Building a Dynamic Lighting Engine for Velvet Assassin

2. Christian Schüler Building a Dynamic Lighting Engine for Velvet Assassin

3. Velvet Assassin • 3rd person stealth game • Formerly known as “Sabotage 1943” • First concepts: late 2000 • Release: April 2009 • Platforms PC, X360 My role: tech. dir. plus lead engine programmer

4. Engine Goals • Must look great! • (of course) • Everything is dynamically lit • Cannot use Lightmaps • Lighting is part of gameplay • If it looks dark, the player should be hidden! • Light sources become game entites.

5. Engine Goals? So, what about ... • … scene visibility • … light influence • … indirect lighting (like radiosity) … if every object can possibly move, even light sources?

6. First Engine (2003)

7. Axioms (2003) • World is a loose octree of objects • Objects are OBB trees of triangles • Multi-pass lighting with stencil shadows • Occlusion culling for visibility • Indirect illumination via bounce lights

8. Shipping Engine (2009)

9. Axioms (2009) • World is a loose octree of objects • Objects are OBB trees of triangles • Hybrid single/multi-pass lighting with shadow maps • Portals for visibility • Indirect illumination via bounce lights + XBox 360 specific optimizations

10. Loose Octrees Thatcher Ulrich (2001): • Cells are overlapping (loose) • Insertion is efficient • No need to rebuild the whole octree if an element moves!  Perfect as spatial index of a dynamic scene!

11. Loose Octrees contd. Extended volume Base cell

12. Loose Octrees contd. Object inserted if inside extended volume  O(1) insertion

13. Loose Octrees contd. Used in finding out • Objects in a view frustum • Objects influenced by a light • Lights influencing an object • Broad phase for ray tests • Gameplay objects in range And everything can be dynamic!

14. OBB Trees Oriented Bounding Box Tree S. Gottschalk et al (1995) • Used on the polygon level • Build as a pre-process over mesh data • Allows efficient ray-mesh and mesh-mesh interference tests

15. OBB Trees contd. Axis aligned … … vs oriented!

16. OBB Trees contd. • Construction: • Principal axes (gaussian point distribution)* • Minimize Box volume • (possibly iterative) *eigenvectors of covariance matrix

17. Hybrid Lighting A hybrid between multi-pass and single-pass forward renderer: • One pass for each primary light • One pass for all secondary lights combined

18. Hybrid Lighting contd. Primary lights • Classic multi-pass (Doom 3 style) • One pass per primary light • Can cast shadows • The light queries for surrounding geometry

19. Hybrid Lighting contd. Secondary lights • Classic single-pass (HL2 style) • Lights collected into one pass • (shader variation based on count) • Can not cast shadows • The geometry queries for surrounding lights • (up to a maximum amount)

20. Hybrid Lighting contd. secondarypoints primary spot

21. Hybrid Lighting contd. primary directional secondarypoints

22. Bounce Light Gives appearance of first bounce indirect light from a surface. Must not illuminate the surface it is placed on. Has a half-sphere influence radius determined by axis. N L axis (N•L) ·f(axis•L)

23. Bounce Light contd. primary spot secondarybounce

24. Bounce Light contd. 2 secondaryambients primaryspot

25. Bounce Light contd. ... and even back in 2003 (it‘s not rocket science)

26. So, for each frame … • Get all primary lights in view • Distribute shadow map pool • Render shadow maps, for each: • Render all objects contained in light frustum • Get all objects in view • Render base pass • For each object, collect nearest N secondary lights (sorted by importance) for the shader • Render additive passes for each … • … primary light: for each object that is in the view and also in the light frustum. That is why you need an efficient spatial index data structure.

27. Fog Zones A.k.a.: There has to be at least one benefit for manual portalization! Here it is: Fog Zones!

28. Fog Zones contd. portal separates fog environments

29. Fog Zones contd. … from the other side

30. Fog Zones contd. Multiply-Add is your friend! (instead of lerping against a constant fog color) C = C0 ∙ T + S C0 original color T fog transmittance S fog in-scatter = (1−T) ∙ CFog traditionally

31. Fog Zones contd. C = ( C0 ∙ TB + SB ) ∙ TA + SA A B portal C = C0 ∙ ( TB ∙ TA)+ ( SB ∙ TA + SA)

32. Fog Zones contd. • Modify T and S of the new environment with T and S from the portal polygon • Calculate fog from the distance of the portal • Repeat recusively

33. Fill Optimization • Only done for XBox 360 • Selected particle effects rendered into off-screen render target at half resolution to save fill rate • (against half resolution depth buffer) • Composited over the final image

34. Fill Optimization contd.

35. Fill Optimization contd. Again, multiply-add solves the math (in the form of pre-multiplied alpha) Off-screen target: CTarget’ = (1−AParticle) · CTarget + CParticle ATarget’ = (1−AParticle) · ATarget + AParticle Compositing: CFrame’ = (1−ATarget) · CFrame + CTarget

36. Multi-threading • XBox 360 needed it; a dual-core PC at least benefits • First thread performs all spatial queries and compiles a “drawlist” • Second thread sets shader registers, render states and submits batches • Most scenes from 300 to 1200 batches/frame

37. The End Questions?