Real-Time Light

The path tracer was about correctness through brute force: shoot enough rays, average enough samples, the right answer falls out eventually. The deferred renderer is its inverse: cheat the rendering equation in clever ways so the right answer falls out at sixty frames per second.

The G-buffer comes first: every fragment writes its position, normal, albedo, and material into screen-space textures so lighting can be computed once per pixel, not once per pixel per light. Shadows arrive as a depth test from the light's point of view, then progressively softened — binary, percentage-closer-filtered, variance-, and finally moment-based — until the penumbras look right. Image-based lighting replaces the analytic light with an HDR environment, pre-filtered offline so a glossy surface can sample roughness-correct reflections at one texture lookup per pixel. Ambient occlusion is added back where the geometry never was: the small darknesses inside corners that direct lighting can't reach. And finally a single indirect bounce, also in screen space, lets one surface tint another by reflected light without ever leaving the current frame's depth buffer.

Every chapter is a different approximation of the same integral the path tracer evaluated honestly. The art of the real-time renderer is knowing which approximations the eye will accept, and where they break.