This image was generated using the method described in this work.  400 directional lights were used to approximate a high-dynamic range environment map.  A standard ray-tracer, augmented with our technique, was then used to compute cast shadows.  Only 4.3% or the shadow rays (400 per pixel) was actually traced.  The other 95.7% were predicted by our coherence-based method.  The rendering time needed to compute this image was about 1/10th the time required to render the image using the unmodified ray-tracer.

Abstract:  This paper addresses the problem of efficiently calculating shadows from environment maps. Since accurate rendering of shadows from environment maps requires hundreds of lights, the expensive computation is determining visibility from each pixel to each light direction, such as by ray-tracing. We show that coherence in both spatial and angular domains can be used to reduce the number of shadow rays that need to be traced. Specifically, we use a coarse-to-fine evaluation of the image, predicting visibility by reusing visibility calculations from four nearby pixels that have already been evaluated. This simple method allows us to explicitly mark regions of uncertainty in the prediction. By only tracing rays in these and neighboring directions, we are able to reduce the number of shadow rays traced by up to a factor of 20 while maintaining error rates below 0.01%. For many scenes, our algorithm can add shadowing from hundreds of lights at twice the cost of rendering without shadows.

The process:

The results:

The links: