ABSTRACT

Current models for feathers in computer graphics are presently insufficient for achieving realistic, physically-based renderings. The complex and directionally-variable visual appearance of feathers originates from the interaction between light and two classes of structure: the feather surface and the subsurface volume. The overall shape of the feather vane and of its component members and the character of their surfaces scatter light according to principles of geometric optics. However, subsurface nano-structure in many feathers generate so-called “structural coloration” , which is a purely physical optics phenomenon and can differ drastically from ordinary coloration mechanisms such as pigmentation. Iridescence, from which many feathers derive their vivid, eye-catching changeable color, is one classification of structural color which varies as a function of viewing angle.

    This thesis presents directional scattering measurements from select samples of structurally-colored bird plumage. The macro-surface, meso-surface, micro-surface, and sub-surface structures each play a role in the observed scattering function over the visible spectrum, producing critical appearance characteristics such as anisotropic reflection and iridescent coloration from a single layer of keratin. Previously published feather models have focused primarily on procedurally generated pennaceous feather geometry. The branching structure was assumed to be opaque and shading limited to rudimentary or idealized geometric refection models, i.e. Lambertian and Phong. As yet, no computer graphics feather model has been based upon or referenced directional scattering measurements. With few exceptions, previous work in iridescence in the field of graphics has tended to focus on non-biological cases, i.e. idealized, multilayer thin films which produce a classic rainbow of color. The only known biological cases have specialized in multi-layer-pair biological interference which produces a single dominant spectral peak.

    Our study will be the first to acquire and provide the necessary data required for an effective feather scattering model. Furthermore, we place our directional scattering measurements in the context of biological morphology, which will facilitate a more accurate and efficient result. We present five case studies in directional scattering and iridescence based on four bird species. The breadth of the phenomena studied is significant for biology as well as for computer science: macroscopic study; diffuse reflectance; specular reflectance of single, double and triple lobes; and iridescence from single layer “thick” film. The results of these analyses should lead to a more thorough understanding of the relationships between specular reflection, color, iridescence, and the structure of a feather itself and will provide a guide to future, predictive, scientific feather scattering models.