Abstract: Photon mapping based on photon maps can efficiently render high-quality
photorealistic images. It stores many photons in photon map to improve rendering qualities of
complex illumination, leading to high memory overhead and long reconstruction time. This paper
presents a grid-based reconstruction of global photon maps. We divide bounding box of photons
into a number of grids. Based on the number of photons, we take a certain rate of photons in grids
to reconstruct global photon map, and ensure conservation of total energy of photons in each grid
before and after reconstruction, which make estimated illumination effects of photon maps before
and after reconstruction almost same. By adding photons in special grids, we effectively reduce
illumination error due to all kinds of geometry biases and enhance rendering effects of direct and
indirect caustic. This paper checks a few noises in rendering pictures by simple method, and adds
a few samples to effectively reduce these noises. Our reconstruction method of photon maps takes
fewer time and ensures viewpoint independence of rendering pictures.

Key words: photon mapping, reconstruction of global photon map, geometry bias, caustic