Rapid movement of heated gases generates high-frequency sounds that are an important part of the sound of fire, but computer-generated images don’t simulate those details. Cornell researchers synthesize low-frequency sounds to match the graphics, then map in the highs based on the sounds of real fire. Photo: Doug James |
Computer-generated
imagery usually relies on recorded sound to complete the illusion.
Recordings can, however, limit the range of sounds you can produce,
especially in future virtual reality environments where you can’t always
know ahead of time what the action will be.
Doug
James, associate professor of computer science at Cornell University,
develops computer algorithms to synthesize sound “on the fly” based on
simulated physics models. His research group has already invented sound
synthesis methods for splashing fluids, fracturing glass and other
solids, and noisy thin shells such as sheet metal.
Now
they have devised methods for synthesizing more realistic sounds of
hard objects colliding and the roar of fire. They report their latest
work at the 2011 ACM SIGGRAPH computer graphics conference in Vancouver,
Canada, Aug. 7-11.
To
synthesize collision sounds, the computer calculates the forces
computer-generated objects would exert if they were real, how those
forces would make the objects vibrate and how those vibrations transfer
to the air to make sound.
Previous
efforts often assumed that the contacting objects were rigid, but “in
reality, there is no such thing as a rigid object,” the researchers say.
Objects
vibrate when they collide, which can produce further chattering and
squeaking sounds. Resolving all the frictional contact events between
rapidly vibrating objects is computationally expensive. To speed things
up, their algorithm simulates only the fraction of contacts and
vibrations needed to synthesize the sound.
“If
you set a box on a flat surface there are many contacts,” James
explained. “We identify a small set of physically faithful contacts, the
simplest set that will get the job done.” Similarly, the algorithm
selects from many different ways the objects can vibrate, ignoring those
that produce inaudible sounds.
Demonstrations
include the sound of a ruler overhanging the edge of table and
“buzzing” when plucked, pounding on a table to make dishes clatter and
ring and the varied sounds of a Rube Goldberg machine that rolls marbles
into a cup that moves a lever that pushes a bunny into a shopping cart
that rolls downhill.
Fire
is animated by mimicking the chemical reactions and fluid-like flow of
burning gases. But flame sounds come from things that happen very
rapidly in the expanding gases, and computer animators do not need to
model those costly details to get good-looking flames.
“We
can simulate the low-frequency sound for flame animations, but not all
the fiery details,” James explained, “so we rely on models based on real
fire sounds to paint the fiery details onto the low-frequency sound.”
Pound your fist on a table and the noise will include chattering and scraping without which synthesized sound will not seem real. Photo: Doug James |
This
keeps the cost down and has the added benefit of providing “style
controls” based on what type of real flame sound is used as input data.
The
researchers demonstrated with a fire-breathing dragon statue, a candle
in the wind, a torch swinging through the air, a jet of flame injected
into a small chamber and a burning brick. The last simulation was run
with several variations of the sound-synthesis method, and the results
compared with a high-speed video and sound recording of a real burning
brick.
The
flame sounds still lack some of the “whoosh” in the middle frequencies,
the researchers noted, and the process is not yet ready for real-time
applications, needing several hours to synthesize fire sound for a short
video clip, but they say more powerful parallel hardware will help.
The
research was supported in part by the National Science Foundation, The
Natural Sciences and Engineering Research Council of Canada and Intel.
