Culling Techniques

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Culling Techniques

• To cull means to select from group
• In graphics context do not process data that
  will not contribute to the final image
• The group is the entire scene, and the
  selection is a subset of the scene that we do not
  consider to contribute

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Culling Overview

• Backface culling
• Hierarchical view-frustum culling
• Portal culling
• Detail culling
• Occlusion culling

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Culling examples
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Backface Culling

• Simple technique to discard polygons that faces
  away from the viewer
• Can be used for
• closed surface (example sphere)
• or whenever we know that the backfaces never
  should be seen (example walls in a room)
• Two methods (screen space, eye space)
• Which stages benefits? Rasterizer, but also
  Geometry (where test is done)

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Backface culling (contd)

• Often implemented for you in the API
• OpenGL glCullFace(GL_BACK)
• How to determine what faces away?
• First, must have consistently oriented polygons,
  e.g., counterclockwise

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2
0
1
0
front facing
back facing
1
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How to cull backfaces
1
2
back
2
eye
1
front
0
0
front
back
screen space
eye space
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View-Frustum Culling

• Bound every natural group of primitives by a
  simple volume (e.g., sphere, box)
• If a bounding volume (BV) is outside the view
  frustum, then the entire contents of that BV is
  also outside (not visible)
• Avoid further processing of such BVs and their
  containing geometry

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Can we accelerate VF culling further?

• Do what we always do in graphics
• Use a hierarchical approach, e.g, the
  scene graph
• Which stages benefits?
• Geometry and Rasterizer
• Bus between CPU and Geometry

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Example of Hierarchical View Frustum Culling
camera
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Culling examples
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Cells and Portals

• Airey90, Teller91, Luebke95

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Cells and Portals

• For each cell, create a list (set) of potentially
  visible objects (or PVS) from any viewpoint in
  the cell.
• During run-time
• Determine cell of the current eye-point and gets
  its PVS.
• Cull down this list by clipping to the viewing
  frustum.
• Render this set.

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Determining the PVS

• Two options
• Static (preprocessing stage)
• Cell-to-cell visibility
• Cell-to-region visibility
• Cell-to-object visibility
• Leads to very large and complicated data
  structures.
• Dynamic (dependent on the viewing frustum)
• Eye-to-cell
• Eye-to-region
• Eye-object

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Cells and Portals(Teller and Sequin, SIG 91)

• Decompose space into convex cells
• For each cell, identify its boundary edges into
  two sets opaque or portal
• Pre-compute visibility among cells
• During viewing (e.g., walkthrough phase), use the
  pre-computed potentially visible polygon set
  (PVS) of each cell to speed-up rendering

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Determining Adjacent Information
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Cell-to-Cell Visibility

• For Each Cell Find Stabbing Tree

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Compute Cells Visible From Each Cell
Linear programming problem
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Cell-to-Region Visibility

• The cell-to-region visibility is a subset of the
  cell-to-cell visibility for the current cell.

• A cell is visible if
• cell is in VV
• all cells along stab tree are in VV
• all portals along stab tree are in VV
• sightline within VV exists through portals

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Cell-to-Region Visibility

• The cell-to-object visibility is a list of those
  objects that are in the visible regions.

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Image Space Cells and Portals (Luebke and
Georges, I3D 95)

• Instead of pre-processing all the PVS
  calculation, it is possible to use image-space
  portals to

• make the computation easier
• Can be used in a dynamic setting

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Top View Showing the Recursive Clipping of the
View Volume
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Portal Culling
Images courtesy of David P. Luebke and Chris
Georges

• Average culled 20-50 of the polys in view
• Speedup from slightly better to 10 times

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Portal culling example

• In a building from above
• Circles are objects to be rendered

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Portal Culling Algorithm

• Divide into cells with portals (build graph)
• For each frame
• Locate cell of viewer and init 2D AABB to whole
  screen
• Render current cell with VF cull w.r.t. AABB
• Traverse to closest cells (through portals)
• Intersection of AABB AABB of traversed portal
• Goto

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Portal overestimation

• To simplify

actual portal
overestimated portal
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Portal Culling Algorithm

• When to exit
• When the current AABB is empty
• When we do not have enough time to render a cell
  (far away from the viewer)
• Also mark rendered objects
• Which stages benefits?
• Geometry, Rasterizer, and Bus
• Source (for Performer-based pfPortal)
  http//www.cs.virginia.edu/luebke/

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Replacing Geometry with Images

• Algorithm
• Select subset of model
• Create image of the subset
• Cull subset and replace with image
• Why?
• Image displayed in (approx.) constant time
• Image reused for several frames

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Portal Images

• Aliaga97

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Simple Example
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Simple Example
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Simple Example
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Creating Portal Images
Ideal portal image would be one sampled from the
current eye position
portal
eye
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Creating Portal Images
Reference COPs
Display one of a large number of pre-computed
images (120)
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Creating Portal Images
Reference COPs
or Warp one of a much smaller number of
reference images
portal
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Example Rendering
Geometry

Final Scene
Image
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Discussion on Object Space

• Visibility culling with large occluders
• good for outdoor urban scenes where occluders are
  large and depth complexity can be very high
• not good for general scenes with small occluders
• Cells and portals
• gives excellent results IF you can find the cells
  and portals
• good for interior scenes
• identifying cells and portals is often done by
  hand

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VISUALIZE fxs Occlusion Culling Algorithm

• Algorithm (extension to OpenGL)
• Scan convert faces of object, typically bounding
  box of complex object, but do not write Z
• Get boolean which says if there was a Z-value
  from scan conversion that was closer than that of
  the Z-buffer (NVIDIA get pixels seen count)
• If seen, render complex object

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VISUALIZE fxs Occlusion Culling Algorithm
(contd)

• Implications
• If an object is occluded, then we gain
  (hopefully) a lot of performance since we only
  scan convert one Bounding Box (BB) instead of the
  entire object
• If BB is not occluded, then we have to render the
  object, and we lose a little performance
• Drawing order matters drawing front-to-back
  gives more occlusion

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Real-Time Rendering?

• In computer graphics, real-time is used in a
  soft way say gt30 fps for most frames
• In other contexts, its a tougher requirement
  the framerate must never be lt30 fps, i.e.,
  constant framerate
• What can we do?
• Reactive LOD algorithm
• Reactive detail culling
• Reactive visual quality

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Resources and Pointers

• http//www.realtimerendering.com
• Journal of Graphics Tools
• http//www.acm.org/jgt/
• source for projected screen area of box
• intersection test routines
• occlusion culling
• http//www.magic-software.com