A Surface-based Appearance Model for Pennaceous Feathers

The appearance of a real-world feather is the result of a complex light interaction with its multi-scale biological structure including the central shaft, branching barbs and interlocking barbules on those barbs. In this work, we propose a practical appearance model for feathers encoded as 2D textures where the overall appearance is a weighted BSDF of the implicit representations of the key biological structures. This BSDF can be applied to any surface and does not require the explicit geometrical modeling of the internal microstructures (barbs and barbules) as in previous works. Our model accounts for the particular characteristics of feather fibers such as the non-cylindrical cross-sections of barbules and the hierarchical cylindrical cross-sections of barbs. To model the relative visibility between barbs and barbules, we derive a masking term for the differential projected areas of the different components of the feather's microgeometry, which allows us to analytically compute the masking between barbs and barbules without costly Monte Carlo integration.