Renderer Design for Multiple Lights

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Renderer Designfor Multiple Lights

• by Wolfgang Engel05/27/08

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Agenda

• What is a Renderer?
• What does the Renderer do?
• Z Pre-Pass Renderer
• Deferred Renderer
• Light Pre-Pass Renderer
• Conclusion

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What is a Renderer

• It is the fundament of your game engine
• The software layer that feeds the graphics card
  with data

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What does the Renderer do?

• First render phase - Shadow pass Cascaded
  Shadows for day light, night-time shadows, cloud
  shadows, character shadows- Z pre-pass /
  G-Buffer update / Light Pre-pass
• Second render phase - Screen-space ambient
  occlusion / local irradiance
• Third render phase- In case of a Light Pre-Pass
  renderer fill up the light buffer with all light
  properties- Global Illumination collects light
  data- Reflections / refractions / environment
  reflections
• Fourth render phase (Opaque objects)- Lighting
  and rendering the opaque objects with shadow data
  sorted by shaders and / or front-to-back (also
  water refraction map)
• Fifth render phase - Dynamic sky-dome
• Sixth render phase (Transparent objects)-
  Lighting and rendering the transparent objects
  with shadow data sorted back-to-front
  alpha-to-coverage objects - Foliage rendering
• Seventh render phase (Particles) - Render
  high-res particles into back-buffer - Render
  low-res particles into smaller render target -
  Composite smaller particle render target with
  main render target
• Eight render phase - MSAA resolve to a
  potentially lower-res render target (for certain
  target platforms) - PostFX (HDR, Depth of Field,
  Depth Fog, Height-Based fog, motion blur, heat
  haze, tear gas, drunk-effect
• Ninth render phase -- Up-scaling of lower res
  render target (for certain target platforms) --
  UI etc.

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Z Pre-Pass Renderer
Render opaque objects
Transparent objects
Depth Buffer
Z pre-pass
Switch off depth write
Forward Rendering
Sort Front-To-Back
Forward Rendering

• First mentioned by John Carmack Carmack
• Shows phase four and six from the overview above
• Separate render path for opaque and transparent
  objects
• Z Pre-Pass
• Layout depth data and hardware culling data
• Utilizes fast double-speed depth writes
• Forward rendering is the common way to render
  objects

Sort Back-To-Front
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Z Pre-Pass Renderer II

• Multi-Light solutions
• One render pass per light lt-gt lots of draw calls
• Render 4 8 lights per drawcall
• Pixel shader can render e.g. up to eight lights
• Requires to split up geometry following the
  amount of lights lt-gt hardware likes low number
  of draw calls
• 2 ½ D light solution with light properties in
  textures
• Store light properties in 2D textures
• Use index texture to index into those light
  property textureslt-gt hardware does not like
  dependent texture reads

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Deferred Renderer
Depth Buffer
Specular /Motion Vec
Normals
Albedo /Shadow
DeferredLighting
Switch off depth write

• First mentioned in Deering
• Shows phase four and six from the overview above
• Separate render path for opaque and transparent
  objects
• G-Buffer Multiple-Render Target (Killzone 2)
  Valient
• Holds material properties mainly specular and
  albedo
• Lighting phase reads DS, RT1 RT3 and renders
  the image per light into RT0 -gt renders as often
  as there are lights

Forward Rendering
Sort Back-To-Front
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Deferred Renderer II

• Reading the G-Buffer for each light -gt lots of
  memory bandwidth -gt solutions
• Scissoring out the 3D bounding box volume of the
  light projected into a 2D rectangle Placeres
• Render for each light convex geometry
• Point light sphere spotlight cone
• If camera is inside light volume only render back
  facing pixels when depth test fails
  (D3DCMP_GREATER instead of D3DCMP_LESSEQUAL )
  Thibieroz04
• Like stencil shadow volumes
• Like above but increment stencil test
• When drawing front-facing light volumes set the
  depth test to D3DCMP_LESSEQUAL and decrement the
  stencil test when the depth test fails
• Only light pixels are rendered where the stencil
  value is greater than or equal to 1
  HargreavesValient.
• G-Buffer holds all material properties -gt the
  restricted number of storage space restricts the
  game to a low variety of materials
• Hardware MSAA is not available on DX9 PC graphics
  hardware and quite expensive on XBOX 360 / PS3

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Light Pre-Pass Renderer
Render opaque objects
Transparent objects
Depth Buffer
Normals
Switch off depth write
Light Buffer
Forward Rendering
Sort Front-To-Back

• Stores normals in a normal buffer and the depth
  values in a depth buffer
• Stores light properties in a light buffer -gt use
  same memory bandwidth optimizations as Deferred
  Renderer
• Renderes forward while re-constructing the
  lighting equation

Forward Rendering
Sort Back-To-Front
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Light Pre-Pass Renderer II

• What are the light properties?
• Color Ambient Shadow Att (N.L DiffColor
  DiffIntensity LightColor R.Vn SpecColor
  SpecIntensity LightColor)
• Properties that depend on the light vector
• N.L
• LightColor
• R.Vn
• Attenuation
• Simple Light Pre-Pass
• LightColor.r N.L AttLightColor.g N.L
  AttLightColor.b N.L AttR.Vn N.L Att
• Spotlight Att represents spotlight factor

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Light Pre-Pass Renderer III

• Simple Pre-Pass Renderer does not allow to
  re-construct the specular term of the lighting
  equation -gt therefore a separate diffuse term
  need to be stored like this
• LightColor.r N.L AttLightColor.g N.L
  AttLightColor.b N.L AttR.Vn N.L Att
• N.L Att
• Now we can do
• (R.Vn N.L Att) / (N.L Att)
• Each pixel of the light buffer represents the
  specular term of all light sources
• Adding a material specular power value can be
  done like this(R.Vn)mn
• Adding a material specular color is done like
  this
• (R.Vn)nm Spec
• Thinking of this specular term as an intensity
  term, we can construct all kind of specular terms
  and multiply it with it.
• Fresnel is just N.V in the forward rendering path

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Light Pre-Pass Renderer IV

• Compared to the Z Pre-Pass Renderer Design
• Less material variety than a Z Pre-Pass renderer
• Probably consumes more memory bandwidth
• Needs higher spec graphics hardware
• It is easier to implement more lights
• Compared to the Deferred Renderer Design
• Light Pre-Pass offers more material variety
• Hardware MSAA is easy to implement
• Memory bandwidth lower -gt reading normal and
  depth buffer for each light is less than reading
  the G-Buffer in the Deferred Renderer
• Cost per light is lower than with a Deferred
  Renderer -gt more lights
• Easy to implement on low spec graphics hardware
  with pixel shader model 1.4 and higher

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Conclusion

• If a game requires lots of dynamic lights, a
  Light Pre-Pass renderer is a good choice to
  achieve this goal
• If minimum hardware capabilities are quite low
  (no programmable pixel shader), the Z Pre-Pass
  Renderer is better

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Thanks
wengel_at_rockstarsandiego.com
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References

• Bavoil Louis Bavoil, Kevin Myers, Deferred
  Rendering using a Stencil Routed K-Buffer,
  ShaderX6
• Calver Dean Calver's article in deferred
  rendering on beyond3d http//www.beyond3d.com/cont
  ent/articles/19/
• Carmack John Carmack ??
• Deering Michael Deering "The triangle processor
  and normal vector shader a VLSI system for high
  performance graphics" SIGGRAPH 1988
• Hargreaves Shawn Hargreaves, Deferred
  Shading, http//www.talula.demon.co.uk/DeferredSh
  ading.pdf
• Placeres Frank Puig Placeres, Overcoming
  Deferred Shading Drawbacks, pp. 115 130,
  ShaderX5
• Thibieroz04 Nick, Thibieroz, Deferred Shading
  with Multiple-Render Targets, pp. 251- 269,
  ShaderX2 Shader Programming Tips Tricks with
  DirectX9
• Thibieroz07 Nick, Thibieroz, Robust
  Order-Independent Transparency via Reverse Depth
  Peeling in DirectX 10, ShaderX6
• Valient Michal Valient, Deferred Rendering in
  Killzone 2, http//www.guerrilla-games.com/public
  ations/dr_kz2_rsx_dev07.pdf