New Microfacet Based Reflection Models

Reflection models (or BRDFs, which stands for Bidirectional Reflectance Distribution Function) are at the heart of computer graphics. This is what determines the perception of what material every object in a computer generated image is made of. There are several strategies to obtain this reflection information.

First, one can measure the BRDF. This, of course, is the most accurate and general (at least in theory) way, provided one has the equipment and material sample available. Short of this, a model of reflectance can be constructed from the first principles, based on physics of light-surface interaction. The result of this is usually rather complex. Finally, one can do what most people do - use some popular empirical model, such as Phong model. These work surprisingly well given the simplicity but usually handle a rather restricted class of surfaces and lack some desirable properties needed to describe the true reflectance correctly.

In this project we explore a class of models which is somewhere in between empirical models and truly physics based ones. These are microfacet models. We were able to make them much more general and easier to use while preserving their nice mathematical properties. The result is that we can generate a whole family of new reflection models for surfaces with non-trivial properties. The paper below describes the details.

From ACM SIGGRAPH 2000:

Ashikhmin, M., Premoze, S., Shirley, P. "A Microfacet Based BRDF Generator" The proceedings of ACM SIGGRAPH 2000, New Orleans, July 23-28, pp. 65-74. (official paper page)

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Based on this work, we were also able to create a new Phong-like reflection model which has better energy behavior, accounts for Fresnel effects and allows anisotropy. An attempt was made to preserve Phong parameters (since most graphics people are quite familiar with what it means to have, say, n=100 Phong model) and make this model as simple and efficient as possible to allow a hardware implementation some day. Who knows, maybe it will become as popular as the original Phong model. A short paper concentrating mostly on how to implement the model is below.

From the Journal of Graphics Tools:

Ashikhmin, M., Shirley, P. "An Anisotropic Phong BRDF Model" Journal of Graphics Tools, v.5, no. 2 (2000), pp.25-32.

Download PDF file (1.8 Mb)

Some images from the paper demonstrating anisotropy, Monte-Carlo friendliness and Fresnel behavior. Click to see a full resolution picture:

A Tech. Report UUCS-00-014 giving more justification to the model and describing its connection to microfacets can be found here (2.0 Mb).