Graphics Reference
In-Depth Information
RJR: What about the workflow when you need to pull a matte on a non green-screen,
natural background?
Lambert: A really good example was a movie set during World War II called
Flags of
Our Fathers
. The productionmade a conscious decision that theyweren't going toput
green or blue screens anywhere. We took on the show knowing that it was going to be
a bunch of roto — people running around, motion blur, and so on. If it's a locked-off
shot, that is, the camera isn't moving, you may be able to get some of the matte with
a difference matte, if you have a clean plate — but that will only get you to a certain
point. The only way to really extract it correctly is to roto it — actually draw Bezier
splines around the foreground. For example, if I had to roto a person off of a natural
background, I'd be drawing separate shapes for the person's head, their arm, their
body, and so on for a bunch of keyframes spread across the sequence. I might put
keyframes every three frames — so go three frames, adjust the splines, three frames,
adjust, play it back and see if it's actually matching, and then apply extra keyframes
just tomake it a bit better. Then you can generate the alpha by adding a softer edge to
the roto curves or looking at their direction and speed of motion, and youmay need to
animate that softer edge onaper-framebasis. It's a very time-consumingprocess! You
would hope that if the production knows in advance that they'll need to extract a per-
son with flowing hair from a plate that they'll put a green screen behind them, since
to extract them otherwise it'll be very labor-intensive and cost half a million dollars.
Scribble-based kinds of approaches tend to work really well on a still frame, but
it's when you go to video imagery where it's changing frame to frame and you have
film grain structure to it, you find that those kind of algorithms jump and around and
flicker.
Sloan: Very often they'll end up having to use a clean plate that is either generated
or captured on set. But sometimes there will be people whose job it is to matte every
wisp of hair that has gotten outside of the blue screen area or for whatever reason
can't be extracted very well. You typically go to a roto artist and they bring everything
they have to bear on the problem — which is typically patience and skill at drawing
those outlines. I have a feeling that whenever people say “we used our proprietary
software to solve this tough matting problem,” that means a bunch of hard-working
roto artists! Sometimes you end up having to make entirely computer-generated
versions of natural foreground elements that for some reason you didn't get properly
in camera and you have to generate from scratch instead of trying to matte.
The natural matting problem is a huge issue for 3D conversion of movies filmed
with a single camera. They basically have to rotoscope everything, pulling out all the
foreground objects and assigning themdifferent depths for the two eyes. Colorization
of old black-and-white movies is a similar problem. Often the heart of 3D conversion
algorithms is a planar tracker that allows you to interactively create and push forward
very precise roto shapes on each frame. The software package called
mocha
is one
very popular example of that kind of planar tracker. The output of it is something
similar to a trimap that the artist can use to synthesize motion blur or some kind of
fall-off to get the final alpha. The holy grail for 3D conversion is “decompositing”—to
take any foreground object, however nebulous or wispy or fragmented, and extract
only it from the scene.
