VRML Scene Graphs on an Interactive Ray Tracing Engine IEEE Virtual Reality 2004 Conference Chicago, IL USA March 30 th, 2004 Andreas Dietrich Ingo Wald Markus Wagner Philipp Slusallek Computer Graphics Group Saarland University

COMPUTER GRAPHIK – UNIVERSITÄT DES SAARLANDES VR 2004VRML Scene Graphs on an Interactive Ray Tracing Engine2 Introduction Today‘s VR systems based on rasterization techniquesToday‘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 technologiesRecent 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 )

COMPUTER GRAPHIK – UNIVERSITÄT DES SAARLANDES VR 2004VRML Scene Graphs on an Interactive Ray Tracing Engine3 Agenda BackgroundBackground –Ray Tracing System OverviewSystem Overview –System Components –OpenRT API Characteristics Implementation IssuesImplementation Issues –Rendering & Shading –Scene Graph Mapping Example ApplicationsExample Applications SummarySummary

COMPUTER GRAPHIK – UNIVERSITÄT DES SAARLANDES VR 2004VRML Scene Graphs on an Interactive Ray Tracing Engine4 Advantages of Ray Tracing Most architectures incorporating sophisticated illumination perform ray tracingMost architectures incorporating sophisticated illumination perform ray tracing –Close modeling of physical light propagation –Shooting of imaginary rays into the scene Advantages of interactive ray tracingAdvantages of interactive ray tracing Physical correctness Plug and play shading Complex scenes (4-16 fps on GHz CPUs)

COMPUTER GRAPHIK – UNIVERSITÄT DES SAARLANDES VR 2004VRML Scene Graphs on an Interactive Ray Tracing Engine5 Problems Common scene graph librariesCommon 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 providingGoal: VRML browser implementation providing –Flexible scene graph handling –Animation, interaction, and model behavior –Benefits of interactive ray tracing

COMPUTER GRAPHIK – UNIVERSITÄT DES SAARLANDES VR 2004VRML Scene Graphs on an Interactive Ray Tracing Engine6 System Components Main componentsMain components –XRML [Bekaert 01] –OpenRT [Dietrich 03] –RTRT [Wald 01/03]

COMPUTER GRAPHIK – UNIVERSITÄT DES SAARLANDES VR 2004VRML Scene Graphs on an Interactive Ray Tracing Engine7 System Components Main componentsMain 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

COMPUTER GRAPHIK – UNIVERSITÄT DES SAARLANDES VR 2004VRML Scene Graphs on an Interactive Ray Tracing Engine8 OpenRT and OpenGL Differences between OpenRT and OpenGL Rendering semanticsRendering 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 instantiationObjects and instantiation –Geometric objects serve as simple containers –Efficient reuse of objects Multi-pass rendering vs. programmable shadingMulti-pass rendering vs. programmable shading –Possible but not necessary due to programmable shader objects

COMPUTER GRAPHIK – UNIVERSITÄT DES SAARLANDES VR 2004VRML Scene Graphs on an Interactive Ray Tracing Engine9 Rendering Rendering controlled by XRML renderer moduleRendering 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

COMPUTER GRAPHIK – UNIVERSITÄT DES SAARLANDES VR 2004VRML Scene Graphs on an Interactive Ray Tracing Engine10 Shading VRML shader node extensionVRML 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 } shader ORTShader { name "Glass" file "libGlass.so" options [ "1f refractionIndex 1.5" ] } Lightbulb of a car headlight

COMPUTER GRAPHIK – UNIVERSITÄT DES SAARLANDES VR 2004VRML Scene Graphs on an Interactive Ray Tracing Engine11 Scene Graph Mapping Acceleration Structures Ray tracing algorithms maintain acceleration structuresRay tracing algorithms maintain acceleration structures –Hierarchical spatial index –Allow for fast geometry queries –May be considered an internal scene graph internal scene graph (OpenRT) Mapping external scene graph (VRML)

COMPUTER GRAPHIK – UNIVERSITÄT DES SAARLANDES VR 2004VRML Scene Graphs on an Interactive Ray Tracing Engine12 Scene Graph Mapping Object Grouping Group Shape nodes into OpenRT geometry objectsGroup Shape nodes into OpenRT geometry objects –Naive solution: Create OpenRT object for every single Shape –Can result in performance loss

COMPUTER GRAPHIK – UNIVERSITÄT DES SAARLANDES VR 2004VRML Scene Graphs on an Interactive Ray Tracing Engine13 Scene Graph Mapping Object Grouping Group Shape nodes into OpenRT geometry objectsGroup Shape nodes into OpenRT geometry objects –Heuristics neccessary –ROUTE statements indicate graph cutting points

COMPUTER GRAPHIK – UNIVERSITÄT DES SAARLANDES VR 2004VRML Scene Graphs on an Interactive Ray Tracing Engine14 Scene Graph Mapping Object Grouping Group Shape nodes into OpenRT geometry objectsGroup Shape nodes into OpenRT geometry objects –Heuristics neccessary –ROUTE statements indicate graph cutting points

COMPUTER GRAPHIK – UNIVERSITÄT DES SAARLANDES VR 2004VRML Scene Graphs on an Interactive Ray Tracing Engine15 Scene Graph Mapping Instancing Instancing possible using DEF/USE mechanismInstancing possible using DEF/USE mechanism –DEF/USE equivalent to pointers –Can link arbitrary nodes OpenRT can only instantiate complete objectsOpenRT can only instantiate complete objects  Use instancing only for Transform nodes

COMPUTER GRAPHIK – UNIVERSITÄT DES SAARLANDES VR 2004VRML Scene Graphs on an Interactive Ray Tracing Engine16 Example Applications Industrial Design Car headlightCar headlight –Physically-correct reflection and refraction –Special reflector and glass shaders –Up to 25 levels of recursion –More than triangles –Only possible with ray tracing ( GHz CPUs)

COMPUTER GRAPHIK – UNIVERSITÄT DES SAARLANDES VR 2004VRML Scene Graphs on an Interactive Ray Tracing Engine17 Example Applications Animated Global Illumination Rotating globe dynamically lit by a moving lampRotating globe dynamically lit by a moving lamp –Indirect diffuse interreflection –Standard VRML scene including animation paths –Global illumination plug-in used as default shader ( GHz CPUs)

COMPUTER GRAPHIK – UNIVERSITÄT DES SAARLANDES VR 2004VRML Scene Graphs on an Interactive Ray Tracing Engine18 Summary Combination of scene graph applications and ray tracingCombination 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 workFuture work –Evaluation of other file formats like X3D –Development of a SG library optimized for RT

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