So the Global Illumination has better results and also cheaper in computation cost?
@tracychen Direct illumination + reflectance + transparency shown on the previous slide only captures a subset of ways that light interacts with the scene. While a rasterizer (our OpenGL pipeline) can simulate direct illumination pretty efficiently, it is much harder to simulate light that bounces around an environment multiple times. This global illumination solution image represents a photorealistic rendering, because it captures all components of light, including light that bounces around the scene N times. However, computing this image tends to be much more expensive! Hence why path tracers are generally pretty slow (as you will see with Assignment 3!).