Refined tri-directional path tracing with generated light portal

The rendering efficiency of Monte Carlo path tracing often depends on the ease of path construction. For scenes with particularly complex visibility, e.g. where the camera and light sources are placed in separate rooms connected by narrow doorways or windows, it is difficult to construct valid paths using traditional path tracing algorithms such as unidirectional path tracing or bidirectional path tracing. Light portal is a class of methods that assist in sampling direct light paths based on prior knowledge of the scene. It usually requires additional manual editing and labelling by the artist or renderer user. Tri-directional path tracing is a sophisticated path tracing algorithm that combines bidirectional path tracing and light portals sampling, but the original work lacks sufficient analysis to demonstrate its effectiveness. In this paper, we propose an automatic light portal generation algorithm based on spatial radiosity analysis that mitigates the cost of manual operations for complex scenes. We also further analyse and improve the light portal-based tri-directional path tracing rendering algorithm, giving a detailed analysis of path construction strategies, algorithm complexity, and the unbiasedness of the Monte Carlo estimation. The experimental results show that our algorithm can accurately locate the light portals with low computational cost and effectively improve the rendering performance of complex scenes.