Over the previous two chapters, we covered various ways to light a scene. The Blinn-Phong light model we used in all those recipes provides a good approximation to how light interacts with the rendered scene's surfaces. However, one major shortcoming of this light model is that it doesn't take into account occlusion by the various scene elements. In other words, it lacks shadows.

In real life, when a light ray hits an opaque surface, it gets reflected or absorbed and does not continue in the same direction. The Blinn-Phong model does not take this into consideration, so every surface inside the light's range will get illuminated even when it is occluded from the light source by other surfaces. Fortunately, shadows can be added to the Blinn-Phong light model as an attenuation factor. This chapter is going to focus on two shadow techniques that can be used for the various light sources featured in the two previous chapters.

In order to better understand how real-world shadows work...