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CHC ++: Coherent Hierarchical Culling Revisited

CHC ++: Coherent Hierarchical Culling Revisited. Oliver Mattausch, Jiří Bittner, Michael Wimmer. Institute of Computer Graphics and Algorithms Vienna University of Technology. CHC++: Fast occlusion culling algorithm. Occlusion Culling. Render only visible geometry → Output sensitivity

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CHC ++: Coherent Hierarchical Culling Revisited

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  1. CHC ++:Coherent Hierarchical Culling Revisited Oliver Mattausch, Jiří Bittner, Michael Wimmer Institute of Computer Graphics and Algorithms Vienna University of Technology

  2. CHC++: Fast occlusion culling algorithm Oliver Mattausch

  3. Occlusion Culling • Render only visible geometry → Output sensitivity • Preprocessing vs.Online occlusion culling Preprocessing: Visibility from a region Oliver Mattausch

  4. Online Occlusion Culling • Query visibility from view point • No preprocessing • Dynamic Scenes • Hardware occlusion queries → # visible pixels Visible? Query bounding box Render geometry Oliver Mattausch

  5. Naive method: Hierarchical Stop & Wait • For each node: Issue query • Visible→ traverse subtree • Invisible→ cull subtree • Problem: Query latency • CPU stalls • GPU starvation Cull Render Render Hierarchical Culling Oliver Mattausch

  6. Previous Work • Coherent Hierarchical Culling (CHC) [Bittner04] • Near Optimal Hierarchical Culling (NOHC) [Guthe06] Oliver Mattausch

  7. CHC • While waiting for query result → traverse / render • Keep query queue • Use coherence, assume node stays (in)visible • For previously visible nodes • Don‘t wait for query result Use result in next frame Issue query Result available? Render geometry Oliver Mattausch

  8. Problems of CHC • Too many queries • Not GPU friendly • Many state changes • Bounding box query (8 vertices per draw call) • Can be slower (!) than view frustum culling (VFC) Most houses visible →Bad view point for CHC Oliver Mattausch

  9. Properties of NOHC • Query only if cheaper than rendering • Mostly better than view frustum culling • Close to self-defined optimum • Hardware calibration step • Complex set of rules • Possible to beat the defined optimum • Can reduce cost of queries • Can further reduce # queries Oliver Mattausch

  10. Improved algorithm: CHC ++ • Reduction of • State changes • Queries • Wait time • Rendered geometry • Keeps simplicity of CHC Oliver Mattausch

  11. Building Blocks of CHC ++ • Query batching • Reduction of state changes • Reduction of CPU stalls • Multiqueries • Reduction of queries • Randomization • Better distribution of queries • Tight bounding volumes • Reduction of queries • Reduction of rendered geometry Oliver Mattausch

  12. Query Batching: State Changes • Switch between render / query mode → Need state change (depth write on / off) • CHC induces one state change per query • Big overhead on modern GPUs! Oliver Mattausch

  13. Query Batching: Previously invisible nodes • Idea: Store query candidates in separate queue • Collect n nodes • Switch to query mode • Query all nodes Query State change State change Query Render mode Query Query Query Candidate queue Query queue Oliver Mattausch

  14. Query Batching: Previously visible nodes • Previously visible nodes • No dependencies (geometry rendered anyway) • Can issue query at any time • Handle them in separate queue • Issue queries to fill up wait time • Very likely no new state change • Issue rest of queries in next frame Oliver Mattausch

  15. Query Batching: Visualization Each color represents a state change CHC: ~100 state changes CHC++:2 state changes(Max. batch size: 50) Oliver Mattausch

  16. Multiqueries: Idea • Node invisible for long time • Likely to stay invisible (e.g., car engine block) • Cover many nodes with single query • Test q invisible nodes by single multiquery • Invisible → saved (q – 1) queries • Visible → must test individually, wasted one query Oliver Mattausch

  17. Multiqueries : Minimize #queries • Use history of nodes • Estimate probability that node will still be invisible in frame n if it was invisible in frame n - 1 • Measurements behave like certain exp() function → sufficient in practice Fitted andmeasured functions Oliver Mattausch

  18. Multiqueries: Greedy Algorithm • While node batch not empty • Add node to multiquery • Use cost / benefit model • Query size optimal → issue multiquery Visualization: Each color represents a multiquery Oliver Mattausch

  19. Queues in CHC++ traversal queue i-queue (invisible nodes) v-queue(visible nodes) Multiquery query queue Oliver Mattausch

  20. Randomization: Assumed Visibility • Test previously visible nodes each frame → queries wasted • Assume visible for t frames • Frame rate drops every t frames Q Q Q Q Q Q Q Q Q Q Q Q Oliver Mattausch

  21. Randomization: Idea • When node becomes visible • Randomize first invocation between [1.. t] • Afterwards, test every t frames Q Q Q Q Q Q Q Q Q Q Q Q Oliver Mattausch

  22. Randomization: Properties • Even distributionover frames • Nodes tested in regular intervals • Very stable for t between 5 - 10 • Tried sophisticated models • But they could not beat the simple randomization! Oliver Mattausch

  23. Tight Bounding Volumes: Idea • Optimization for bounding volume hierarchy (BVH) • For each node → query bounding boxes of children(using single query) Child boxes invisible → Cull node, saved 2 queries Box visible → Traverse node Oliver Mattausch

  24. Tight Bounding Volumes for Leaves • Subdivide deeper than actual hierarchy depth → tight bounds also for leaves • Better adjusts to shape of objects Tight bounds shown in red Oliver Mattausch

  25. Tight Bounding Volumes: Properties • Bounds for interiors reduce queries • Bounds for leaves reduce geometry (without overhead of deeper hierarchy) • More boxes per query not relevant for hardware Oliver Mattausch

  26. Results Pompeii (6M triangles) Powerplant (12M triangles) Oliver Mattausch

  27. Results: Powerplant (12M triangles) Oliver Mattausch

  28. Results: Powerplant (12M triangles) CHC + Batching + Randomization + Tight Bounds +Multiqueries Oliver Mattausch

  29. Results: Pompeii (6M triangles) Oliver Mattausch

  30. Conclusions • Hierarchical culling algorithm based on CHC • Kept simplicity of CHC • Improved several issues of CHC • Always better than view frustum culling • Up to 2 – 3 times speedup • Better than optimum as defined by NOHC • Drawback: several parameters Oliver Mattausch

  31. Multiqueries: Vienna (1M triangles) Oliver Mattausch

  32. Questions? THANK YOU FOR YOUR ATTENTION! Oliver Mattausch

  33. Batching: Render engine integration • Rendering in modern engines • Collect visible objects in render queue • Sort by materials • Render everything at once • Rendering single nodes is inefficient (CHC) • With batching: • Traverse render queueonly before switch to query mode Oliver Mattausch

  34. Future Work • (Semi-)automatically find optimal values for parameters • Calibrate parameters during representative walkthrough • Remove overhead also for difficult view points Oliver Mattausch

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