Abstract: Focusing on the difficulty in interactively rendering physically correct soft shadows existing in global illumination effects, this study proposes an algorithm for producing approximate soft shadows, which can generate alterable penumbra reflecting spatial positions of occluders. Firstly, a binary light-visibility map is generated by using occluding detection method based on ray tracing and taking the center of light source as a reference point. Secondly, a way that adaptively determines the kernel size of spatial smoothing filter is proposed. Then, visibility factors that change smoothly from visible region to non-visible region are obtained by a method of adaptive visibility filtering. The filtering method can solve the problem of visibility infiltration by using mask. Finally, the visibility factors are used to dynamically modulate the direct illumination of the associated visible points in the process of ray tracing. The highly realistic soft shadow effects are produced by adding the modulated direct illumination and ordinary indirect illumination together. Experimental results show that the algorithm is fully compatible with ray tracing and hence is easy to compute indirect illumination including mirrored visual effects. The soft shadows in images generated by our method look like physically-correct soft shadows. The rendering speed of our method is beyond 30 frames per second for all test scenes featuring specular reflections, hence satisfying the requirements of interactive applications.

Key words: soft shadows, variable penumbra, light-source visibility, ray tracing, adaptive filtering