1. Visibility II CS 446: Real-Time Rendering & Game Technology
David Luebke
University of Virginia

2. Demo Time: Matt Spear NOTE: I’ve reorganized the next few demos:
Feb 21: Paul Tschirhart
Feb 23: Erin Golub
Feb 28: Sean Arietta
Mar 2: Jiayuan Meng
If this is a problem, let me know!
Real-Time Rendering David Luebke

3. Assignment 3 Go over it Groups will be assigned before C.O.B. today…
Give your preferences on the forum!
Feel free to augment these by saying what you like about each idea
Real-Time Rendering David Luebke

4. Recap: Visibility Calculations
Motivation: avoid rendering redundant geometry
Huge speedup, especially for indoor environments
Basic idea: don’t render what can’t be seen
Off-screen: view-frustum culling
Occluded by other objects: occlusion culling
Real-Time Rendering David Luebke

5. Recap: Potentially Visible Set
Our goal: quickly eliminate large portions of the scene which will not be visible in the final image
Not the exact visibility solution, but a quick-and-dirty conservative estimate of which primitives may be visible
Z-buffer& clip this for the exact solution
This conservative estimate is called the potentially visible set or PVS
Real-Time Rendering David Luebke

6. Recap: Cells & Portals
Occlusion culling technique specialized for architectural models (buildings, cities, catacombs)
These divide naturally into cells
Rooms, alcoves, corridors…
Transparent portals connect cells
Doorways, entrances, windows…
Notice: cells only see other cells through portals
Real-Time Rendering David Luebke

7. Cells & Portals Idea: Cells form the basic unit of PVS
Create an adjacency graph of cells
Starting with cell containing eyepoint, traverse graph, rendering visible cells
A cell is only visible if it can be seen through a sequence of portals
So cell visibility reduces to testing portal sequences for a line of sight…
Real-Time Rendering David Luebke

8. Cells & Portals A D E F B C G H A E B C D F G H
Real-Time Rendering David Luebke

9. Cells & Portals A D E F B C G H A E B C D F G H
Real-Time Rendering David Luebke

10. Cells & Portals A D E F B C G H A E B C D F G H
Real-Time Rendering David Luebke

11. Cells & Portals A D E F B C G H A E B C D F G H
Real-Time Rendering David Luebke

12. Cells & Portals A D E F B C G H A E B C D F G H
Real-Time Rendering David Luebke

13. ? Cells & Portals A D E F B C G H A E B C D F G H ?
Real-Time Rendering David Luebke

14. Cells & Portals A D H F C B E G X
Real-Time Rendering David Luebke

15. Cells & Portals
View-independent solution: find all cells a particular cell could possibly see:
C can only see A, D, E, and H A D H F C B E G A D E H
Real-Time Rendering David Luebke

16. Cells & Portals
View-independent solution: find all cells a particular cell could possibly see:
H will never see F A D H F C B E G A D E B C G
Real-Time Rendering David Luebke

17. Cells and Portals Questions: The key insight:
How can we detect whether a given cell is visible from a given viewpoint?
How can we detect view-independent visibility between cells?
The key insight:
These problems reduce to eye-portal and portal-portal visibility
Real-Time Rendering David Luebke

18. Recap: “Luebke/Georges” algorithm
Depth-first adjacency graph traversal
Render cell containing viewer
Treat portals as special polygons
If portal is visible, render adjacent cell
But clip to boundaries of portal!
Recursively check portals in that cell against new clip boundaries (and render)
Each visible portal sequence amounts to a series of nested portal boundaries
Kept implicitly on recursion stack
Real-Time Rendering David Luebke

19. Recap: “Luebke/Georges” algorithm
Recursively rendering cells while clipping to portal boundaries is not new
Visible-surface algorithm (Jones 1971): general polygon-polygon clipping
Elegant, expensive, complicated
Conservative overestimate (pfPortals): use portal’s cull box
Cull box = x-y screenspace bounding box
Cheap to compute, very cheap to intersect
Real-Time Rendering David Luebke

20. Recap: “Luebke/Georges” algorithm
How badly does the cull box approximation overestimate PVS?
A: Not much for most architectural scenes
Note: Can implement mirrors as portals with an extra transformation!
Some clipping & Z-buffering issues
Must limit recursion
Real-Time Rendering David Luebke

21. Cells & Portals: Old Skool
Show the video…
Real-Time Rendering David Luebke

22. Creating Cells and Portals
Given a model, how might you extract the cells and portals?
Airey: k-D tree (axis-aligned boxes)
Teller: BSP tree (general convex cells)
Luebke: modeler/level designer (arbitrary cells)
Problems and issues
Running time
Free cells
Intra-wall cells
Real-Time Rendering David Luebke

23. Cells and Portals: Discussion
Good solution for most architectural models
Use the simplest algorithm that suffices for your needs:
pfPortals-style algorithm: lightweight view-dependent solution, reasonably tight PVS, no preprocess necessary (except partition)
Teller-style algorithm: even tighter PVS, somewhat more complex, can provide view-independent solution for prefetching
Real-Time Rendering David Luebke

24. General Occlusion Culling
Clearly cells and portals don’t work for all models…
Trees in a forest
A crowded train station
Other specialized visibility algorithms exist
From colonoscopy to cityscapes…
Need general occlusion culling algorithms:
Aggregate occlusion
Dynamic scenes
Non-polygonal scenes
Real-Time Rendering David Luebke

25. Image-Space Occlusion Culling
Many general occlusion culling algorithms use an image-space approach
Idea: solve visibility in 2D, on the image plane
Real-Time Rendering David Luebke

26. Hierarchical Z-Buffer
Replace Z-buffer with a Z-pyramid
Lowest level: full-resolution Z-buffer
Higher levels: each pixel represents the max depth of the four pixels “underneath” it
Basic idea: hierarchical rasterization of the polygon, with early termination where polygon is occluded
Real-Time Rendering David Luebke

27. Hierarchical Z-Buffer
Idea: test polygon against highest level first
If polygon is further than distance recorded in pixel, stop—it’s occluded
If polygon is closer, recursively check against next lower level
If polygon is visible at lowest level, set new distance value and propagate up
Real-Time Rendering David Luebke

28. Hierarchical Z-Buffer
Z-pyramid exploits image-space coherence:
Polygon occluded in a pixel is probably occluded in nearby pixels
HZB also exploits object-space coherence
Polygons near an occluded polygon are probably occluded
Real-Time Rendering David Luebke

29. Hierarchical Z-Buffer
Exploiting object-space coherence:
Subdivide scene with an octree
All geometry in an octree node is contained by a cube
Before rendering the contents of a node, “test render” the faces of its cube (i.e., query the Z-pyramid)
If cube faces are occluded, ignore the entire node
Real-Time Rendering David Luebke

30. Hierarchical Z-Buffer
HZB can exploit temporal coherence
Most polygons affecting the Z-buffer last frame will affect Z-buffer this frame
HZB also operates at max efficiency when Z-pyramid already built
So start each frame by rendering octree nodes visible last frame
Real-Time Rendering David Luebke

31. Hierarchical Z-Buffer: Discussion
HZB needs hardware support to be really competitive
Hardware vendors haven’t entirely bought in:
Z-pyramid (and hierarchies in general) a pain in hardware
Unpredictable Z-query times generate bubbles in rendering pipe
But we’re getting there…
ATI HyperZ
Similar technology in NVIDIA
Both “under the hood”, not exposed to programmer
At the user level, hardware now supports occlusion queries
Real-Time Rendering David Luebke

32. Modern Occlusion Culling
Support from hardware would be nice
Want an “occlusion test”: would this polygon be visible if I rendered it?
How could you use such a test?
Test portal polygons before rendering adjacent cell
Test object bounding boxes before rendering object
Yay! GL_HP_OCCLUSION_TEST extension
Problems:
CPU/GPU synchronization == bad
Might want to know “how visible” is the polygon
Real-Time Rendering David Luebke

33. Modern Occlusion Culling
GL_ARB_OCCLUSION_QUERY to the rescue
Non-blocking query
“Is this occlusion query done yet?”
Multiple queries in flight
Returns number of fragments visible
Note: can actually render object or not
Still lots of issues for efficient culling
Real-Time Rendering David Luebke

34. 111 uses for Occlusion Queries
Occlusion culling (duh)
Others?
Approximate culling
LOD size estimation
Lens flare effects
Transparency
Collision detection (!)
Convergence testing
Real-Time Rendering David Luebke