Specular materials reflect lights when objects are viewed from certain angles. Unfortunately, even the Fresnel reflection, which is one of the most accurate models, does not correctly reflect lights from nearby objects. The lighting models examined in the previous chapters took into account only light sources, but ignored light that is reflected from other surfaces. With what you've learned about shaders so far, making a mirror is simply not possible.
Global illumination makes this possible by providing PBR shaders with information about their surroundings. This allows objects to have not just specular highlights, but also real reflections, which depend on the other objects around them. Real-time reflections are very costly and require manual setting up and tweaking in order to work. When done properly, they can be used to create mirror...
