Oren-Nayar Diffuse Reflectance Model

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This demo renders the Utah teapot with a surface resembling clay or plaster. It uses the Oren-Nayar reflectance model for diffuse surfaces. The model borrows the Torrance and Sparrow idea that surfaces are composed of tiny microfacets. Each microfacet can have a different orientation, so each one contributes to the output radiance. The model empirically accounts for masking/shadowing, microfacet distribution and interreflection from these microfacets. A Guassian distribution gives good results as an approximation of the microfacet distribution, so this demo allows for modifying the standard deviation parameter (to change the variance). The Oren-Nayar model does not account for specular reflection, focusing strictly on diffuse reflection. The model is widely used in the industry today.