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

Real-Time Rendering 2 David Luebke Happy Valentines Day!

Real-Time Rendering 3 David Luebke Bittersweets: Dejected sayings I MISS MY EX PEAKED AT 17 MAIL ORDER TABLE FOR 1 I CRY ON Q U C MY BLOG? REJECT PILE PILLOW HUGGIN ASYLUM BOUND DIGNITY FREE PROG FAN STATIC CLING WE HAD PLANS XANADU 2NITE SETTLE 4LESS NOT AGAIN

Real-Time Rendering 4 David Luebke Bittersweets: Dysfunctional sayings RUMORS TRUE PRENUP OKAY? HE CAN LISTEN GAME ON TV CALL A 900# P.S. I LUV ME DO MY DISHES UWATCH CMT PAROLE IS UP! BE MY YOKO U+ME=GRIEF I WANT HALF RETURN 2 PIT NOT MY MOMMY BE MY PRISON C THAT DOOR?

Real-Time Rendering 5 David Luebke Demo Time: Ken Arthur

Real-Time Rendering 6 David Luebke Homework The pitch, round 2 –Many ideas way too hard Art assets (Much) character animation Elaborate AI/gameplay –Some of my favorites so far: Gallery Mariokart Pirates (but just the ships) Combat billiards Orb (a la marble madness) Waverace –Too hard: Total War (Full-blown) RPG Ninjas UVA v. VaTech –Yes to: Terrain LOD, mocap data, NPR, Shadows –No to: Combat animations, AI, big worlds –Out-of-the-box ideas?

Real-Time Rendering 7 David Luebke Project preview Next 48 hours: –Pitch a few more ideas on forum –Submit “signup sheet” by List 3 project ideas you’d be happy to work on I assign groups on Thursday based roughly on interest overlap –Note: I assign groups, not project ideas! –Assignment 3: Groups build a basic engine Groups propose their final idea –Later may reorganize groups, allow 3 rd -party engines

Real-Time Rendering 8 David Luebke 5 Steps to Efficient Rendering Step 1: reduce #vertices/triangle Step 2: reduce overhead of each vertex Step 3: recognize the existence of the vertex cache Step 4: display lists (deprecated re: performance) Step 5: learn about the hardware Meta-step 0: profile and analyze code

Real-Time Rendering 9 David Luebke Other things you should know Most interesting stuff in OpenGL isn’t in red book –Extension registry Extensions you should know: –ARB_multitexture –EXT_framebuffer_object –ARB_occlusion_query –ARB_point_sprite –ARB_texture_rectangle –ARB_draw_buffers –ARB_texture_float (_framebuffer_float, _half_float_pixel, etc) –ARB_depth_texture –ARB_vertex_buffer_object (_pixel_buffer_object)

Real-Time Rendering 10 David Luebke Other things you should know Most interesting stuff in OpenGL isn’t in red book –Extension registry –Developer sites Find demos, tools, tutorials, presentations, white papers

Real-Time Rendering 11 David Luebke Visibility Calculations Motivation –Avoid rendering redundant geometry Complements LOD, potentially even more powerful –Basic idea: don’t render what can’t be seen Off-screen: view-frustum culling Occluded by other objects: occlusion culling

Real-Time Rendering 12 David Luebke Motivation The obvious question: why bother? –Off-screen geometry: solved by clipping –Occluded geometry: solved by Z-buffer The (obvious) answer: efficiency –Clipping and Z-buffering take time linear to the number of primitives

Real-Time Rendering 13 David Luebke The Goal 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 14 David Luebke View-Frustum Culling An old idea (Clark 76): –Organize primitives into clumps –Before rendering the primitives in a clump, test a bounding volume against the view frustum If the clump is entirely outside the view frustum, don’t render any of the primitives If the clump intersects the view frustum, add to PVS and render normally

Real-Time Rendering 15 David Luebke Efficient View-Frustum Culling How big should the clumps be? –Choose minimum size so: cost testing bounding volume << cost clipping primitives –Choose minimum size so primitives can be rendered efficiently At least 500 triangles/clump on today’s hardware –Organize clumps into a hierarchy of bounding volumes for more efficient testing If a clump is entirely outside or entirely inside view frustum, no need to test its children

Real-Time Rendering 16 David Luebke Efficient View-Frustum Culling What shape should bounding volumes be? –Spheres and axis-aligned bounding boxes: simple to calculate, cheap to test – Oriented bounding boxes converge asymptotically faster in theory –Lots of other volumes have been proposed Capsules, ellipsoids, k-DOPs –…but most use spheres or AABBs.

Real-Time Rendering 17 David Luebke Cells & Portals Goal: walk through 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 18 David Luebke Cells & Portals An example:

Real-Time Rendering 19 David Luebke 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 20 David Luebke Cells & Portals A D H F CB E G H BCDFG EA

Real-Time Rendering 21 David Luebke Cells & Portals A D H F CB E G H BCDFG EA

Real-Time Rendering 22 David Luebke Cells & Portals A D H F CB E G H BCDFG EA

Real-Time Rendering 23 David Luebke Cells & Portals A D H F CB E G H BCDFG EA

Real-Time Rendering 24 David Luebke Cells & Portals A D H F CB E G H BCDFG EA

Real-Time Rendering 25 David Luebke Cells & Portals A D H F CB E G H BCDFG EA ? ?

Real-Time Rendering 26 David Luebke Cells & Portals A D H F CB E G H BCDFG EA X X

Real-Time Rendering 27 David Luebke 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 CB E G A D H E

Real-Time Rendering 28 David Luebke Cells & Portals View-independent solution: find all cells a particular cell could possibly see: H will never see F A D H F CB E G A D CB E G

Real-Time Rendering 29 David Luebke 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

Real-Time Rendering 30 David Luebke Cells and Portals: History Airey (1990): view-independent only –Portal-portal visibility determined by ray-casting Non-conservative portal-portal test resulted in occasional errors in PVS –Slow preprocess –Order-of-magnitude speedups

Real-Time Rendering 31 David Luebke Cells and Portals: History Teller (1993): view-independent + view-dependent –Portal-portal visibility calculated by line stabbing using linear program Cell-cell visibility stored in stab trees View-dependent eye-portal visibility stage further refines PVS at run time –Slow preprocess –Elegant, exact scheme

Real-Time Rendering 32 David Luebke Cells and Portals: History Luebke & Georges (1995): view-dependent only –Eye-portal visibility determined by intersecting portal cull boxes –No preprocess (integrate w/ modeling) –Quick, simple hack –Now-archaic public-domain library: pfPortals

Real-Time Rendering 33 David Luebke pfPortals 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 34 David Luebke pfPortals 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