Implementing One-Click Caustics in Corona Renderer

Abstract

This paper describes the implementation of a fully automatic caustics rendering solution in Corona Renderer. The main requirement is that the technique be completely transparent to the user, should not need any parameter setting at all, and be fully integrated into the interactive and progressive rendering workflow. We base our approach on an efficient subset of the vertex connection and merging algorithm, specifically a multiple importance sampling combination of path tracing and photon mapping. We rely on Metropolis sampling to guide photon paths into the relevant parts of the scene. While these underlying ideas have appeared in existing research work, numerous previously unaddressed issues and edge cases arise when one applies these ideas in practice. These include unreliable convergence of the Metropolis sampler in scenes with many light sources of different sizes and intensities, the ''caustic in a stadium'' problem (i.e., efficient rendering of small caustics in extremely large scenes), etc. We present the solutions we have developed to address such issues, yielding what we call ''one-click caustics rendering''. User feedback suggests that our approach substantially improves usability over methods previously implemented in comercially available software, all requiring the user to set various technical parameters.