Occlusion Culling

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Occlusion Culling

• I4B06 ???
• I4B12 ???

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Indoor scenes
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Viewing position change
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Visibility culling

• Goal
• To bring the cost of rendering a large scene down
  to the complexity of the visible portion of the
  scene.
• Visible set
• Conservative visibility
• We may classify an occluded object as visible but
  may never classify a visible object as occluded.
• PVS potentially visible set
• Hierarchical scene

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Visibility culling techniques

• View frustum culling
• Back-face culling
• Occlusion culling

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Taxonomy of occlusion culling techniques

• Point vs. Region
• Image precision vs. Object precision
• Cell-and-portal vs. Generic scenes

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Additional criteria

• Conservative vs. approximate
• All vs. subset of occludes
• Individual vs. fused occluders
• Need of precomputation
• Treatment of dynamic scenes

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Aspect graph in 2D

• Principle
• Subdivide
• Visibility space partition (VSP)
• Visual event
• The boundary of a VSP region
• End points
• Supporting lines
• Separating lines

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Many Aspect region
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Cell-and-portal from-region visibility

• Based on the characteristics of architectural
  scenes ? cells and portals
• Key concepts
• Conservative visibility
• PVS
• Densely occluded environments

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Important Points

• Subdivided into convex cells
• Adjacency graph
• View volume
• Dynamically culling ? view volume of the
  observer, producing a superset of the visible
  scene data.

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A cell is visible

• It is in the View Volume
• All cells along stab tree are in the view volume
• Sightline within the view volume exists through
  portals

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Cells and portals adjacency graph and stab tree
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Point-based object-precision method

• Point-based Cells and Portals technique based on
  from-region methods Luebke and Georges
• No pre-computing for each cell
• Simple and effective
• Rendering the initial cell and its portals
• Any remaining visible geometry has lie inside the
  projection of those portals

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Cells and Portals
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Large convex occluders

• Using of temporal coherence by taking a subset
  of visual events
• Separating and Supporting planes
• The basic idea ? aspect graph
• Linearized portion of the aspect graph ( as the
  user moves)
• The silhouette edges and vertices of the relevant
  primitives

Coorg and Teller
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Large convex occluders (Improved)

• View-dependent set of occluders
• Object hierarchy
• Visibility test
• Pre-selected convex occluders
• Bounding boxes of the hierarchy
• Edges of the occluder vertices of the occludee

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Culling using shadow frusta

• Concept
• A viewer cannot see the occludee if it is inside
  the shadow .
• Dynamically choosing a set of occluders
• ? Producing shadow frusta
• ? Culling the bounding boxes a hierarchy of
  objects
• Scene Test top-down

Hudson et al.
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Point-based image-precision techniques

• Key concept
• Subsequent objects can be culled away quickly by
  the already-filled parts of the images
• Finite resolution
• More robust than object-precision
• Numerical precision problems

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Point-based image-precision algorithm

• Hierarchical Z-buffer
• Hierarchical occlusion map
• Ray casting
• Approximate volumetric visibility
• Directional discretized occluders
• Occluder shadow footprints

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Ray casting

• The simplest forms
• Never rendering an occluded part of the scene
• High complexity

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Directional discretized occluders

• Both HZB and HOM
• DDOs simple concept
• Replace complex occluder with simpler object

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Approximate volumetric visibility

• Using volumetric representation
• Density of geometry in the environment ? (
  probabilistic value )
• The volume opacity between regions (related to
  density)

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Occluder shadow footprints

• 2.5D scene ( terrain or height field )
• Using z-buffer
• Wedges ( delimit the occluder shadow )
• Footprint
• An object is Hidden

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Shadow Footprint ???
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Generic from-region visibility

• Region or view cell
• Sampling view points
• Unacceptable flicking artifacts
• Needing Effective occluder
• Occluder fusion

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Umbra Virtual occluder
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Conservative visibility preprocessing using
extended projections

• An extension of image-precision methods
• Using Projection
• Extended projection
• Allowing occluder-fusion

Durand et al.
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Visible or Occluded
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Virtual occluders

• View-dependent convex object
• Represent the aggregate occlusion for a given
  cell
• Applying an Effective from-region culling
  technique
• To Efficiently compute a PVS

Koltun et al.
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Virtual occluders
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Occluder fusion for urban walkthroughs

• Observation
• It is possible to compute a conservative
  approximation of the umbra from a set of discrete
  point samples placed on the view cells boundary.
• Shrinking an occluder by ?

Wonka et al.
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5 sample points in view cell
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Occlusion Culling Algorithms

• Select from the whole scene that we want to
  render, and the selection is limited to those
  portions of the scene that re not considered to
  contribute to the final image.
• The z-buffer selects and renders only those
  objects which are visible

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Occlusion culling is useful
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General occlusion culling

• 1 OcclusionCullingAlgorithm (G) 2
  ORempty3 for each object g in G4
    if(isOccluded(g,OR))5     Skip(g)6
    else7     Render(g)8     Update(OR,g)9
    end
• 10 end

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Occlusion Culling for Outdoor

• Outdoor environments
• Closed-in locations

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Bounding Volumes

• A simple geometrical object roughly representing
  the volume of a real objects geometry
• The most suitable geometric objects for bounding
  volumes are spheres and boxes
• ( The goal of using a combination of both is
  speed )
• Node Volumes

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Bounding box and bounding sphere
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The View Frustum
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View Occluders
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Occluder
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Visibility in Difference Bounding
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Hierarchy Tree Example Octree
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Hierarchy Tree Example Octree
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Hierarchical Z-Buffer

• Replace Z-buffer with a Z-pyramid
• Lowest level full-resolution Z-buffer
• Higher levels derive from lower level

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Hierarchical Z-Buffer

• A Z-pyramid represents depth buffer in
  hierarchical fashion
• Depth at each pixel at a given level represents
  the max of the depth of the four corresponding
  pixels at next level
• Z-pyramid constructs until the farthest one
  derived

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Hierarchical Z-buffer
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Hierarchical Z-Buffer

• Organize scene into an octree
• Before rendering the polygons associated with a
  node in the octree, test if the nodes bounding
  box is visible
• Recursion

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Hierarchical Z-Buffer
1 ProcessOctreeNode(OctreeNode N)2
if(isOccluded(NBV, ZP)) then return3 for each
primitive p in N4    tileInto(p, ZP)5 end6
for each child node C in N in front-to-back
order7    ProcessOctreeNode(C)8 end
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Hierarchical Occlusion Maps

• Two major differences from HZB
• Separates occluders from occludees
• Divide occlusion test into two part
• an depth test and a overlap test

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Hierarchical Occlusion Maps
Depth
Overlap
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Hierarchical Occlusion Maps

• Representation of projection for overlap test
  occlusion map
• Generate by rendering occluders

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Occlusion Maps
Occlusion Map
Rendered Image
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