Graphics Reference
In-Depth Information
knowing what the final background will be. You can make temporary mattes for
people to use in between, but for a fully finished professional look to an edge in
a composite, you have to create the matte with the background in mind. The best
case is when the background has a similar luminance to the original plate. If I'm
compositing the foreground onto a darker background, I know there are going to be
certain problems around the edges. If I'm compositing onto a brighter background,
like a a flaring light or bright explosion, I know the background will show through the
foreground where the alphas aren't exactly 1, and that I'm going to have to do extra
work to actually get the matte. So from just a single image you're never going to be
able to pull a perfect key.
It's also important for the overall pipeline to do things procedurally as opposed to
painting alphas in by hand, since if something else changes in the composite, your
paintstrokes may not be valid anymore. When you know you're the very final step
and it's got to go out because the client's waiting to take the shot away on a hard
drive, then yes. If it's the first comp and I know there are going to be fifty iterations of
the background with camera moves and changes in color correction, then I'm going
to do it procedurally because in the long run it'll save me time.
Geoghegan: The matte could be a combination of whatever keying algorithms come
with the software package, or it could be rewiring red, green, and blue channels, or
crunching or clamping them down— theoretically anything can be a key. If a certain
keyer gets me part of the way on one foreground character, I'll keep that part and try
another approach somewhere else. It's like the matte is its own composite, and the
sooner you start seeing it that way, the better off you're going to be. It really takes a
lot of time and practice to be efficient and develop an eye for what approach to use
in each part of a plate. For example, I may say, for this guy's hair, I know this keyer
works really well for me, so I'll do a garbage matte just to affect the hair; then on the
left side of his face I can key off the red channel and the luminance, and the other
side it'll be easiest if I roto it by hand, and so on.
When it comes to keying, denoising is probably the most vital part of it. If you
just cycle through the channels in video taken from a digital movie camera, you
can often see the simulated grain. This is added to make digital video look more
like film. Our eyes love grain when we're watching movies; it's great, but if you
pull a key on a plate with film grain, you're going to end up with “chewing” and
“chatter” in the matte. You should start off with a very nice denoised plate where
you're not sacrificing detail, and generate the key using the best starting point
possible.
On the other hand, you probably won't use the denoised foreground in composit-
ing because you'll notice something's off if everything in the background is grained
and the foreground isn't. In general, you have to process the foreground after extract-
ing it. For example, some characters in the foreground may be a little further back
from the main camera than others, and you may want to sharpen up their faces. Or
the skin color of an actor may be off due to a combination of makeup and lighting,
so you may do a gentle hue shift to bring it back to a good flesh tone. You may need
to do a lot of things to really make the foreground live in the new environment; it's
never as easy as an “A over B comp.”
