The Sky Is Not (Quite) a Blue Screen

The Sky Is Not (Quite) a Blue Screen

Study the actual sky (there may be one nearby as you read
this) or even better, study reference images, and you may
notice that the blue color desaturates near the horizon,
cloudless skies are not always so easy to come by, and even
clear blue skies are not as saturated as they might sometimes
seem.
Some combination of a color keyer, such as Keylight, and
a hi-con luminance matte pass or a garbage matte, as
needed, can remove the existing sky in your shot, leaving
nice edges around the foreground. Chapter 6 focuses on
strategies for employing these, and Chapter 7 describes
supporting strategies when keys and garbage mattes fail.
The fi rst step of sky replacement is to remove the existing
“sky” (which may include other items at infi nite distance,
such as buildings and clouds) by developing a matte
for it. As you do this, place the replacement sky in the
background; a sky matte typically does not have to be as
exacting as a blue-screen key if the replacement sky is also
fundamentally blue, or if the whole image is radically color
corrected.

Infinite Depth

A locked-off shot can be completed with the creation of
the matte and a color match to the new sky. If, however,
there is camera movement in the shot, you might assume
that a 3D track is needed to properly add a new sky
element.
Typically, that’s overkill. Instead, consider
. When matching motion from the original shot, if
anything in the source sky can be tracked, by all means
track the source.
. If only your foreground can be tracked, follow the
suggestions in Chapter 8 for applying a track to a
3D camera: Move the replacement sky to the distant
background (via a Z Position value well into four or
fi ve digits, depending on camera settings). Scale up to
compensate for the distance; this is all done by eye.
. A push or zoom shot (Chapter 9 describes the difference),
may be more easily re-created using a tracked 3D
camera (but look at Chapter 8 for tips on getting away
with a 2D track).
The basic phenomenon to re-create is the scenery at
infi nite distance that moves less than objects in the foreground.
This is the parallax effect, which is less pronounced
with a long, telephoto lens, and much more
obvious with a wide angle. For the match in Figure 13.6, a
still shot (no perspective) was skewed to match the correct
angle and tracked in 2D; the lens angle was long enough
and the shot brief enough that they got away with it. A
simpler example is included on the disc.