Lightmaps

• Raytracing, but rendered offline and baked into texture data
  • Generated as part of level data -- e.g. you hit 'save' in a level editor, lightmaps are a section of saved data
• At runtime, just use multitexturing and multiply diffuse texture sample with lightmap texture sample
• Best for static geometry and non-moving light sources

Full scene without lightmaps (before)

Full scene with lightmaps (after)

Stencil Buffer Shadows

• Most notably used in Doom 3
• Very precise shadows, very few artifacts/glitches from long distances
• Best used in outdoor, open scenes

Stencil Buffer Shadows

• Given a light source, render outwards
• Occluding geometry creates a shadow volume

Shadow Mapping

• Most common implementation of real-time shadows for the last few years
• More flexible in terms of scene variety (indoor, outdoor, whatever)
• Bad precision, prone to low resolution and artifacting issues ("shadow acne") without precautions

Shadow mapping basic algorithm:

• For each light source, render scene into a depth map using the light as a camera
  • Doesn't need to be full resolution, e.g. 256x256

Original scene rendered from "light camera" and not "real camera"

Shadow Mapping algorithm

• Project/transform each shadow map created this way onto the scene rendered by the real camera

• Test shadow map depth against real camera depth:
  • If shadow map depth < real camera depth, darken pixel

Shadow Mapping issues

• Shadow Acne
  • Problem: Low resolution and floating-point precision causes discontinuities, jagged "false" shadows
  • Fix: Add some padding to depth test, e.g. shadow map depth has to be less than real depth plus some margin

Real-time Raytracing

• Most accurate - not approximations like other shading/lighting techniques, but literal emulation of light physics
• Absurdly costly without shortcuts
  • Rendering a few seconds of final animation footage can take days of processing
  • Getting it to work at 30-60 frames per second is the holy grail
    • Solves other issues, too, like transparency or refraction or reflection

Hybrid Ray-tracing

• Mix of traditional rasterization game engines (e.g. deferred rendering) and ray tracing
• Render frame, ray trace only what's accumulated in the G-buffer and not entire scene

Hybrid ray-tracing

Path tracing

Ray-tracing, but with fewer bounces -- approximates with a "russian roulette" early termination

http://www.youtube.com/watch?v=FbGm66DCWok