[info] eurekalert: SIGGRAPH: bottom up rendering of non-homogeneous particles in absorbing mediums

[Alejandro Dubrovsky]

(
http://www.eurekalert.org/pub_releases/2007-08/uoc--cgs080607.php
)

Contact: Daniel B. Kane
dbkane at ucsd.edu
858-534-3262
University of California - San Diego
Computer graphics spills from milk to medicine
Rendered images of different types of coastal and oceanic waters of
increasing (left to right) mineral and algal content. Atlantic,
Mediterranean, Baltic and North Sea.
Click here for more information.

A new UC San Diego computer graphics model capable of generating
realistic milk images based on the fat and protein content will likely
push the field of computer graphics into the realms of diagnostic
medicine, food safety and atmospheric science, according to a new study.

“Computer graphics is no longer just about pretty pictures and realism
for the sake of aesthetics. We have harnessed the math and physics
necessary to generate realistic images of a wide range of natural
materials based on what they are made of. With our approach, computer
graphics can contribute to a handful of pressing problems,” said Henrik
Wann Jensen, a UC San Diego computer science professor and Academy Award
winning computer graphics researcher. Jensen created the model with two
colleagues from the Technical University of Denmark – Niels Jørgen
Christensen, an associate professor, and Jeppe Revall Frisvad, a Ph.D.
student.

On August 8, 2007, the new graphics research will be presented at the
Association for Computing Machinery’s SIGGRAPH conference, the premier
annual conference for the graphics and interactive techniques community.

If you tell the new computer graphics model how much fat and protein you
want in your milk, the model will spit out the information you need to
create a life-like milk image by determining how light will interact
with your specified ratio of milk fats and proteins. Similarly, if you
specify the concentration of algae and different minerals in a sample of
ocean water, the same theoretical model will render the color of the
water.

The new work extends well beyond milk and ocean water to a wide range of
materials called “participating media.” The word “participating” refers
to the fact that some of the light that hits the material is absorbed
and not reflected.

In addition to creating images based on what the material is made of,
the authors used the milk example to show that the new model can work
backwards and determine how much fat and protein a sample of milk
contains, based on just a digital picture of the milk.

“Putting the model in reverse, grocery stores could identify spoiled
meats, contaminants or other food safety issues – if a particular food
problem consistently and detectably changed the light scattering
properties of the food,” said Jensen.

The model has already provided insights into the mystery of
“bottle-green” icebergs. In the SIGGRAPH paper, the authors show that
their model agrees with the claim that bottle-green icebergs are, in
fact, clean (non-green) icebergs that appear blue during the day but
turn green as the sun sets.
A blue iceberg at noon turns green in the evening. These bottle green
icebergs are one of nature's peculiarities. The authors simulated the
properties of the green iceberg by including...
Click here for more information.

The new research eliminates a long-standing roadblock and describes a
way to use theoretical math and physics to generate realistic computer
graphics of materials that absorb some light and are not made of perfect
spheres. In other words, the paper marks a vast extension of the
Lorenz-Mie theory, which is a complete solution to Maxwell’s equation
for scattering of electromagnetic waves by a homogenous, spherical
particle embedded in a non-absorbing medium. The Lorenz-Mie theory has
been around for more than a century and was introduced to graphics in
1995. In computer graphics, it has been used to compute the optical
properties of certain paints, plastics, clouds, atmospheric phenomena
and other media that can be described as spherical particles embedded in
a non-absorbing medium. The theory, however, has not been used to render
the vast class of participating media, until now. In the past,
determining the optical properties of participating media necessary for
rendering realistic images has required extensive measurements and/or
trial-and-error manual adjustments.

The new model eliminates the restrictions of the Lorenz-Mie theory for
participating media such as milk, in which the embedded particles
(primarily proteins, fat and vitamin B2) are not perfect spheres and the
host medium (water) absorbs light.
Rendered images of the components in milk as well as mixed
concentrations. From left to right, the glasses contain: Water, water
and vitamin B2, water and protein, water and fat,...
Click here for more information.

“If you are trying to generate realistic computer graphics, it is
intuitive to specify what the material is made of, like the amount of
fat and protein in the milk. In the past, we had to do a lot of
observing and measuring to determine how the milk would scatter and
absorb light,” said Jensen. “With our theoretical approach, a much wider
range to people will be able to play a role in creating realistic images
of natural materials.”

“We can visualize what specific particles – like milk fats or proteins –
look like on their own. We can also visualize what a medium would look
like if one particle type were missing. Having knowledge about the
visual effect caused by each type of particle is incredibly valuable in
many different contexts,” explained Jeppe Revall Frisvad, a Ph.D.
student from the Technical University of Denmark.

The new work may also help Jensen and colleagues extend their
theoretical model for human skin, work that earned Jensen an Academy
Award in 2004.

In 2006, at SIGRAPH, Jensen and UCSD computer science graduate student
Craig Donner published a paper describing a model that generates
realistic looking skin based on specified amounts of hemoglobin and
melanin.

Jensen is now looking to extend the skin shading model so that it can
predict the appearance of skin based on a detailed description of dermal
structures other than hemoglobin and melanin. Understanding how
structures within the skin scatter and absorb light could be important
to the doctors who are using light to treat skin cancer and other skin
diseases, Jensen explained.

Some of the inspiration for their model being presented at SIGGRAPH 2007
came from equations that physicists studying coated soot particles
derived to deal with shortcomings in the Lorenz-Mie theory. Jensen and
the students from Denmark also called upon their mother tongue, Danish,
to pick apart a little-known paper published in 1890 in Denmark by the
physicist Ludvig Lorenz, one of the two namesakes of the Lorenz-Mie
theory. The report clearly outlined the math and many of the assumptions
that are central to Lorenz-Mie theory.

“Going through his assumptions carefully, we hit a series of ‘ah-hah!’
moments in which we saw ways to address some of the assumptions that had
made it difficult to generalize the Lorenz-Mie theory beyond perfect
spheres,” said Jensen.

Once their new theoretical model predicted the relevant physics for
light hitting participating media, the researchers took the next step
and made the results useful for computer graphics by determining how the
light would actually scatter and be absorbed. It is this information
that is used to render realistic images.

“We have the first complete, bottom-up theoretical model that addresses
the shortcoming of the Lorenz-Mie theory for participating media. It
allows us to render computer graphics for absorbing materials and with
non-spherical particles based on the contents of the material. I can’t
wait to see how others implement our model,” said Jensen, who is making
the model available to other researchers.
###

Media Contact:
Daniel B. Kane
UCSD Jacobs School of Engineering
858-534-3262 (office)
858-926-8664 (cell)
dbkane at ucsd.edu


Contacts for the computer scientists:

Henrik Wann Jensen
Henrik at cs.ucsd.edu

Niels Jørgen Christensen
njc at imm.dtu.dk

Jeppe Revall Frisvad
jrf at imm.dtu.dk

“Computing the Scattering Properties of Participating Media Using
Generalized Lorenz-Mie Theory,” by Jeppe Revall Frisvad, Niels Jorgen
Christensen at the Technical University of Denmark and Henrik Wann
Jensen at University of California, San Diego

The paper is available at:
http://graphics.ucsd.edu/~henrik/papers/