Graphics Topics in VR

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Graphics Topics in VR

• By Shaun Nirenstein

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Overview

• Impact of graphics/geometric algorithms on VR
• Visibility in VR
• Rendering Topics in VR
• Collision Detection
• Animation (I wont talk about this)

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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)

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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

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Visibility

• Obvious application is to manage geometric
  complexity
• Dont render what you cannot see
• Less obvious application is to manage global
  complexity
• Dont simulate invisible objects
• Can X see Y (e.g. AI)
• Dont 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

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Visibility 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

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From Point Visibility

• Occluder shadow volumes

Invisible
Occluder
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Occluder Fusion (from point)

• Occluder shadow volumes

Invisible
Occluder
Invisible???
Invisible
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Occluder Fusion (from point)

• Cells and portals

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Occluder Fusion (from point)

• Cells and portals

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Occluder Fusion (from point)

• Cells and portals

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Problems

• 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

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From Region Visibility

• Occluder shadow volumes

Invisible
Occluder
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From Region Visibility

• Area light source analogy

Separating lines
Penumbra
Umbra
Supporting lines
Penumbra
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From Region Visibility

• Area light source analogy

Umbra?
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Occluder Fusion (from region)

• Cells and portals

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Occluder Fusion (from region)

• Cells and portals

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Occluder Fusion (from region)

• Cells and portals (visible volume)

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Occluder Fusion (from region)

• Cells and portals (visible volume)

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Occluder Fusion (from region)

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

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Finding a stabber (2D)

• Find a line which separates two set of points

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Finding a stabber (2D)

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

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Finding a stabber (2D)

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

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Finding a stabber (2D)

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

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Finding a stabber (3D)

• Also solved with duality
• Lines in 3D go to points in 5D Pluecker
  coordinates

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Constructing Cells/Portals

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

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Constructing Cells/Portals

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

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Soft and Hard Shadows

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

Hard
Soft
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Soft and Hard Shadows

• Soft shadows ! Blurring!!!

Light
Occluder
Ground
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Soft 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

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Lightmaps

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

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Global 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

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Photon Mapping
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Photon Mapping
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Photon Mapping
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Collision Detection

• Why?
• Need to know if movement results in solid bodies
  colliding
• Physics
• Visibility

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Collision Detection

• Broad Phase
• Conservative only test whether or not further
  testing(narrow phase) is required
• Uses bounding volumes and bounding hierarchies

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Collision Detection

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

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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

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Collision Detection(cont.)

• 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

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Collision Detection(cont.)

• Convex Hull
• Accurate
• Conservative
• bounding planes
• faster, but less accurate

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Collision Detection(cont.)

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

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Collision Detection(cont.)

• 8-dop

45o
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Collision Detection(cont.)

• Hierarchies AABB
• Test top down

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Collision Detection(cont.)

• Hierarchies OBBTree
• Test top down

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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