Graphics Topics in VR

Collaborative Visual Computing LabDepartment of Computer Science

University of Cape Town

Graphics Topics in VRGraphics Topics in VR

By Shaun Nirenstein

OverviewOverview

• Impact of graphics/geometric algorithms on VR

• Visibility in VR

• Rendering Topics in VR

• Collision Detection• Animation (I won’t talk about this)

Impact of graphics/geometric algorithms on VR

• Constrains the VR• Visibility

• Scene complexity• Performance

• Shadows• Cues• Soft shadows offer more information about the light

source• Shadows are difficult(slow), soft shadows are more

difficult(slow)

Impact of graphics/geometric algorithms on VR

• Constrains the VR (cont.)• Illumination models

• Defines the set of surface materials available• Global vs. local illumination• What can be done at real-time?

– Global specular?– Global diffuse?– Arbitrary BRDF (Bidirectional Reflectance Distribution

Function)?

• Collision detection• How accurate?• How fast?

• (Agent/Avatar) Animation• Scripted, captured, simulated

VisibilityVisibility• Obvious application is to manage geometric

complexity• Don’t render what you cannot see

• Less obvious application is to manage global complexity• Don’t “simulate” invisible objects• Can X see Y (e.g. AI)• Don’t illuminate invisible objects• Predictive cache management!!!

• Could Y be visible from X soon– Invisible geometry, textures, bump maps, vertex/pixel

programs do not have to be resident

Visibility AlgorithmsVisibility Algorithms

• From-point techniques:• What is visible from view point X

• From-region techniques:• What is visible from view region R• I.e. Y is visible from R, if there exists a

point P in R, which can see Y• Can bind a visible set to time

From Point VisibilityFrom Point Visibility

• Occluder shadow volumesInvisible

Occluder

Occluder Fusion (from point)Occluder Fusion (from point)

Occluder

Invisible

Invisible???

• Cells and portals

ProblemsProblems• Cells and portals

• Only work (well) for “architectural scenes”

• Occluder Shadows• Fusion is difficult• Only works well for a small number of occluders

• Other techniques offer various improvements and trade-offs• Hierarchical Occlusion Maps• Occlusion bit testing• Hierarchical Z-Buffer• Many more…

From Region VisibilityFrom Region Visibility

Occluder

• Area light source analogy

Separating lines

Supporting lines

Penumbra

• Area light source analogy

Umbra?

Occluder Fusion (from region)Occluder Fusion (from region)

• Cells and portals (visible volume)

• Cells and portals (visible cell iff. stabbing line does exist)

Finding a stabber (2D)Finding a stabber (2D)• Find a line which separates two set of points

Finding a stabber (2D)Finding a stabber (2D)• Find a line which separates two set of points• Left and right sets (defined by orientation)

Finding a stabber (2D)Finding a stabber (2D)• Find a line which separates two set of points• Left and right sets (defined by orientation)• Solved using duality

Finding a stabber (3D)Finding a stabber (3D)• Also solved with duality• Lines in 3D go to points in 5D Pluecker coordinates

Constructing Cells/PortalsConstructing Cells/Portals• BSP Tree – Splits volume until leaves are convex• Portals are sides of leaf nodes which do not correspond to scene polygons

Constructing Cells/PortalsConstructing Cells/Portals• Optimal portal finding is an open problem• Portals / Cells (sectors) are usually defined by hand

Soft and Hard ShadowsSoft and Hard Shadows• Hard Shadows – small (points) or far• Soft Shadows – area/volume light source

Hard Soft

Soft and Hard ShadowsSoft and Hard Shadows• Soft shadows != Blurring!!!

Occluder

Ground

Soft and Hard ShadowsSoft and Hard Shadows• Algorithms

• Point sources are easy!• Clip shadow volume against scene• Ray tracing• Shadow map• Shadow volume

• Soft shadows are difficult!• Soft shadow volumes• Point sampling of area light source• Ray tracing

• Hard and soft shadows may be pre-computed by sampling surface geometry and applying lightmaps

LightmapsLightmaps• Can be used for shadows, lighting, diffuse illumination• Need to be recomputed when geometry and/or lights move

Global vs. Local IlluminationGlobal vs. Local Illumination• Global looks better – local is faster• Trade offs between complexity, performance, ability to move

geometry, quality, etc.• What do games do?

• Radiosity preprocess into light maps• Light grid for dynamic objects• Bump mapping is applied in addition• Many passes: textures, bumps, lightmaps, shadow passes, etc.

• Doom III?• No global illumination• Not necessary for the “feel” of the game• Allows for dynamic hard shadows (cast BY everything ONTO

everything)• Photo-realism does not add as much to mood as shadows in this

context

Photon MappingPhoton Mapping

Collision DetectionCollision Detection

• Why?• Need to know if movement results in solid

bodies colliding• Physics• Visibility

• Broad Phase• Conservative – only

test whether or not further testing(narrow phase) is required

• Uses bounding volumes and bounding hierarchies

• Narrow Phase• Accurate• Triangle-triangle intersection• Segment-triangle intersection• Bounding-volume triangle

intersection

Collision Detection(cont.)Collision Detection(cont.)

• What makes a good bounding box?• Easy to test for collision• Tightly represents the model – minimal

extraneous volume• More information implies better potential• What about moving objects?

• On the fly creation?• Conservative updates• Tracking nearest geometry

• AABB (Axially aligned bounding boxes)• Two points in 3D (6 scalars)

• BS (Bounding Sphere ) • Point and radius (4 scalars)

• Not always a good fit – may require OBB (Oriented bounding box)• Three points in 3D (9 scalars )• Principal component analysis

• Convex Hull• Accurate

• Conservative• bounding planes• faster, but less accurate

• k-dops (discrete oriented polytopes)• Accurate – more planes• AABB == 6-dop

• 8-dop

• Hierarchies – AABB

• Test top down

• Hierarchies – OBBTree

• Test top down

Collision Detection(cont.)Collision Detection(cont.)• Polytope-polytope intersection (narrow

phase)• Assuming non-containment:

Intersection is equivalent to the existence of an edge of one polytope which intersects the face of another

Containment

Graphics Topics in VR

Documents

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