Andreas Dietrich Ingo Wald Markus Wagner Philipp Slusallek Computer Graphics Group

1. VRML Scene Graphs on anInteractive Ray Tracing EngineIEEE Virtual Reality 2004 ConferenceChicago, IL USAMarch 30th, 2004 Andreas Dietrich Ingo Wald Markus Wagner Philipp Slusallek Computer Graphics Group Saarland University http://graphics.cs.uni-sb.de

2. Introduction • Today‘s VR systems based on rasterization techniques • Quantitative evaluation difficult • Sophisticated Lighting not possible or requires too much tricks • Recent advances in interactive ray tracing technologies • High-quality image generation at interactive frame rates • Ray tracing will play a larger role in industrial visualization • Scene graph driven VR Application based on ray tracing VRML Browser ( vv ) VRML Scene Graphs on an Interactive Ray Tracing Engine

3. Agenda • Background • Ray Tracing • System Overview • System Components • OpenRT API Characteristics • Implementation Issues • Rendering & Shading • Scene Graph Mapping • Example Applications • Summary VRML Scene Graphs on an Interactive Ray Tracing Engine

4. Advantages ofRay Tracing • Most architectures incorporating sophisticated illumination perform ray tracing • Close modeling of physical light propagation • Shooting of imaginary rays into the scene • Advantages of interactive ray tracing Physicalcorrectness Plug and play shading Complex scenes (4-16 fps on 24 1.8 GHz CPUs) VRML Scene Graphs on an Interactive Ray Tracing Engine

5. Problems • Common scene graph libraries • Targeted towards rasterization / OpenGL • Ray tracing features cannot be adequately exploited • Same with standard graphics file formats, like VRML • Goal: VRML browser implementation providing • Flexible scene graph handling • Animation, interaction, and model behavior • Benefits of interactive ray tracing VRML Scene Graphs on an Interactive Ray Tracing Engine

6. System Components • Main components • XRML [Bekaert 01] • OpenRT [Dietrich 03] • RTRT [Wald 01/03] VRML Scene Graphs on an Interactive Ray Tracing Engine

7. System Components • Main components • XRML [Bekaert 01] • OpenRT [Dietrich 03] • RTRT [Wald 01/03] • OpenRT API • Supports all features of interactive ray tracing • Syntactically similiar to OpenGL • But some semantic differences VRML Scene Graphs on an Interactive Ray Tracing Engine

8. OpenRT and OpenGL Differences between OpenRT and OpenGL • Rendering semantics • Unlike OpenGL’s drawing approach OpenRT uses objects • Shader objects are bound to geometry objects • Complete scene has to be defined prior to rendering • Objects and instantiation • Geometric objects serve as simple containers • Efficient reuse of objects • Multi-pass rendering vs. programmable shading • Possible but not necessary due to programmable shader objects VRML Scene Graphs on an Interactive Ray Tracing Engine

9. Rendering • Rendering controlled by XRML renderer module • Custom-made based on original OpenGL module • Geometry definition calls largely reused • OpenRT specific shading and lighting calls • No immediate-mode rendering • Scene graph traversal just for scene specification • Rendering after traversal in one single step VRML Scene Graphs on an Interactive Ray Tracing Engine

10. Shading • VRML shader node extension • Standard VRML limited to simple Blinn-Phong shading • Support for arbitrary shading necessary • New nodes ORTAppearance, ORTShader appearance ORTAppearance { material Material { diffuseColor 1 0 0 } shader ORTShader { name "Glass" file "libGlass.so" options [ "1f refractionIndex 1.5" ] } } Lightbulbof a car headlight VRML Scene Graphs on an Interactive Ray Tracing Engine

11. Scene Graph MappingAcceleration Structures • Ray tracing algorithms maintain acceleration structures • Hierarchical spatial index • Allow for fast geometry queries • May be considered an internal scene graph Mapping external scene graph (VRML) internal scene graph (OpenRT) VRML Scene Graphs on an Interactive Ray Tracing Engine

12. Scene Graph MappingObject Grouping • Group Shape nodes into OpenRT geometry objects • Naive solution: Create OpenRT object for every single Shape • Can result in performance loss VRML Scene Graphs on an Interactive Ray Tracing Engine

13. Scene Graph MappingObject Grouping • Group Shape nodes into OpenRT geometry objects • Heuristics neccessary • ROUTE statements indicate graph cutting points VRML Scene Graphs on an Interactive Ray Tracing Engine

14. Scene Graph MappingObject Grouping • Group Shape nodes into OpenRT geometry objects • Heuristics neccessary • ROUTE statements indicate graph cutting points VRML Scene Graphs on an Interactive Ray Tracing Engine

15. Scene Graph MappingInstancing • Instancing possible using DEF/USE mechanism • DEF/USE equivalent to pointers • Can link arbitrary nodes • OpenRT can only instantiate complete objects • Use instancing only for Transform nodes VRML Scene Graphs on an Interactive Ray Tracing Engine

16. Example ApplicationsIndustrial Design • Car headlight • Physically-correctreflection andrefraction • Special reflectorand glass shaders • Up to 25 levelsof recursion • More than 800.000triangles • Only possible withray tracing (16 1.8 GHz CPUs) VRML Scene Graphs on an Interactive Ray Tracing Engine

17. Example ApplicationsAnimated Global Illumination • Rotating globe dynamically lit by a moving lamp • Indirect diffuseinterreflection • Standard VRMLscene includinganimation paths • Global illuminationplug-in used asdefault shader (16 1.8 GHz CPUs) VRML Scene Graphs on an Interactive Ray Tracing Engine

18. Summary • Combination of scene graph applications and ray tracing • Flexible scene graph handling • Animation, interaction, and model behaviour • VRML97 support • Physical correctness • Plug and play shading • Complex scenes • Future work • Evaluation of other file formats like X3D • Development of a SG library optimized for RT VRML Scene Graphs on an Interactive Ray Tracing Engine

19. Questions ?For more information seehttp://graphics.cs.uni-sb.dehttp://www.openrt.dehttp://www.intrace.com