Simulation - ANSYS

How are the requirements for simulation in motion
picture animation different than for engineering
simulation involving real-world problems?
The differences are enormous and primarily relate to the
end goal. For motion picture effects, the goal is to create
something that satisfies the visual expectations of the
director. If the fluid behaves inaccurately, or even unphysically,
that is not a problem if it fits within the look and story
of the film. Frequently, the story is about an unusual situation,
so unusual fluid behavior fits with that theme.
How do the fluid physics engines driving animation
software differ from those driving commercial
engineering simulation software?
At the core of the simulation code, the differences
between our simulation algorithms and those in commercial
systems are not that great. We work toward solving the
Navier–Stokes equations through a mix of Eulerian gridded
methods, Lagrangian smooth particle hydrodynamics
methods and some others that are proprietary. The two
codes diverge in the way the simulations are accessed or
driven. Animation artists work in specialized software packages
designed for their specific workflow and have the
ability to create imaginative methods of forcing the fluid and
applying boundary conditions. Also, there are a number of
post-simulation tools available either to alter the simulation
results to enhance or suppress behaviors or to use the simulation
to drive geometry, particles and volumes. We rarely
use raw simulation output in final imagery.
How much time does it take in terms of processing,
person-hours, etc., to produce a challenging fluid
simulation-based animation?
As with all visual effects elements, simulations are set
up, run and revised incrementally over an extended period,
because the goals for the simulation evolve during the
course of the work. For a single iteration, the artist sets up
the simulation conditions (initial fluid state, boundaries,
objects, forcing) and sends multiple versions of the simulation
conditions to the simulation farm, producing three to 20
variations on a simulation per day. Early in development,
setup may take a day to accomplish, but as the work
evolves, setup time is reduced to just a few minutes. By running
multiple simulations simultaneously, the artist can vary
factors such as forcing strength, viscosity,
accuracy, etc., over a short period of
time. The goal is to have a daily
cycle of simulation results, at
least for testing and development.
Late in the development,
www.ansys.com
OUTSIDE THE BOX
the simulations may be run with higher resolution or over
more frames if the scene requires it.
What fluid phenomenon is the hardest to simulate for
visualization in motion picture animation and why?
We have been able to realistically simulate liquids,
gases and fires with our tools. The most difficult problem is
to simulate very large regions of fluid at very high resolution.
Limited computing resources ultimately limit our ability to
simulate big and small scales simultaneously, which is one
of the most difficult problems in fluid dynamics as a whole.
We continue to improve our tools and hardware in order to
simulate greater volume and detail, but we also have
evolving tools for the post-processed enhancement of
simulations. These tools, implemented in a proprietary
scripting language, are based on a combination of physical
reasoning and artistic inspiration.
Can you provide an example of a challenging fluid
simulation from a recent movie?
You might not expect it, but there was a hard simulation
problem in Alvin and the Chipmunks. For a concert scene,
the chipmunks performed near and on top of a cauldron
with dry ice vapors, which had to be simulated using our
CFD system. Since the camera was very close in many
shots, we needed to simulate at high resolution, resulting in
about one week of simulation time. Also, the chipmunks
danced in the vapor, inducing compromises to the simulation
stability. In the end, since the effect was a minor
priority, we simulated at a lower resolution and then
enhanced the simulation output to achieve a nice visual
quality for the final result. ■
Jerry Tessendorf of Rhythm & Hues works on an animation sequence with fluid simulation
for Alvin and the Chipmunks.
ANSYS Advantage • Volume II, Issue 2, 2008 53