at the last minute, it can substantially lower
operator blood pressure.
Chroma Key Tips and Tricks
Some useful tips for how to get your production
ready for a flawless chroma key, and some tricks
for how to deal with difficult shadows.
The Set
In every set design, care must be taken to ensure
there are no visible seams, joints or hard
corners that will appear on camera. Paint should
be chosen that is a close as possible to the exact
color vector that will be used, typically Green
or Blue. If the paint pigment is not precise and
several batches of paint or repaint are required,
the uneven color result will make the perfect
key impossible. There are several vendors of
chroma key specific paints and set materials
you can work with. Ross Video works closely
with ProCyc (
) in the USA, we
whole-heartedly recommend their products.
The set should be sized to allow ample
separation between foreground talent and the
chroma background wall; ideally at least 6-to-8
feet of separation. The further the distance, the
easier it is to reduce unwanted color spill and
achieve good lighting separation.
Lighting
Lighting configuration will naturally vary with
set design and choice of lighting equipment, but
there are some simple rules that will help
achieve the best results:
• Light the chroma set independent of the
talent: light the walls and floor to achieve
flat, even illumination.
• Light the talent independently: use back and
side lighting to minimize color spill from the
walls and floor. Use floor or stand mounted
front fill lights to eliminate areas of spill
reflected from the floor.
• Don’t light the background too brightly: it
should ideally be half a stop or more below
the level of the foreground subject (about 50
- 75% of the foreground on a waveform
monitor).
• Don’t rely on eyesight: when assessing the
set lighting and background quality and
finish - eyesight will mislead you. Look at the
camera output on a waveform/vectorscope
and ensure that the chroma background
reads as an even signal without variation.
Camera and Lenses
Resolution: Cameras naturally vary in
performance; the ideal camera has a
high-resolution imager with very low noise. This
is critical for an excellent final result. Medium
resolution imagers can produce great results in
normal video productions, but rely on relatively
high-detail settings to enhance image edges.
Detail-enhanced edges are undesirable in
chroma keying. These edges will appear as
bright or dark key halos, which cannot be
removed without compromising overall key
quality.
Imager/Sensor Noise: Noise is always present
in the video image but can be minimized by
choice of sensor type and gain settings. Image
noise has a significant effect on chroma keying
as the chroma levels in a normal video image
are much lower than the luma levels - as a
result, the chroma information contains
proportionately more noise. High noise-levels
will be most obvious in dark areas of a
developed chroma key and can be very difficult
to remove without compromising key quality.
The better the noise performance of the camera
- the better the end result.
Lens: Select a camera lens based on the set size
and production needs. Some lower-cost lenses
can exhibit chromatic aberration - colored halos
- around edge transitions in certain lens regions.
This has a highly undesirable effect on edge
quality.
Detail: Avoid high levels of detail regardless of
camera type; if detail is necessary to achieve the
image quality desired, then the best choice is to
use the chroma key Edge Softness control to
soften the edge and conceal the aberrations
caused by detail setting.
Shadow Tricks
Developing a shadow from the chroma key set
can give you a more realistic final output, but
this is very difficult to do. As previously
mentioned, noise lurks in the shadows and can
look quite nasty. But all is not lost, very
acceptable shadows are achievable with a nifty
keying trick.
1.
Setup the chroma key as usual; don’t worry
about resolving the shadow, just make the
Ultrix Acuity Operation Manual (v10.1) — Keying • 45