Unbiased Photon Gathering In Participating Media

The realistic participating media rendering algorithm based on physical optics simulation has been an important direction of research in the field of computer graphics. The simulation of smoke, snow, sky and other scenes for the movie effects industry enhance the viewing of realistic great-ly, which covers the physical optics, expertise in mathematics like statistical probability theory, geometry, computer rendering technology and other subjects. Nowadays the field of media ren-dering includes Bidirectional path tracing (referred to as BDPT), Photon mapping and a series of sophisticated algorithms system. The Photon mapping algorithm due to its efficient rendering rate is widely used, but the algorithm itself does not guarantee unbiased. And therefore Unbiased pho-ton gathering(referred to as UPG) methods have been proposed by to solve the biased items of the Photon mapping algorithm recently. But its unbiased method applies only to the scene without par-ticipating media, most photon mapping algorithm is employed to the participating media scene. So the problem to be solved in our paper is how to adapt the UPG to the participating media rendering.We achieve the UPG algorithm in Mitsuba Renderer. The method uses the traditional bidi-rectional path tracing and photon mapping algorithm corresponding to the vertex connection and merging algorithm, to improve reuse of the subpath for acceleration of rendering. The main idea of unbiased improvement:convert the biased estimate formula into precise unbiased items by rewrit-ing and sampling. In order to adapt the method into the media rendering, we improve method from four aspects:transform of subpath sampling ways, transform of theoretical formula, impor-tance sampling of unbiased estimation term and discussion of the media border situation. Then we present the the framework of UPG+BDPT joint algorithm for participating media in our imple-mentation.Finally, the results of the rendering experiment fully demonstrated our media rendering UPG+BDPT joint algorithm in rendering speed and unbiasedness is obviously superior to the traditional bidirectional path tracing and photon mapping algorithms.