Renderer Design for Multiple Lights

1. Renderer Designfor Multiple Lights by Wolfgang Engel05/27/08

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

3. What is a Renderer • It is the fundament of your game engine • The software layer that feeds the graphics card with data

4. 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.

5. 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

6. Z Pre-Pass Renderer II • Multi-Light solutions • One render pass per light <-> 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 <-> 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 textures<-> hardware does not like dependent texture reads

7. Deferred Renderer Normals Specular /Motion Vec Albedo /Shadow Depth Buffer 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 -> renders as often as there are lights Forward Rendering Sort Back-To-Front

8. Deferred Renderer II • Reading the G-Buffer for each light -> lots of memory bandwidth -> 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 [Hargreaves][Valient]. • G-Buffer holds all material properties -> 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

9. Light Pre-Pass Renderer Render opaque objects Transparent objects Normals Depth Buffer 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 -> use same memory bandwidth optimizations as Deferred Renderer • Renderes forward while re-constructing the lighting equation Forward Rendering Sort Back-To-Front

10. Light Pre-Pass Renderer II • What are the light properties? • Color = Ambient + Shadow * Att * (N.L * DiffColor * DiffIntensity * LightColor + R.V^n * SpecColor * SpecIntensity * LightColor) • Properties that depend on the light vector • N.L • LightColor • R.V^n • Attenuation • Simple Light Pre-Pass • LightColor.r * N.L * AttLightColor.g * N.L * AttLightColor.b * N.L * AttR.V^n * N.L * Att • Spotlight: Att represents spotlight factor

11. Light Pre-Pass Renderer III • Simple Pre-Pass Renderer does not allow to re-construct the specular term of the lighting equation -> therefore a separate diffuse term need to be stored like this • LightColor.r * N.L * AttLightColor.g * N.L * AttLightColor.b * N.L * AttR.V^n * N.L * Att • N.L * Att • Now we can do • (R.V^n * 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.V^n)^mn • Adding a material specular color is done like this • (R.V^n)^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

12. 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 -> 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 -> more lights • Easy to implement on low spec graphics hardware with pixel shader model 1.4 and higher

13. 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

14. Thanks wengel@rockstarsandiego.com

15. 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/content/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/DeferredShading.pdf • [Placeres] Frank PuigPlaceres, “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/publications/dr_kz2_rsx_dev07.pdf