Occlusion Culling
©Yiorgos Chrysanthou, 1999-2001, Anthony Steed, 2004
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Overview

Graphics Pipeline Problems
• Speed up techniques

Occlusion Culling
• Image-based
• Object-based
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What’s wrong with the standard
graphics pipeline?

It processes every polygon therefore it
does not scale
• Sometimes polygons than pixels
• Pixels are being drawn multiple times

According to the statistics, the size of
the average 3D model grows more than
the processing power
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Speed Up Techniques

Roughly three classes:
• Visibility culling
– Avoid processing anything that will not be visible in (and
thus not contribute to) the final image
• Levels of detail
– Generate several representations for complex objects
are use the simplest that will give adequate visual result
from a given viewpoint
• Image based rendering
– Replace complex geometry with a texture
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The Visibility Problem


Select the (exact?)
set of polygons
from the model
which are visible
from a given
viewpoint
Average number of
polygons, visible
from a viewpoint, is
much smaller than
the model size
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Visibility Culling


Avoid rendering polygons or objects not
contributing to the final image
We have three different cases of non-visible
objects:
• those outside the view volume (view volume
culling)
• those which are facing away from the user (back
face culling)
• those occluded behind other visible objects
(occlusion culling)
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Visibility Culling
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View Volume Culling

If a spatial subdivision exists, it can be
used for view volume culling
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Occlusion Culling

Image-space methods
• Generally analyse the partially completed image
to determine whether or not to render complete
objects

Object-space methods
• Provide methods for determining if sections of the
model need rendering due to what can be seen in
the view volume

A very active topic of research in last 5 years,
so there are many, many techniques
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Image-Based Occlusion

Occlusion maps
• If you can order the polygons front to back,
then the image on the screen tells you
areas which are obscured
• If, e.g. we find that one quarter of the
screen is covered then we need not render
anything in that quarter
• The problem devolves to finding good
occluders, and occludees
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Testing for Occlusion


If the box surrounding a node is not visible then
nothing in it is either
The faces of the box are projected onto the image
plane and tested for occlusion
occluder
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In Image Space



Render some set of
objects as normal
(foreground)
Read the image
back
For other objects
• Find bounding box
• Test if those pixels
covered yet or not
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In Practice


Reading back the image is very slow with
current graphics hardware
Fortunately recent graphics hardware can do
occlusion queries for you
• Instead of rendering it tells you whether or not any
pixels would be drawn

Of course have to make sure that doing the
occlusion test is no more expensive that
rendering the actual object!
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Cells and Portals
(Teller and Sequin, SIGGRAPH 91)
Decompose space into convex cells
 For each cell, identify its boundary
edges into two sets: opaque or portal
 Precompute visibility among cells
 During viewing (eg, walkthrough
phase), use the precomputed potentially
visible polygon set (PVS) of each cell to
speed-up rendering

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Determining Adjacency Information
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For Each Cell Find Stabbing Tree
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Compute Cell Visible From Each
Cell
Linear programming problem:
S•L  0,
S•R  0,
LL
RR
Find_Visible_Cells(cell C, portal sequence P, visible cell set V)
V=V  C
for each neighbor N of C
for each portal p connecting C and N
orient p from C to N
P’ = P concatenate p
if Stabbing_Line(P’) exists then
Find_Visible_Cells (N, P’, V)
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Eye-to-Cell Visibility

The eye-to-cell visibility of any observer is a
subset of the cell-to-cell visibility for the cell
containing the observer

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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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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Summary



Faster hardware encourages big models
Real-time is maintained by visibility culling, level
of detail, image-based rendering
Occlusion, part of visibility culling relies on
either
• Being able to identify good occluders (image-based
occlusion)
• Being able to structure the model to allow analytic
solutions (object-based occlusion)
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