Recap: General Occlusion Culling l When cells and portals don’t work… –Trees in a forest –A...

Recap:General Occlusion Culling

When cells and portals don’t work…– Trees in a forest– A crowded train station

Need general occlusion culling algorithms:– Aggregate occlusion – Dynamic scenes– Non-polygonal scenes

Recap: Image-SpaceOcclusion Culling

Most general occlusion culling algorithms use an image-space approach– Idea: solve visibility in 2D, on the image plane– Note: gives up view-independent visibility

Recap: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

Recap: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

Recap: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

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) unfriendly to

hardware– Unpredictable Z-query times generate bubbles in

rendering pipe

But there are promising trends:– ATI’s HYPER-Z technology: one-level HZB– 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_NV_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 Supports object-space coherence,

temporal coherence Still lots of issues for efficient cullingWrong

Modern Occlusion Culling

GL_ARB_OCCLUSION_QUERY – Deprecates NV_OCCLUSION_QUERY– Fixes a potential race condition

111 uses for NV_OCCLUSION_QUERY

Occlusion culling (duh) Others?

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

NV_OCCLUSION_QUERY:Details

Go to NVIDIA presentation…

Occlusion Culling:Grab Bag

Portal textures – Dan Aliaga and others

From-region visibility– Generalizes view-independent cell-portal problem

Linear-time view-independent portal cull box approach?– Nina Amenta and others

Modeling issues for cells-and-portals– Daniel Cohen-Or and others

Advanced Texturing:Stupid Texture Tricks

David Luebke

University of Virginia

Point Sprites

A point sprite is a screen-aligned textured quad placed by rendering a single vertex – Ideal for particle systems

– When GL_POINT_SPRITE_NV is enabled: Point antialiasing state ignored – all points quads Points are rendered with point width as usual Tex coords of points can be replaced by special “point sprite” tex

coords s,t,r between 0,1 Tex coord r is special, can set to zero (default) or use to ‘play through’

an animation stored in 3D tex– Enable this with COORD_REPLACE_NV

For hardware acceleration on GF3, set r to 0 (default) and enable coordinate replacement for tex unit 3 only

– See http://www.nvidia.com/dev_content/nvopenglspecs/GL_NV_point_sprite.txt– See http://www.codesampler.com/oglsrc.htm; search for “point sprite”

Billboards

A point sprite is a certain kind of billboard– Billboards are textured polygons (usually quads)

that rotate to face the viewer Build a rotation matrix for each billboard Screen-aligned billboard: quad is parallel to the screen and

has a constant up vector Similar to old-school 2D sprites Useful for text, HUDs, lens flare, etc Build rotation matrix with camera u, n = -v, r=uXv

World-oriented billboard Sprite’s native up vector not always appropriate World-oriented billboard: use the world up vector Still faces viewer n = -v, r=uXv

Billboards

Viewpoint-aligned– v = vector to eye (Fig. 8.5)– Resulting quads capture perspective distortions

across view-frustum– Ex: clouds (Fig 8.6)

Axially-alligned– Rotates around a fixed world-space axis– Ex: trees (Fig 8.7)– Problem: from above, look like cardboard

cutouts

Imposters

An imposter is a billboard created on the fly to “cache” rendered imagery– Once rendered, cost of rendering an imposter

is just a single textured quad– Can use for a few frames before updating– Can use for a few instances of the object

Works best on distant objects (why?) Great example: portal textures

– But what’s wrong with this idea?

Depth Sprites

Can render an object with a depth texture, so depth buffer affects and is affected by the rendering (Fig 8.16)– Can also do depth-affected lighting – Needs to be orthogonal or nearly orthogonal

to look right– Under the right circumstances, might allow

imposters with dynamic geometry Ex: portal texture with monster moving around in

the textured room

Imposters Continued

Depth meshes– UNC MMR system

Multitexturing

Modern hardware can read from multiple textures at once, even with mipmapping

Detail texturing Light mapping Bump mapping