| Eureka!
You just had a great idea for your video, but
how are you going to place your talent inside
a lavish landscape when your available surroundings
are dull and dreary? That's where special effects
come in. Thanks to chromakey, you can have your
talent trudge across the frozen tundra, drive
through the streets of Paris or stand 10 feet
from a hungry lion, without leaving the comfortable
confines of the studio. You can also shrink your
talent down to miniature proportions so that they
can walk around a product or through a giant landscape.
While
the basic premise of using chromakey is pretty
simple to understand, lighting for chromakey can
be tricky. Even in high-profile studio broadcasts,
you may notice something odd about a scene that
gives the chromakey away. In extreme situations,
a bad key can leave a fuzzy halo around the talent.
In this column, we will take a look at various
uses of chromakey and how you should light the
set and talent. |
Using
Chromakey - Lighting Tips
Creating
a clean chromakey is only half the battle. The
other is making it look as real as possible. One
important way to accomplish this is to carefully
study (or plan) the footage or scene you will
use as a background.
Ask
yourself, "Is it indoors or outdoors? What
time of day is it? Where is the primary or key
light coming from? What kind of light is it; is
it hard or soft, bright or dark? What is the mood
of the scene? How large should the talent or object
be to look real in the image? Does the background
move and, if so, which direction and how fast?
Should there be wind, rain, or some other effect
added to the shot to make your subject blend with
background scene?"
Once
you have asked yourself these questions, it is
a simple matter of matching the talent or object
you plan to shoot with the background you will
key in later. |
|
Chromakey
Lighting Basics
When
lighting a scene for chromakey, keep a couple
of things in mind. First, light always reflects
back the color of the surface on which it shines.
If you shine a light on a green wall, it will
reflect green. This may seem obvious, but don't
forget that this reflected green light is not
just being reflected back to the camera lens and
your eyes, but is also spilling onto your talent.
The subject will be bathed in a pool of reflected
green light, exactly the color you are trying
to key out. For this reason, it is very important
to move your subject as far from the backdrop
as possible and to make sure that there is a strong
white backlight to wash out any light reflected
off the chromakey background (see Figure 1).
The
other major issue is the lighting of the background
itself. You need to light the background as evenly
as possible, so the color range and brightness
of the wall will appear as uniform and narrow
as possible. The computer uses a very small range
of colors for the keying effect. The smaller the
range of color, the cleaner the chromakey. To
get an even light across the background, use lighting
instruments that offer diffused, even light. Depending
on the size of your background, you can usually
use a couple of lights set up above the background,
shining on the surface from about a 60-degree
angle (see Figure 2). By lighting at a steep angle,
most of the light will reflect onto the floor
and not the talent. Make sure that there are no
shadows or hotspots. If you do not have large,
diffused lights, use a scrim or gels to diffuse
smaller instruments. It is always best to have
a floor that is either the same color as the chromakey
or is non-reflective, so that it doesn't pick
up the background light. |
Lighting
If
the background footage that you'll key in was
shot indoors, it should rather simple to match
the color and direction of the lighting. Make
sure your talent's key and fill lights come from
the same direction as those in the scene. Flag
the key and fill so that they do not spill extra
light on the chromakey wall and create bright
spots. Keep in mind that you can have the key
and fill appear as dramatic as you wish. Just
because you shoot against a bright green background
doesn't mean the shot won't turn out to be dramatic.
Turn off the lights that illuminate the wall while
you set up your key, back and fill lights, to
achieve the desired effects. This will also help
you find those places where you need to flag the
lights to keep them off the background (see Figure
3).
Outdoor
Lighting
If
the background footage that you'll key in was
shot outdoors, make sure you know what direction
the sun was coming from and the prevailing type
of sky. If it is an overcast day, set up a very
diffused light and fill the talent with even light.
If it is a bright, sunny day, light the talent
with a small, bright light that creates hard shadows.
Be careful to duplicate the direction and angle
of the sun. Also, be sure to flag the light, so
that none falls on the background. For those partly
cloudy days, have a crew member occasionally move
a small, narrow flag across the light to give
the impression of passing clouds. Add a quiet
fan that blows in the same direction as the wind
in the background. It's amazing how the addition
of a little wind and some sound effects pulls
the audience right into the scene and allows them
to accept the scene as real.
If
the background moves, determine if the light changes
in the scene and then how the talent should move
to match the background motion.
Putting
It into Action
The
scene: a motorcycle drive through the country
on a bright spring day. To begin the preparation
for the shot, shoot the required background footage.
Once
you are back in the studio, park your motorcycle
in front of the green chromakey background and
light it to match the brightness of the day's
footage. If it is a bright, sunny day, use only
one very bright, hard light, coming from the direction
and angle of the sun in your scene and with white
cards bounce fill light just out of camera shot.
Make sure that no green light reflects on the
cycle. Remember, if it is green, it will become
transparent.
Using
bounce cards, light the rider's face. You can
supplement the light by adding gelled fluorescent
lamps that lay on the ground hidden behind an
artificial shrub. Get someone to rock the cycle
to match the type of road in the scene. Also,
play the road footage for the talent, so he knows
when to lean. Be careful with shiny surfaces.
They will reflect back the studio lights. Add
a little wind to blow on your talent. Later, when
you edit, add the background footage, some cutaways
and some road noise and you have a very convincing
road trip. |
