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

Visibility II

CS 446: Real-Time Rendering& Game Technology

David LuebkeUniversity of Virginia

Real-Time Rendering 2 David Luebke

NOTE: I’ve reorganized the next few demos:

Feb 21: Paul Tschirhart Feb 23: Erin GolubFeb 28: Sean AriettaMar 2: Jiayuan Meng

If this is a problem, let me know!

Demo Time: Matt Spear


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


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


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


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


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…


Cells & Portals

A

D

H

FCB

E

G

H

B C D F G

EA


Cells & Portals

A

D

H

FCB

E

G

H

B C D F G

EA


Cells & Portals

A

D

H

FCB

E

G

H

B C D F G

EA


Cells & Portals

A

D

H

FCB

E

G

H

B C D F G

EA


Cells & Portals

A

D

H

FCB

E

G

H

B C D F G

EA


Cells & Portals

A

D

H

FCB

E

G

H

B C D F G

EA

?

?


Cells & Portals

A

D

H

FCB

E

G

H

B C D F G

EA

X

X


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

FCB

E

G

A

D

H

E


Cells & Portals

• View-independent solution: find all cells a particular cell could possibly see:

H will never see F

A

D

H

FCB

E

G

A

D

CB

E

G


Cells and Portals

• Questions:– 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


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



• 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



• 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


Cells & Portals: Old Skool

• Show thevideo…


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


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


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


Image-Space Occlusion Culling

• Many general occlusion culling algorithms use an image-space approach

• Idea: solve visibility in 2D, on the image plane


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



• 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



• 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



• 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



• 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


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


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


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


111 uses for Occlusion Queries

• Occlusion culling (duh)• Others?

– Approximate culling– LOD size estimation– Lens flare effects– Transparency– Collision detection (!)– Convergence testing

Documents

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