TUTORIAL
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HOW TO...
FINE-TUNE YOUR KEYING
Keying using a greenscreen backdrop is a fundamental After Effects technique. J J Johnstone reveals how to avoid the pitfalls
If your initial key isn t
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strong enough because of
an unevenly lit screen you will
need to up your tolerance. This
will probably create transparency
problems with your actors. Try
duplicating your layer so you have
the same layer playing
simultaneously on top of itself.
Now darken or lighten your
bottom layer according to the
transparent areas. You will
probably now need to up your
tolerance on one of the two layers
to avoid light halos.
J J JOHNSTONE runs the Rockadee design collective in
Portland, Oregon. He works with greenscreen daily on If your actor has a light
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projects that blur the lines between film and design. halo around him, and
Find out more about his work at www.rockadee.com. turning up your tolerance eats
away at the clean edges, try
clipping your black/white mattes
and softening your key s edges. If
Now that affordable and such as the Foundry s Keylight,
your problem isn t edges, but
consumer-friendly Ultimatte s Advantedge or Red
glasses or other objects are
compositing applications are Giant s Key Correct Pro, which
reflecting your screen colour,
freely available to everyone, can be much more effective.
duplicate your layer and delete
keying is becoming ever more Once you have a good key the key, then mask around the
specific article. Make the mask
important. There are now started, try some of these steps
loose and feather it as much as
keying plug-ins available for to fine-tune it for perfectly
possible. This will prevent the
just about every application, so realistic composites.
shiny objects from keying out.
you can effectively key any
project in just a couple of steps.
Lighting is very important TOP TIP: CHANGE BACKGROUNDS
For colour issues, spill
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suppression is usually your
for keying, and at one time or When working with a keyer, try
best bet, although the results of
another we have all ended up frequently changing your
this technique are usually subtle
with a hard shadow or poor background colour. Switch back
at best. When using spill
backlighting. No matter what and forth between darker and
suppression, set your key first
problems you run into, there are lighter shades, always using a
using a mid to darker tone of the
a couple of different ways to colour instead of black and white.
green/blue of the screen. Set your
avoid transparent actors or light This will troubleshoot the key normally, then turn it off and
add your spill suppressor. Set
halos around your characters. possibility of weird light halos or
your suppressor to the lightest
Using basic keying plug-ins colour spills appearing. If you find
possible shade of the screen and
(chroma-key) won t always give trouble areas, especially with
then turn your key back on. Adjust
you a good key from the start. shadows or highlights, try
your suppressor from there.
Instead, try utilising software masking around the area to
available for most applications, isolate it and work from there.
58 April 2006
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