray areas that matter.
Good Habits and Best Practices
Before we get into detail about specifi c keying methods
and when to use them, I’ll share some top-level advice to
remember when creating any kind of matte.
. Introduce contrast. Use a bright, saturated, contrasting
background (Ctrl+Shift+B/Cmd+Shift+B) such as yellow,
red, orange, or purple (Figure 6.1). If the foreground
is to be added to a dark scene, a dark shade is
okay, but in most cases bright colors better reveal matte
problems. Solo the foreground over the background
you choose.
. Protect edge detail. This is the name of the game (and
the focus of much of this chapter); the key to winning
is to isolate edges as much as possible and focus just on
them so as to avoid crunchy, chewy mattes.
. Keep adjustments simple, and be willing to start over.
Artists spend hours on keys that could be done more
effectively in minutes, simply by beginning in the right
place. There are many complex and interdependent
Color Keying
Color keying was devised in the 1950s as a clever meansto combine live-action foreground footage and backgrounds
that could come from virtually anywhere. What
was once a fragile and expensive proposition is now fully
mainstream; whole fi lms such as 300, now rely on this
technique, while the Colbert Report invites anyone with a
computer to try the “Green Screen Challenge” (and runs
entries from none less than John Knoll).
The process goes by many names: color keying, blue
screening, green screening, pulling a matte, color differencing,
and even chroma keying—a term from analog
color television, the medium defi ned by chroma and heavily
populated with weather forecasters.
The purpose of this chapter is to help you not only with
color keying of blue- and green-screen footage but with all
cases in which pixel values (hue, saturation, and/or brightness)
stand in for transparency, allowing compositors to
effectively separate the foreground from the background
based on color data.
All of these methods extract luminance information that
is then applied to the alpha channel of a layer (or layers).
The black areas become transparent, the white areas
opaque, and the gray areas gradations of semi-opacity; it’s
the gray areas that matter.
Good Habits and Best Practices
Before we get into detail about specifi c keying methods
and when to use them, I’ll share some top-level advice to
remember when creating any kind of matte.
. Introduce contrast. Use a bright, saturated, contrasting
background (Ctrl+Shift+B/Cmd+Shift+B) such as yellow,
red, orange, or purple (Figure 6.1). If the foreground
is to be added to a dark scene, a dark shade is
okay, but in most cases bright colors better reveal matte
problems. Solo the foreground over the background
you choose.
. Protect edge detail. This is the name of the game (and
the focus of much of this chapter); the key to winning
is to isolate edges as much as possible and focus just on
them so as to avoid crunchy, chewy mattes.
. Keep adjustments simple, and be willing to start over.
Artists spend hours on keys that could be done more
effectively in minutes, simply by beginning in the right
place. There are many complex and interdependent
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