Acquisition of Multispectral Bidirectional ReflectanceDistribution Functions

Colin Braley, Jason LawrenceUniversity of Virginia

Background

Goal

SetupBRDF BasicsA BRDF describes how an opaque material reflects light. BRDFs are used in computer graphics to model the appearance of virtual objects. For a ray of light arriving in the direction ωi, the BRDF tells us how much light is reflected back in direction ωo. In general, a BRDF is a 5 dimensional function. We consider the more common isotopic BRDF, which is a function of 4 variables.

Measured BRDFs of real world materials allow users in areas such as visual effects and architecture to create highly realistic images of virtual worlds.

Spectral RenderingHumans posses tristimulus vision. As a result of this remarkable property, the sensation of color is describable by only 3 numbers. The RGB colorspace is one such description.

Computer graphics software aims to simulate the behavior of light to generate synthetic images. Commonly this simulation is done on RGB colors. This is incorrect, and a full spectral simulation should be used. Conversion to RGB images for human viewing should be a final step.

RGBRendering

FullySpectral

Rendering

Our goal is to measure the spectral BRDFs of many real world materials. Available BRDF data is sparsely sampled in either the spectral or spatial domain. We hope to produce the first database of many multispectral BRDFs, with dense sampling in both the spatial and angular domains.

We use an image based technique, first proposed by Marschner et al[2]. This allows us to take images of spherical targets, and convert each pixel into a measurement. This reduces acquisition time, as multiple measurements are captured in parallel.

We take many images of a spherical material sample as a light source moves around a 180 degree arc.

Related Work1 - "A Data-Driven Reflectance Model", Wojciech Matusik et al., ACM Transactions on Graphics 22, 3(2003), 759-769.2 – “Image-based BRDF Measurement” Stephen R. Marschner et al. Applied Optics, vol. 39, no. 16 (2000).

For calibration, we have to measure the following: 1) Spectral transmission of our camera and LCTF 2) Spatial sensitivity of our camera 3) Spectrum of our light source

Calibration

Spectral ReconstructionGiven 30 spectral measurements, we need to reconstruct the underlying spectrum.

12 bit QImaging Retiga 4000R CCD camera

Varispec VISLiquid CrystalTunable Filter

Top View

Spectral Transmission

Our camera is equipped with a Liquid Crystal Tunable Filter(LCTF) to selectively choose which wavelengths to allow into the camera.

Spatial Sensitivity

Images of Isotropic BRDFs from the MERL BRDF Database[1]