Importance Sampling for Production Rendering

Importance sampling provides a practical, production-proven method for integrating diffuse and glossy surface reflections with arbitrary image-based environment or area lighting constructs. Here, functions are evaluated at random points across a domain to produce an estimate of an integral. When using a large number of sample points, the method produces a very accurate result of the integral and provides a strong basis for simulating complex problems such as light transport. Frequently, using the necessary number of samples to reach the exact result is too computationally expensive and fewer samples are evaluated at the cost of visual noise, or variance, within the image. Importance sampling offers a means to reduce the variance by skewing the samples toward regions of the illumination integral that provide the most energy. For instance, the direction of specular reflection or a bright light source within an environment more likely represent the final value of the integral than a random sample. The variance can be reduced more efficiently by combining multiple components of the illumination integral, such as the lighting and material function, to determine where to sample, which is the principle of Multiple Importance Sampling (MIS). As an alternative to the noise in importance sampling, Filtered Importance Sampling (FIS) can provide fast integration, where the lighting environment look-ups are prefiltered to give a smoother result with a significantly smaller number of samples. Importance sampling, MIS and FIS have various practical implications. In this quarter-day course, we cover the necessary background for using Monte Carlo-based techniques for direct lighting and explain how various visual effects companies use these shading methods in their production pipelines.