The Rendering Technology of KillZone 2 (Michal Valient)

How we made Killzone 2 run @ 30FPS

• Deferred shading
• Diet for render targets
• Dirty lighting tricks
• Rendering, memory and SPUs

Deferred shading

• not forward rendering
• Geometry pass – fill the GBuffer (all material info for lighting)
• loading depth map, normal / bump map, albedo (diffuse color and texture), shininess (reflective materials)
• Lighting pass – accumulate info (only light, no textures)

GBuffer

• RGBA FP16 buffers proved to be too much
• Moved to RGBA8
  • 4xRGBA8 + D24S8 – 18.4mb
  • 2xMSAA (Quincunx) – 36.8mb
• Memory reused by later rendering stages
  • Low res pass, post processing, HUD
• View space position computed from depth buffer
• Normal.z = sqrt(1 – Normal.x2 – Normal.y2)
  • No neg z, but does not cause problems
  • 2xFP16 compressed to RGBA8 on write
• Motion vectors – screen space
• Albedo – material diffuse color
• Roughness – specular exponent in log range
• Specular intesity – single channel only
• Sun Shadow – pre-rendered sun shadows (offline light map)
  • Mixed with real-time sun shadows
• Lighting accumulation buffer (LAB)
  • Geometry pass fills in indirect lighting terms
    • Stored in lightmaps and IBLs
    • Adds ambient color, scene reflections
  • Lighting pass adds contribution of each light
• Glow – contains HDR luminance of LAB
  • Used to reconstruct HDR RGB for bloom

Lighting pass

• Most expensive pass
  • 100+ dynamic lights per frame
  • 10+ shadow casting lights per frame
  • AA means more of everything
• Optimization
  • Avoid hard work
  • Work less for MSAA
  • Precompute sun shadow offline
  • Approximate

Avoid hard work

• Don’t run shaders
  • Use early z/stencil cull unit
  • Depth bounds test is the new cool
  • Enable conditional rendering
• Optimized light shaders
  • For each combination of light features
• Fade out shadows for small lights
• Remove small objects from shadow map

Lighting pass and MSAA

• MSAA facts
  • Each sample has to be lit
  • Samples of non-edge pixel are equal
• KZ2 solution – in shader supersampling
  • Run at 1280x720 not 2560x720
  • Light two samples in one go

Shadow map filtering distribution

• Motivation
  • Define filtering quality per pixel rather than per sample.
• Split filter coordinates into disjoint sets
  • One set per pixel sample
• MSAA is almost as fast as non-MSAA

Sunlight

• Fullscreen directional light
• We divide screen into depth slices
• Each depth slice is lit separately
  • Different shadow properties
  • Used depth bounds test
• Use sun shadow from GBuffer
  • Stencil mark pixels completely in shadow
    • Skip expensive sunlight shader
  • Also mixed with real-time shadows

Sunlight rendering – Fake MSAA

• Used only in distance pixels
• Cut down lighting cost
  • Run lighting equation on closest sample only
• Is this wrong?
  • Its a hack
  • Works correctly against background
  • The edges are still partially anti-aliased
  • Distant scenery is heavily post processed

Sunlight – shadow map rendering

• Generate shadow map for each depth slice
• Common approach
  • Align shadow map to view direction
  • Pros – max shadow map usage
  • Result – shadow map shimmering
• Fix
  • Remove shadow map rotation
    • Align shadow maps to world instead of view
    • Remove sub-pixel movement
    • Cons – unused shadow map space

GPU driven memory allocation system

Push Buffer building

• Multiple SPUs building PB in parallel
• Additional SPUs generating data
  • Skinning, particles – VB
  • IBL interpolation – textures
• Common solutions
  • Ring Buffering
    • Issue with out of order allocations
  • Double Buffering
    • Too much memory

KZ2 render memory allocator

• Fixed mem pool
  • 22MB block – split into 256k blocks
• Each block has associated AllocationID
  • Specified by client during allocation
  • Only whole block can be allocated
• Global FreeID identify free blocks
  • Updated as RSX consumes ‘Free’ marker
• Lockless, out of order, memory allocation
  • From PPU and/or SPU
  • Simple table walk (fast!)
• Allows immediate memory reuse
  • WE generate push-buffer just in time for RSX
  • Block can be reused right after RSX consumption
• Can allocate memory for skinning early…
  • and still free at correct point in frame