Addressing grazing angle reflections in Phong models

The Phong illumination model is used extensively as it is simple with few parameters. It is however often challenging to fit such a single lobed model to the bidirectional reflectance distribution function (BRDF) of a real material, especially at grazing angles (Fig. 1). The fitting issues are in shortcomings of the model, in choosing error function, and in initial guess sensitivity [Matusik et al. 2003]. In previous work [Ngan et al. 2005], these issues were bypassed by using two specular lobes, by ignoring very grazing angles (>80°), and, in cases of unsatisfactory fitting quality, by manually restarting the fitting procedure with a different initial guess. In this work, we also fit Phong models to the BRDFs measured by Matusik et al. [2003], but we focus on the difficult grazing angles. Our result is a new Phong variant that fits better to a broader range of materials, and, for this model, we address the above-mentioned fitting issues.