On Radiance materials - Measurements

Getting materials into Radiance

Most commonly, material parameters are guessed with minimal assumptions about the physical material properties. Comparing the rendered image with reality and fine tuning parameters with some experience works pretty well and effective. At least for scenes with fixed lamps and fixed observer.
To come up with a material model that behaves well for various incident and observer directions proves difficult without help from physics and measured data.

Raw data

To model surfaces, rho_dh and tau_dh are only useful as first approximation, sine they don't specify what the angular distribution of the reflected/transmitted light is. Sadly, in most cases that's what is solely available from manufactures. Special measurement devices (e.g. pab gonio-photometer or FhG-ISE gonio-photometer) give the full information needed, or more exact, they measure the BRTF of the surface in question averaged over the solid angles of a detector and light source at specific angles xⁱ_in, x^j_out.
Gonio-Photometers differ in angular resolution, solid angles of source and detector, dynamic range of the detector and whether they do relative or absolute measurements. Broadly scattering materials are easy to measure, shiny materials with very "peakish" BRTFs are trickier and need higher classes of instrument precision.
Measurements are typically done by adjusting the incident direction and taking measurements for different outgoing angles, than stepping to the next incident direction and repeating the process. Outgoing resolution can be very fine, up to 100000 directions on the hemisphere. The more "peakish" the BRTF, the finer the angular resolution needed to resolve the peaks. The incident directions depend on symmetries of the BRTF and vary typically in steps of 5-10^o.
For one incident direction a file with measured BRTF values of the above mentioned ISE-FhG gonio-photometer looks rather trivial:

#using beam file: pab-aluA-st1:beam.mpc.refint 0.063626
#measurements done at Fraunhofer Institute for Solar Energy Systems, FhG-ISE
#contact [email protected] for details
#...
#small beam, no-lense detector
#input aperture used: def
#detector aperture used: def
#incident angle theta [degree]:
30
#incident angle phi [degree]:
#0
#backgrounds: left side lamp on/off, right side lamp on/off, [uA]
#background2: 2.73701e-06 2.57384e-06
#theta phi beam-ref org-data minus-background/beam-ref cosine-out-corrected brtf voronoi-cell-size
95.5176 119.535 9.9e+37 5.14761e-06 -1.60952e-05 -0.000167394 -0.00263091 0.0351119
95.526 240.467 9.9e+37 1.96535e-06 -1.92775e-05 -0.000200187 -0.0031463 0.0351942
96.2589 235.494 9.9e+37 2.20671e-06 -1.90361e-05 -0.000174609 -0.0027443 0.0110056
96.2645 124.505 9.9e+37 5.79507e-06 -1.54477e-05 -0.000141569 -0.00222501 0.0110376
96.9904 230.514 9.9e+37 2.04376e-06 -1.91991e-05 -0.000157753 -0.00247938 0.0119793
97.0222 129.481 9.9e+37 0.000205099 0.000183856 0.00150389 0.0236364 0.0120372
...

with the outgoing direction given as theta_out, phi_out in column 1 and 2 and the absolute BRTF in column 7.
Directly using raw measured data in Radiance (e.g. as brightdata) is not advisable for two reasons: raw data files are large and more important, they don't provide a sound way to interpolate between directions: Interpolation for x_out is typically done by indexing into an array and works only for data values on a regular x_out grid, which is either very coarse or even larger than the original measured data. ¹
Interpolation for x_in is even harder: The shape of the BRTF for an x_in given e.g. as (theta_in, phi_in)=(45,0) is only the average of the BRTF measured at (40,0) and (50,0), if the BRTF is totally flat and the material is ideal diffuse. For all other materials, the BRTF shape changes, typically its maximum in x_out moves with x_in, and the interpolation would have to be done in a much smarter way. By the way, the same problem holds for all compression techniques of the BRTF in x_out using a non-physical motivated, general method, e.g. spherical harmonics or wavelets.

Fitting of BRTF models

The idea described in 1995 by the author is quite simple and works in two steps: First a reasonably decent model of the BRTF and its dependency on incoming and outgoing directions is fitted with a minimum of parameters to the measured BRTF in x_out, while x_in is kept constant. This works for data measured on a regular grid or on an adaptive one, and results in some (approx 2-6) parameters for each x_in. For consecutive x_in, e.g. (theta_in,phi_in)=(40,0),(50,0),(55,0) , these parameters should be well behaved and depend smoothly on variations in theta_in, phi_in . Interpolation between x_in is thereby solved.
Obviously, this depends on a good choice of the "reasonably decent model" in the first place. This choice is not easily automated for new classes of materials with different BRTF types, which is the main drawback of this idea.

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¹ Radiance could, in theory, use the indexing function of brightdata to generate an index with a higher angular resolution in specific regions of the hemisphere, but I don't see a decent reason someone wants to take the hassle of synchronizing the mapping function and the data values in a general and automatic way, especially because the next problem is much trickier and important anyway.

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Last modified Monday September 02, 2002