Pixar faced some unique challenges in bringing their Scottish river to
life in the film "Brave". Not only did they have to construct,
simulate and render a convincing and beautiful mountain river with a
waterfall, they also had to have a girl and a huge bear interact with
it. At their presentation during the "Wild Rides" talk at SIGGRAPH
2012 they gave some insights into how they approached this situation.
One main problem with simulating any body of freely flowing water is
that the simulation resolution, the fineness of detail, needed to
provide a convincing interaction with character animation is very
expensive. The coarseness of what might work to capture the feel of
the overall river set location wouldn't be nearly fine enough to
capture the nuances of how the water needs to flow around the
characters. And the resolution needed to capture the character
interactions would be prohibitively expensive to use on the larger
scale. Pixar solved this with the tried and true technique of divide
and conquer.
How they approached this was ingenious.
They defined regions where they needed higher resolution, for instance
around the characters, and then defined what they called a "Windowed
Simulation" in that spot. Essentially it involves plopping a finer
detail resolution simulation into the middle of the larger scale
simulation and using the conditions of the larger to control the
smaller. Basically the boundary around the finer simulation gets data
from the surrounding simulation and interpreted it as a series of
faucets and drains that maintain the flow of water into and out of the
region. They did this using the "PhysBam" physics computing engine.
They found they had to add some additional artistic controls, like
virtual hot-tub jets, to make all of the mathematical accuracy look
good though. The result was a river of water that looked as though it
had been simulated at an impossibly high resolution, and with even
greater flexibility.
Scott Singer can be reached at: scott.m.singer@gmail.com.