Hierarchical Occluders for Interactive Rendering of Large Models

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Hierarchical Occluders for Interactive Rendering
of Large Models

• Avneesh Sud

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Motivation

• The Dream of Viewing the Double Eagle at 20 fps
  in good fidelity

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Occlusion Culling

• Hidden Surface Removal methods are not fast
  enough for massive models on current hardware
• Occlusion Culling avoids rendering primitives
  that are occluded by another part of the scene
• Occlusion Culling techniques are ideally output
  sensitive runtime is proportional to the size
  of exact visibility set

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Related Work

• Hierarchical Z-Buffer
• Image space occlusion culling method Greene93
• Build a layered Z-pyramid with a different
  resolution of the Z-buffer at each level
• Allows quick accept/reject
• Hierarchical LODs
• Simplification Culling Approximate entire
  branch of the scene graph by an HLOD
• Can we use HLODs as occluders/occludees?

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Parallel Occlusion Culling

• Bill Baxters idea of using separate pipelines
  for occlusion tests and rendering Spring 2000
• Use the HZB for rendering acceleration

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Hierarchical Parallel Occlusion Culling

• Build an HLOD representation of the model and
  find a set of hierarchical occluders for culling
• Build a HZB from the hierarchy of occluders in
  the occlusion pipeline
• Test the objects against the HZB and pass the
  visible nodes to the rendering pipeline for
  displaying at the appropriate LOD

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HPOC System

• 4 Threads
• Software Culling
• Hardware Culling
• Display
• Load
• LOD selection on pixel error
• Display and Culling threads in synch or asynch
  modes

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HPOC System
Coarse HZB
Coarse HZB
All Nodes Camera
VFC
VFC
SW Culler
Maybe Occluded
Visible
Build HZB
Occlusion Pipeline
HW Culler
Histogram Test
Z-Buffer
Visible Nodes
Renderer
Display Pipeline
Render
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Partitioning

• Repartition the model for better visibility
  coherence Karl Hillesland Fall 00
• Worked well with small disconnected objects (e.g.
  split Double Eagle)
• Fixed bug to work with connected meshes

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Partitioning

• Even then splits could be lopsided

Torpedo Room Leaf Object 232627 faces of 705917
Try splitting across different axes if a split
across the longest axis fails !
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Constructing the Scene Graph

• Build an AABBTree hierarchy
• Create HLODs from geometry at leaf nodes
• Use GAPS to simplify geometry at each node
• Need better terminating conditions for GAPS

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Scene Graph AABBTree

• While the partitioning is smart, AABBTree
  subdivides across longest axis

Rightmost leaf node of the AABBTree Most of the
geometry inside the shell is grouped into 1 node.
Use a different partitioning hierarchy
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Occluder/Occludee Selection

• Currently use same nodes for culling and display

Bad Occludee Selection Close up top view of the
double eagle HPOC is rendering 323 objects and
160k polygons while there are 4 objects and 370
polygons visible.
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Further Work

• Better partitioning, HLOD construction
• Memory management
• Using coarser HLOD occlusion bounding volumes,
  finer LODs for occludee selection
• Timing Analysis

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Lessons

• Working with massive models is a pain
• Multithreaded massive model rendering is a bigger
  pain
• Good Occlusion Culling is a tough problem