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UVA CS 445 - Real­Time Rendering & Game Technology

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Real-Time Rendering & Game TechnologyDemo TimeRecap: Level of DetailSlide 4Level of Detail: The Big QuestionsSlide 6Slide 7Some BackgroundTraditional Approach: Discrete Level of DetailDiscrete LOD: AdvantagesSlide 11Discrete LOD: DisadvantagesDrastic Simplification: The Problem With Large ObjectsDrastic Simplification: The Problem With Small ObjectsDrastic SimplificationContinuous Level of DetailContinuous LOD: AdvantagesSlide 18Slide 19View-Dependent LOD: ExamplesSlide 21Slide 22Slide 23View-Dependent LOD: AdvantagesAn Aside: Hierarchical LODAn Aside: Hierarchical LODSummary: LOD FrameworksChoosing LODs: LOD Run-Time ManagementChoosing LODsSlide 30Choosing LODs Maintaining constant frame rateChoosing LODs: Maintaining constant frame rateChoosing LODs: Funkhouser & Sequin, SIGGRAPH 93The Big QuestionMeasuring FidelityMeasuring ErrorMeasuring Visual FidelityPerceptual LODPerception 101: Contrast Sensitivity FunctionYour Personal CSFMeasuring Visual ErrorReal-Time Rendering & Game TechnologyCS 446/651David LuebkeDemo TimeNew card not installed yet so…Recap: Level of DetailThe basic idea:249,924 polys 62,480 polys 7,809 polys 975 polysRecap: Level of DetailThe basic idea:Level of Detail:The Big QuestionsHow to represent and generate simpler versions of a complex model?Courtesy Stanford 3D Scanning Repository69,451 polys 2,502 polys 251 polys 76 polysLevel of Detail:The Big QuestionsHow to evaluate the fidelity of the simplified models?Courtesy Stanford 3D Scanning Repository69,451 polys 2,502 polys 251 polys 76 polysLevel of Detail:The Big QuestionsWhen to use which LOD of an object?Courtesy Stanford 3D Scanning Repository69,451 polys 2,502 polys 251 polys 76 polysSome BackgroundHistory of LOD techniques–Early history: Clark (1976), flight simulators–Handmade LODs  automatic LODs–LOD run-time management: reactive  predictive (Funkhouser)LOD frameworks–Discrete (1976)–Continuous (1996)–View-dependent (1997)Traditional Approach: Discrete Level of DetailTraditional LOD in a nutshell:–Create LODs for each object separately in a preprocess–At run-time, pick each object’s LOD according to the object’s distance (or similar criterion)Since LODs are created offline at fixed resolutions, we call this discrete LODDiscrete LOD:AdvantagesSimplest programming model; decouples simplification and rendering–LOD creation need not address real-time rendering constraints–Run-time rendering need only pick LODsDiscrete LOD:AdvantagesFits modern graphics hardware well–Easy to compile each LOD into triangle strips, display lists, vertex arrays, …–These render much faster than unorganized triangles on today’s hardware (3-5 x)So why use anything but discrete LOD?Answer: sometimes discrete LOD not suited for drastic simplificationSome problem cases:–Terrain flyovers–Volumetric isosurfaces–Super-detailed range scans–Massive CAD modelsDiscrete LOD:DisadvantagesDrastic Simplification: The Problem With Large ObjectsCourtesy IBM and ACOGDrastic Simplification: The Problem With Small ObjectsCourtesy Electric BoatDrastic SimplificationFor drastic simplification:–Large objects must be subdivided–Small objects must be combinedDifficult or impossible with discrete LODSo what can we do?Continuous Level of DetailA departure from the traditional discrete approach:–Discrete LOD: create individual levels of detail in a preprocess–Continuous LOD: create data structure from which a desired level of detail can be extracted at run time.Continuous LOD:AdvantagesBetter granularity  better fidelity–LOD is specified exactly, not chosen from a few pre-created options–Thus objects use no more polygons than necessary, which frees up polygons for other objects –Net result: better resource utilization, leading to better overall fidelity/polygonContinuous LOD:AdvantagesBetter granularity  smoother transitions–Switching between traditional LODs can introduce visual “popping” effect–Continuous LOD can adjust detail gradually and incrementally, reducing visual popsCan even geomorph the fine-grained simplification operations over several frames to eliminate pops [Hoppe 96, 98]Continuous LOD:AdvantagesSupports progressive transmission–Progressive Meshes [Hoppe 97]–Progressive Forest Split Compression [Taubin 98]Leads to view-dependent LOD–Use current view parameters to select best representation for the current view–Single objects may thus span several levels of detailView-Dependent LOD: ExamplesShow nearby portions of object at higher resolution than distant portionsView from eyepointBirds-eye viewView-Dependent LOD: ExamplesShow silhouette regions of object at higher resolution than interior regionsView-Dependent LOD:ExamplesShow more detail where the user is looking than in their peripheral vision:34,321 trianglesView-Dependent LOD:ExamplesShow more detail where the user is looking than in their peripheral vision:11,726 trianglesView-Dependent LOD:AdvantagesEven better granularity–Allocates polygons where they are most needed, within as well as among objects–Enables even better overall fidelityEnables drastic simplification of very large objects–Example: stadium model–Example: terrain flyoverAn Aside: Hierarchical LODView-dependent LOD solves the Problem With Large ObjectsHierarchical LOD can solve the Problem With Small Objects–Merge objects into assemblies–At sufficient distances, simplify assemblies, not individual objects –How to represent this in a scene graph?An Aside:Hierarchical LODHierarchical LOD dovetails nicely with view-dependent LOD–Treat the entire scene as a single object to be simplified in view-dependent fashionHierarchical LOD can also sit atop traditional discrete LOD schemes–Imposters [Maciel 95]–HLODs [Erikson 01]Summary: LOD FrameworksDiscrete LOD–Generate a handful of LODs for each objectContinuous LOD (CLOD)–Generate data structure for each object from which a spectrum of detail can be extractedView-dependent LOD–Generate data structure from which an LOD specialized to the current view parameters can be generated on the fly. –One object may span multiple levels of detailHierarchical LOD–Aggregate objects into assemblies with their own LODsChoosing LODs:LOD Run-Time ManagementFundamental LOD issue: where in the scene to allocate detail?–For discrete LOD this equates to choosing which


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UVA CS 445 - Real­Time Rendering & Game Technology

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