My master thesis on fluorescence in bidirectional rendering
Fluorescence - the effect of a photon being absorbed at one wavelength and re-emitted at another - is present in many common materials such as clothes and paper. Yet there has been little research in rendering fluorescence, and previous projects are all based on a simple forward path tracer.
This thesis shows how fluorescence can be simulated by a bidirectional path tracer. We propose a simple model for fluorescent BRDFs that can be parameterized by physical measurements of fluorescent spectra, and demonstrate how this BRDF can be included in a bidirectional path tracer. The proposed algorithm samples vertices and wavelengths at the same time and connects vertices to produce a geometrically valid path. If the path contains fluorescent surfaces, the wavelengths of the camera and light sub-path will not match, and the algorithm resets wavelengths along the geometric path to generate a valid path with consistent wavelengths and a well defined measurement contribution for each wavelength combination. We also present an alternative approach where camera and light sub-paths are sampled independent of wavelengths and then are evaluated for the full spectrum instead of one random wavelength per path segment.