Realistic image synthesis in computer graphics relies heavily on global illumination (GI) to simulate indirect lighting. Computing GI remains one of the most computationally demanding tasks. Traditional approaches like Instant Radiosity (photon mapping) generate virtual point lights (VPLs) to approximate indirect lighting, but become inefficient in complex scenes due to reliance on uniform light sampling. As the number of VPLs grows, this method leads to increased noise and reduced performance.
In this paper we explore the integration of ReSTIR DI (Reservoir-based Spatiotemporal Importance Resampling for Direct Illumination) with photon-mapped VPLs to improve the efficiency and quality of global illumination. By leveraging ReSTIR’s ability to reuse and resample light paths spatially and temporally, the aim is to address the scalability issues of uniform sampling and enable practical, real-time GI rendering.
The approach is evaluated using three test scenes of varying complexity: Cornell Box, Sahur, and a detailed living room scene. Performance metrics include root mean squared error (RMSE) comparison against a reference and average frame times across different sampling methods. Results demonstrate that ReSTIR with photon mapping achieves significantly lower RMSE values and faster convergence compared to uniform sampling, with an average RMSE improvement of 57.4% across all tested scenes. The method shows particularly strong improvements in visually complex scenes, with ReSTIR consistently producing the cleanest results at 1 sample per pixel. However, the integration within the current implementation introduces computational overhead, with frame times increasing by 4-7 times compared to uniform sampling, and occasional blotting artifacts in regions with high VPL density. Despite these limitations, the work establishes that ReSTIR DI can be successfully extended to handle photon-mapped global illumination, providing a foundation for improving VPL-based rendering efficiency while maintaining visual quality.