Architectural walkthroughs require fast global illumination algorithms and also accurate results from certain viewpoints. This paper introduces a global illumination method that combines several strategies to meet the contradicting criteria of architectural walkthroughs. The methods include parallel and perspective ray-bundle shooting and ray shooting. Each method is designed to randomly approximate the effect of the light transport operator. Parallel ray-bundle tracing transfers the radiance of all points parallel to a randomly selected global direction, with perspective ray-bundles we can shoot the radiance of a single patch in all directions, and ray shooting transfers the radiance of a randomly selected point at a randomly selected direction. These strategies are of complementary character since each of them is effective in different illumination conditions. The proposed algorithm is iterative and the steps realized by different methods that randomly follow each other. In each step, the applied strategy is selected randomly according to the properties of the current radiance distribution, thus we can exploit that the used strategies are good in different conditions. The formal framework of their combination is the stochastic iteration. Although the final result is the image, i.e. the algorithm is view dependent, a rough approximation of the radiance function is stored in object space, that can allow fast movements at reasonable storage requirements and also speed up Monte-Carlo simulations which result in the final image. The method is also suited for interactive walkthrough animation in glossy scenes since when the viewpoint changes, the object space radiance values remain valid and the image quickly adapts to the new situation.
Global illumination, stochastic iteration, finite-element
techniques, Monte-Carlo methods.