Acquiring Scattering Properties of Participating Media by Dilution

Srinivasa G. Narasimhan (CMU) Mohit Gupta (CMU) Craig Donner (UCSD)
Ravi Ramamoorthi (Columbia University) Shree Nayar (Columbia University) Henrik Wann Jensen (UCSD)

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 input (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 very few methods that measure or estimate these properties for media that are of relevance to computer graphics. In this paper, we present 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 estimation algorithm. Furthermore, unlike previous approaches that require complicated or separate measurement setups for different types or properties of media, our method and setup can be used to measure media with a complete range of absorption and scattering properties from a single HDR 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 forty commonly found materials, that can be immediately used by the computer graphics community. We can also create realistic images of combinations or mixtures of the original measured materials, thus giving the user a wide flexibility in making realistic images of participating media.


"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 Transactions on Graphics (SIGGRAPH 2006),
August, 2006.




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 was 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.


Original Photographs of Dominantly Scattering Materials:     

The acquired images of the 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 – heavy scattering results in blurring of the bulb image. The extent of blurring is proportional to the amount of scattering exhibited by the medium.


Original Photographs of Dominantly Scattering Materials:         

Colored bulb image and absence of glow signify highly absorbing media – the color of the bulb is an indicator of the relative properties across different color channels – red, green and blue.



Estimation Quality:  

Here we compare plots of captured image brightness to profiles reconstructed using the estimated parameters. The accuracy of fits for a variety of media, having a wide range of scattering and absorption properties, indicates accurate estimation of parameters for all three color channels – red, green and blue.



The estimated parameters can be readily used to render photo-realistic images at arbitrary concentrations of the materials with multiple scattering, using a standard physics based volumetric rendering algorithm.  These images have been rendered using Volumetric Monte-Carlo Path Tracing algorithm.


Renderings: 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.


Renderings: Dissolved Powders

Similarly, powdered materials dissolved in water can be rendered using the recovered parameters. Minor noise in the renderings is due to Monte-Carlo Under-sampling.



Renderings: with "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 to reproduce realistic appearance of many liquids. Notice the bright red color of Merlot wine, and the soft yellow of chardonnay.  


Renderings: Blending Materials

By mixing parameters for 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.



Transition between Materials:

By scaling and blending scattering parameters, we can freely adjust material concentration, and interpolate between measured materials, simulating their mixture. This video shows the gradual transition from wine to water to milk to espresso.


Increasing Material Concentrations:

For each media, 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 increasing concentration. On the other hand, Red wine is a highly absorbing liquid, showing only a saturation of the bulb color with increasing concentration, and no blurring.