Visibility Driven Out-of-Core HLOD RenderingPatrick CozziThe University of Pennsylvania

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Project History

Procedurally generated model of Pompeii: ~1.4 billion polygons.Image from [Mueller06]

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Project History

Boeing 777 model: ~350 million polygons.Image from http://graphics.cs.uni-sb.de/MassiveRT/boeing777.html

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Contents

Previous Work View Frustum and Occlusion Culling

Hardware Occlusion Queries (HOQ) Level of Detail (LOD)

Hierarchical Level of Detail (HLOD) Out-of-Core Rendering (OOC)

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Contents Continued

Implementation Work Vertex Clustering [Rossignac93] HLOD Tree Creation Primary Contribution: OOC Rendering

Results Future Work Demos throughout

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View Frustum Culling

Can be slower than brute force. When?

culled

rendered

culled

culled

rendered

rendered

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View Frustum Culling

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View Frustum Culling

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View Frustum Culling

Demo

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Occlusion Culling

Effective in scenes with high depth complexity

culled

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Occlusion Culling

From-region or from-point Most are conservative Occluder Fusion Difficult for general scenes with

arbitrary occluders. So make simplifying assumptions: [Wonka00] – urban environments [Ohlarik08] – planets and satellites

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Hardware Occlusion Queries

From-point visibility that handles general scenes with arbitrary occluders and occluder fusion

How? Use the GPU

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Hardware Occlusion Queries

Disable color and depth write

Color Buffer Depth Buffer

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Hardware Occlusion Queries

Disable color and depth write Render BV using HOQ

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Hardware Occlusion Queries

Disable color and depth write Render BV using HOQ Enable color and depth writes

Color Buffer Depth Buffer

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Hardware Occlusion Queries

Disable color and depth write Render BV using HOQ Enable color and depth writes Render object based on HOQ

results

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Hardware Occlusion Queries

class IQueryOcclusion{public: virtual void Begin() = 0; virtual void End() = 0; virtual bool IsResultAvailable() = 0;

virtual unsigned int NumberOfSamplesPassed() = 0; virtual unsigned int NumberOfFragmentsPassed() = 0;};

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Hardware Occlusion Queries

class IQueryOcclusion{public: virtual void Begin() = 0; virtual void End() = 0; virtual bool IsResultAvailable() = 0;

virtual unsigned int NumberOfSamplesPassed() = 0; virtual unsigned int NumberOfFragmentsPassed() = 0;};

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Hardware Occlusion Queries

class IQueryOcclusion{public: virtual void Begin() = 0; virtual void End() = 0; virtual bool IsResultAvailable() = 0;

virtual unsigned int NumberOfSamplesPassed() = 0; virtual unsigned int NumberOfFragmentsPassed() = 0;};

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Hardware Occlusion Queries

CPU stalls and GPU starvation

Draw o1 Draw o2 Draw o3

Draw o1 Draw o2 Draw o3

CPU

GPU

Query o1

Query o1

Draw o1

Draw o1

-- stall --

-- starve --

CPU

GPU

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Is Culling Enough?

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Is Culling Enough?

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Is Culling Enough?

Demo

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Level of Detail

Generation: less triangles, simpler shader

Selection: distance, pixel size Switching: avoid popping

Discrete, Continuous, Hierarchical

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Discrete LOD

3,086 Triangles 52,375 Triangles 69,541 Triangles

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Discrete LOD

Demo

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Discrete LOD

Not enough detail up close

Too much detail in the distance

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Continuous LOD

edge collapse

vertex split

Image from [Luebke01]

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Hierarchical LOD

1 Node3,086 Triangles

4 Nodes9,421 Triangles

16 Nodes77,097 Triangles

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Hierarchical LOD

1 Node3,086 Triangles

4 Nodes9,421 Triangles

16 Nodes77,097 Triangles

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Hierarchical LOD

visit(node) { if (computeSSE(node) < pixel tolerance) { render(node); } else { foreach (child in node.children) visit(child); } }

Node Refinement

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Hierarchical LOD

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Hierarchical LOD

New Problem: Cracks

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Hierarchical LOD

Demo

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HLOD + Culling

visit(node){ if (node overlaps view frustum) { // ... }}

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HLOD + Culling

visit(node){ if (node overlaps view frustum) { render node’s BV with HOQ if (query.NumberOfFragmentsPassed() > 0) { // ... } }}

Render front to back!

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HLOD + Culling + VMSSEvisit(node){ if (node overlaps view frustum) { render node’s BV with HOQ if (query.NumberOfFragmentsPassed() > 0) { if (computeVMSSE(node, query) < tolerance) { render(node); } else { // ... } } }}

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VMSEE: Virtual Multiresolution SSE Relative Visibility =

# pixels visible / # possible pixels visible

VMSSE = f(SSE, Relative Visibility)

VMSSE

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Optimized HLOD Refinement Driven by HOQs [Charalambos07]

Exploit spatial and temporal coherence for scheduling HOQs.

Predict refinement based on node’s relative visibility from previous frame

VMSSEiest = SSEi * biasi-1

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Optimized HLOD Refinement Driven by HOQs [Charalambos07]

Example prediction Refinement stopped for this node in

previous frame VMSSEi

est < threshold ? Stop : Refine Stop:

Issue query Render without checking query

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Implementation Work

3 HLOD algorithms including [Charalambos07]

Vertex Clustering HLOD Tree Creation OOC Rendering

Load/Unload Rules Rendering Replacement Policy Multithreading

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Vertex Clustering [Rossignac93]

Fast: expected O(n) Robustness: arbitrary topology Capable of drastic simplification “Easy to code” OOC extensions [Lindstrom00]

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Vertex Clustering [Rossignac93]

1. Compute per-vertex weights

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2. Assign vertices to clusters

3. Identify highest weighted

vertex in each cluster

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Vertex Clustering [Rossignac93]

1. Compute per-vertex weights

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0.8

2. Assign vertices to clusters

3. Identify highest weighted

vertex in each cluster

4. Collapse and remove

degenerate triangles

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Vertex Clustering [Rossignac93]

3,086 Triangles 52,375 Triangles 69,541 Triangles

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Vertex Clustering [Rossignac93]

Questionable Fidelity Hard to control output Conservative Error Metric

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HLOD Tree Creation

Input Model (.ply, .obj) Target triangles per leaf node Maximum tree depth

Output 1 file per node Normals computed at runtime

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HLOD Tree Creation

Top-down Root node:

Full AABBLowest Detail

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HLOD Tree Creation

Splitting Planes

2 Planes 3 Planes

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HLOD Tree Creation

Splitting Planes

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HLOD Tree Creation

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visit(node) { if ((computeSSE(node) < pixel tolerance) || (not all children resident)) { render(node); foreach (child in node.children) requestResidency(child); } else { foreach (child in node.children) visit(child); } }

Previous Work: Out-of-Core

Based on [Ulrich02]

Prefetch

Need all children

To render

To refine

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Previous Work: Out-of-Core

[Varadhan02] Requires full skeleton in memory No occlusion culling No front-to-back sorting

Image From [Varadhan02]

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Previous Work: Out-of-Core

[Corrêa03] PLP in separate thread Requires full skeleton in memory No LOD

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Out-of-Core

Replacement Policy? LRU? Can’t refine when one child is removed

Remove deepest child in parent’s tree?

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OOC Rendering

Benefits of our algorithm No full HLOD skeleton Works with HOQs Refinement with a subset of children Replacement policy maximizes detail

near the viewer Multithreaded

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OOC Rendering: Load/Unload

HLOD tree on disk

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OOC Rendering: Load/Unload

Subset of HLOD tree in memory

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OOC Rendering: Load/Unload

Load node -> load children skeletons

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OOC Rendering: Load/Unload

Only unload dynamic leafs

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OOC Rendering: Load/Unload

Only unload dynamic leafs

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OOC Rendering: Load/Unload

Nodes don’t need all their children in memory

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OOC Rendering: Load/Unload

Result: If a node is not a skeleton, none of its

ancestors are skeletons. In other words, if a node has geometry loaded, so does all of its ancestors.

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OOC Rendering: Load/Unload

Never Happens:

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OOC Rendering: Rendering

Modify in-core HLOD Add request queue:

Stop refinement at skeleton node Push node onto request queue Ensure parent safety Render subset of parent’s geometry

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OCC Rendering: Subset of Parent

Use OpenGL clipping planes

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OCC Rendering: Subset of Parent

Without clipping planes

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OCC Rendering: Subset of Parent

Demo

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OCC Rendering: Node Replacement

Replacement List (only dynamic leafs)

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OCC Rendering: Node Replacement

Replacement List Partitions

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OCC Rendering: Node Replacement

Start Frame

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OCC Rendering: Node Replacement

Add Node

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OCC Rendering: Node Replacement

Render Node

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OCC Rendering: Node Replacement

Move to safety

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OCC Rendering: Node Replacement

Suggest Removal Node

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OCC Rendering: Multithreading

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Low Memory

Demo

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Selected Results (lol)

Load Time 10 Blocks in Pompeii 5,646,041 triangles

Time in seconds

Full model 5.2

Out-of-Core 0.05

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Selected Results

View 1 View 2

Zoomed out rendering

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Selected Results

View 1 View 2

Brute Force 63 fps5,646,041 triangles

63 fps5,646,041 triangles

HLOD - SSE 1,415 fps161,742 triangles

881 fps302,337 triangles

HLOD - Naive VMSEE 1,060 fps140, 458 triangles

300 fps260,007 triangles

HLOD - Scheduled VMSSE

1,176 fps140, 458 triangles

588 fps270,774 triangles

Zoomed out rendering

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Selected Results

Zoomed In Rendering

View 3 View 4

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Selected Results

Zoomed In Rendering

View 3 View 4

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Selected Results

Zoomed In RenderingView 3 View 4

Brute Force 62 fps5,646,041 triangles

62 fps5,646,041 triangles

HLOD - SSE 128 fps2,541,434 triangles

98 fps3,222,701 triangles

HLOD - Naive VMSEE 180 fps346,901 triangles

320 fps46,765 triangles

HLOD - Scheduled VMSSE

210 fps601,730 triangles

232 fps103,844 triangles

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Statistics

Lines of Code GUI: 420 Unit Tests: 1,720 HLOD Creation: 4,600 Rendering: 4,500

Time Spent Coding: 8 weeks “fulltime.” 3 last

spring, 5 this fall. Plus reading, writing, slides, and

logistics.

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Future Work

Improve tree creation Polygonal simplification Splitting planes

Fill cracks Optimal disk layout Better occlusion performance

Multiple volumes or occlusion-preserving low LOD

Optimize use of clipping planes

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Future Work

Don’t require ancestors to have geometry loaded. Much better use of memory More complicated rendering More rendering artifacts

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Future Work

Cache Management Aggressively remove nodes Replacement Policy: Average detail

instead of best up close

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Future Work

Multithreading Multiple load threads

Fault tolerance, increase throughput Compute thread(s)

Compute normals Decompress (/ recompress) Vertex cache optimize?

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Future Work

True Usefulness Textures Picking on individual objects Test with truly massive models

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Future Work

Today Mad Mex Hour Happy. Now – 6:30pm

Saturday, February 7th

Graduation Party. My House. 3pm.

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