In Chapter 3, Lights, we learned how to illuminate our scene by mimicking light. We did so by leveraging various shading and light reflection techniques that use two main components: specular and diffuse. Although we've been modeling materials with specular and diffuse in computer graphics for a long time, these techniques produce results that are not very realistic. For example, changing the specularity of a material doesn't change the diffuse:
The preceding screenshot demonstrates that changing the two parameters of specular intensity and specular hardness only changes the whitish part of the reflection. The blue diffuse reflection doesn't change at all—that's not how our physical world works! So, in applications aiming for more realistic effects, an artist would be tasked with manually tuning these values for each material...