A System and Method for Measuring Spatially Varying Surface Appearances with a Study of Feathers

Real-world materials, particularly biological structures such as feathers exhibit complex appearances that vary spatially across their surfaces. The field of computer graphics provides a means of understanding such surfaces through material modeling which uses both analytical models and data acquired from light-surface interactions. There are many efforts within the past decade in measuring materials for graphics, but common limitations in these works include not accounting for spatially varying properties and reliance on neural networks and synthetic datasets. Feathers from modern birds present diverse appearances due to how light interacts with their unique hierarchical microstructures. Variations in those structures lead to a variety of appearances at the macroscale that inherently require a graphical model which varies spatially across the surface. In order to acquire data to help create such models, I developed software and methods to use a motorized, universal stage for sample orientation within a custom spherical illumination system, the Variable Illumination Sphere (VarIS). I also propose a pipeline for predicting, measuring, and rendering spatially varying appearances using VarIS or other systems such as goniophotometers. I take inspiration from Dupuy & Jakob's adaptive-parameterization technique in measuring Bidirectional Reflectance Distribution Functions (BRDFs), extending their approach to support spatially varying measurements from VarIS. I additionally present methods for analyzing and preprocessing data using distribution metrics and clustering that can reduce necessary capture time and storage. I acquired spatially varying appearances of a collection of feather samples across all major systematic groups of living birds, performing an analysis and fitting of the material data to achieve a relatively lightweight representation of feather light-scattering behaviors. My work presents an approach to capture complex, spatially varying appearances more efficiently and provides data for efforts in other disciplines to further understand the appearance of feathers.