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The visual world around us displays a rich set of volumetric effects
due to participating media. The appearance of these media is governed
by several physical properties such as particle densities, shapes and
sizes, which must be provided (directly or indirectly) to a rendering
algorithm to generate realistic images. While there has been
significant progress in developing rendering techniques (for instance,
volumetric Monte Carlo methods and analytic approximations), there are
few methods for measuring the scattering properties of a medium.
In this project, we have developed a simple device and technique for
robustly estimating the properties of a broad class of participating
media that can be either (a) diluted in water such as juices,
beverages, paints and cleaning supplies, or (b) dissolved in water such
as powders and sugar/salt crystals, or (c) suspended in water such as
impurities. The key idea is to dilute the concentrations of the media
so that single scattering effects dominate and multiple scattering
becomes negligible, leading to a simple and robust algorithm for
estimating the scattering parameters. Furthermore, unlike previous
approaches that require complicated or specialized measurement setups
for different types or properties of media, our method and setup can be
used to measure media with a wide range of absorption and scattering
properties from a single high dynamic range photograph. Once the
parameters of the diluted medium are estimated, a volumetric Monte
Carlo technique may be used to create renderings of any medium
concentration and with multiple scattering. We have measured the
scattering parameters of 40 commonly found materials that can be
immediately used for rendering.
We can also create realistic images of mixtures of the original
measured materials, thus giving the user significant flexibility in
creating realistic images of participating media. This project was done
in collaboration with Craig Donner and Henrik Wann Jensen at UC San Diego, and Ravi Ramamoorthi and Shree Nayar at Columbia University.
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Publications
"Acquiring Scattering Properties of Participating Media by Dilution,"
S. G. Narasimhan, M. Gupta, C. Donner, R. Ramamoorthi, S. K. Nayar, and H. W. Jensen,
ACM Trans. on Graphics (also Proc. of ACM SIGGRAPH),
Jul, 2006.
[PDF]
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Pictures
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Experimental Setup:
This picture shows the
measurement apparatus used in our experiments. The 25 cubic cm tank is
made of transparent anti-reflection coated glass and contains the
scattering medium (for example, dilute milk). The material is
illuminated by a small frosted bulb fixed to the side of the tank. A
12-bit Canon EOS-20D digital camera was used to capture approximately
orthographic images of the tank.
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Original Photographs of Dominantly Scattering Materials:
These
acquired images of a diluted set of liquids are indicative of the
scattering properties of the corresponding media. For example, highly
scattering media show a glow around the bulb since light scattering
results in blurring of bulb radiance. The extent of blurring increases
with the amount of scattering exhibited by the medium.
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Original Photographs of Dominantly Absorbing Materials:
The
negligible amount (or absence) of glow in these images indicate that
the media are highly absorbing. The color of the bulb is an indicator
of
how the absorption varies across the three color channels – red, green
and blue.
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Estimation Quality:
Here we compare plots of
captured image brightness to profiles reconstructed using the estimated
parameters. The accuracy of the fits (for the three color channels) for
a variety of media, having a wide range of scattering and absorption
properties, indicates the robustness of the
estimation algorithm. |
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Renderings of Strongly Absorbing Materials:
A set of strongly absorbing liquids are lit by a single directional
source in order to highlight the bright caustics. Caustic effects are
created using Photon Mapping. Notice the bright color of the caustics
and the liquid itself, characteristic of strongly absorbing media. The
images have been tone-mapped to reproduce the full dynamic range
visible to the human eye. |
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Renderings of Dissolved Powders:
Similarly, powdered materials dissolved in water can be rendered using the recovered parameters.
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Renderings with a "Kitchen" Environment Map:
In daily life, participating media are usually viewed in complex
lighting environments. These renderings use Debevec et al.’s kitchen
environment map. Notice the bright red color of the Merlot wine, and
the soft yellow color of the Chardonnay. |
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Renderings of Blending Materials:
By mixing parameters of materials -- milk and espresso, in this case --
we can obtain the light brown color of light coffee. Note that a simple
interpolation of images doesn’t produce the desired result. |
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Renderings of 4
liquids at their measured concentrations.
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Renderings of the above 4
liquids at their natural concentrations.
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Videos
(Video Result Playlist)
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SIGGRAPH 2006 Video:
This video is a compilation of the main results of this project.
(Apple Quicktime 6.0). |
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Transition between Materials:
By scaling and blending scattering parameters, we can freely adjust
material concentrations and interpolate between measured materials,
simulating their mixture. This video shows the gradual transition from
wine to water to milk to espresso.
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Increasing Material Concentrations:
For each
medium, we capture images at several different degrees of dilution.
This video shows the scattering and absorption effects as the
concentrations of wine and milk are increased in our experimental
apparatus. Milk is a highly scattering medium. Hence, we observe an
increase in blurring with concentration. On the other hand, red wine is
a highly absorbing liquid, exhibiting saturation, rather than blurring,
of the bulb color with increasing concentration. |
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