The interaction of light with materials results in a rich visual experience that can bring wonder and puzzlement to children and adults alike. Light can follow a complex path before reaching our eyes, reflecting off objects or bending to yield shiny reflections, rainbows, or the bewildering patches of light known as caustics. Physically based rendering seeks to faithfully simulate this complex process through numerical integration and the modeling of light-matter interaction. But it offers more than a formidable technical challenge, it also provides us with a fascinating behind-the-scenes perspective on the genesis of our visual experience.
The difficulty of light simulation stems from the very complexity of the physical process. While the simulation of light reaching objects directly from a light source is simple enough, it only accounts for a fraction of the light illuminating our world. Much light bounces off objects in a recursive way, and can have a significant impact even after many bounces, especially if the objects it reflects off are very shiny or translucent. Initially, computer graphics rendering avoided this complexity and only considered direct illumination, relying on various hacks such as fake additional lights to make the results look acceptable. Most early CGI in movies such as Toy Story and Jurassic Park did not take into account indirect illumination and relied instead on the artists' ability to fake believable lighting.