In this update I want to show the results of switching to a physically based shading model and share some of the resources that I used to make the transition. The image on the left is with shadows and reflection turned on while the image on the right is with only reflection turned on. In order to get familiar with physically based shading, I spent a good amount of time finding/reading resources about it. I began my research by thoroughly reading Real-Time Rendering: Third Edition Chapter 7: Advanced Shading. That chapter gives an excellent description of lighting concepts and BRDFs (Bidirectional Reflectance Distribution Functions). Additionally, I read Alex Tardif's blog  post where he describes his own process on implementing a physically based shading. Alex also provides some extremely valuable references, in particular: the SIGGRAPH 2013 course notes (Background: Physics and Math of Shading and Real Shading in Unreal Engine 4) as well as the Graphics Rants: Specular BRDF Reference (it was useful to see all the formula on a single page). I found Simon's Tech Blog and Filmic Worlds blog useful as well. I also had the opportunity to talk with Dr. Marc Olano, a professor from University of Maryland, Baltimore County, about BRDFs, microfacet theory, and energy conservation. He helped me better wrap my head around the concepts and talking to him motivated me to dig even deeper into these BRDFs. Overall, it has been a very rewarding experience and I'm still finding new resources to learn from.

I use the following functions to generate the terms for the BRDF:

• Fresnel Reflectance Function - Schlick Approximation
• Normal Distribution Function - Trowbridge-Reitz (with Burley roughness value adjustment)
• Geometry Function - Schlick Approximation of Smith (GGX) (with Burley roughness value adjustment and UE4 remapping of k = alpha / 2)

P.S. I increased the resolution for the shadow and reflection maps to 1024 x 1024 for the above images.