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Hierarchical Radiosity for Dynamic Environments. Presented by Konstantinos Georgiadis. Abstract. Previous Work. Dynamic Environments. Taxonomy of Links. Taxonomy of Links. Object - Object. Creation is of hard complexity. Object - Environment. Creation is of hard complexity. - PowerPoint PPT Presentation
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Presented by Konstantinos Georgiadis
Hierarchical Radiosity for
Dynamic Environments
AbstractThis method extends the Hierarchical Radiosity approach for environments whose geometry and surface attributes changes dynamically.
• It maintains the accuracy of the mesh• It handles changes in geometry
Major contribution
s
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Previous Work
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• View independent, but still only for static environmentsRadiosity
• No mechanism for mesh unrefinement because:• It is impossible to maintain the mesh history• No control over accuracy
Progressive
Radiosity
• Handles refinement/unrefinement of the mesh but:• The accuracy of the mesh is not maintained• It doesn’t handle changes in geometry
Hierarchical
Radiosity
Dynamic Environments
Surface Attribute Manipulation
• Changes in the light source or in the reflectance of a surface• Geometry not effected.
SolutionAdjust the mesh to maintain accuracy
Geometry Manipulation• Changes in geometry affect the
form factors• Still, most links remain
unchanged
SolutionA taxonomy of links reduces form factors
recomputationUtrecht University - Advanced
Graphics
Taxonomy of LinksScene surfaces are divided into two groups
Object A movable group of surfaces
For example, a chair.
Environment Rest surfaces including other objects & lights
sources
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Taxonomy of LinksThe links are classified into three
categories
Object-Object Object-Environment Environment-Environment
• Updated• Deleted• Created• Or remain unchanged
Links can be:
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Object - ObjectIn few occasions, rotation may cause self occlusion. The modification of object-object links is of moderate complexity.
Remain unchanged
Become occluded
Delete link
Become visible
Create link
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Creation is of hard complexit
y
Object - EnvironmentThe modification of such links is of easy complexity, because the dynamic object is always known.
Become occluded
Delete link
Become visible
Create link
Visibility unchanged
Update link
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Creation is of hard
complexity
Environment - EnvironmentThe modification of such links is of hard complexity. An exhaustive search must be performed to identify them.
Become occluded
Delete link
Become visible
Create link
Visibility unchanged
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Creation is of hard
complexity
Mesh Optimization
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The mesh (un)refinement depends on:
Brightness Occlusion
The unrefinement of the mesh is called mesh folding and it introduces two new interactions :
Ghost linksShadow links
Motivation
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Changes in the emissive power of the light sources affects the brightness of the scene. If
brightness :
IncreaseMesh refinement
Decrease Mesh
unrefinement?
SolutionUse Ghost
links
The reposition of objects leaves under-refined and overly refined
mesh elements due to occlusion.
Under-refined areas
Automatically meshed Overly refined areas
How to remove them?
SolutionUse Shadow
links
Ghost Link Creation
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Information retained by a Ghost link
Form factor Visibility information
Ghost links are created during the refinement process. Instead of discarding information from energy links, store it in ghost links. • No further computational cost• Storage cost moderate
Shadow Link Creation
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Information retained by a Shadow link
Receiver patch
Source patch
A set of occluding objects
Form factor
Shadow links are created during refinement process, when partial visibility causes sufficient decrease in energy to terminate subdivision. • The number of shadow links is optimal
Global Mesh Folding with Ghost Links
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Energy delta Difference between new/old attribute values.
Computed for each color channel.
Positive deltaPatch refining
(Energy increase)
Negative deltaPatch folding
(Energy decrease)
Conservative delta Ensure optimal mesh by invoking folding for any negative
delta
Global Mesh Folding with Ghost LinksMesh folding Performed in two steps:
Step 1: Delete links When energy is decreased, ghost links become energy links. Energy links between patch children get deleted.
Step 2: Delete patches Patches with no children than do not contribute energy to the scene get deleted. Accomplished through use count.
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If (use count = 0) {delete all patches}
Local Mesh Folding with Shadow Links
Local mesh folding similar to global mesh folding; only shadow links are used instead of ghost links.
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Shadow clean-up Solution
Recompute the form factors of the energy links from the light source to the receiver patches of the shadow links.
Results
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Original Image Middle Image Final
Image
Original Image Middle Image Final
Image
Ghost links
Shadow links
Scene PartitioningThe objects of a dynamic environment are organized by using:
Hierarchical Bounding Volume (HBV)
Object Clustering
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Furthermore, the objects can be manipulated by using:
Motion Volume
Hierarchical Bounding VolumeA hierarchical bounding volume (HBV) is a data structure that groups individual bounding volumes into larger and larger volumes.
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This method uses an automated HBV in order to :
Remove a bounding volume subtree from hierarchy
Add and remove clustered objects
Object ClusteringObject clustering defines a class of movable objects and aids a ray-acceleration scheme which is important for visibility test.
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The sub-objects of all dynamic objects are clustered together into sub-HBV.• When selected the clustered object
is removed from scene• When released it is transformed
and re-inserted back into the scene Scene modeled as an n-tree
HBV built from this tree
The Motion Volume
The motion volume is a scene-partitioning bounding volume which encloses the original and the repositioned object. • It consists of a collection of plane-sets that enclose the
range of motion of the object
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Creating the Volume
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The eight vertices of the original bounding box transform
to eight new vertices.
An enclosure around the sixteen points yields the
motion volume.
The motion volume is created
by using:
• the bounding box of the dynamic object • the transformation matrix that takes the
object to its new position
Object ClippingCull links that have not been affected by an object's motion using: • The motion volume • A modified 3D Cohen-Sutherland clipping
algorithm
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• Check the outcodes of pairs of objects to decide if their links can be excluded from the search. Reject objects that lie on the same side of a motion volume plane.
Methodology:
Object Visibility
The remaining pairs of clustered objects will either be:• mutually visible (links may exist)• completely occluded (no link exist)
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The outcode test is irrelevant to object-to-object visibility, thus a visibility test is required.
Surface VisibilityLinks between visible objects are limited to particular surfaces. Some surfaces can be rejected based on their orientation.
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Method Determine if two surfaces are visible
Examine the surfaces of each object For every surface normal N, examine the projection onto N of the other object's maximum bounding vertex in the direction of N. If the projection lies in the positive half space of the surface then surfaces are visible and their links must be recomputed.
Updating the Links
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Step 1Modify all links
between pairs of visible surfaces
Step 2Refine the scene
Step 3Create shadows using adaptive
subdivision.
Advantages - Drawbacks- New mesh folding methods- Novel scene-partitioning technique to speed the update of a system
- The update time for most scenes is still too slow to be interactive.- The modification and creation of object-object links are naively solved
Utrecht University - Advanced Graphics
Utrecht University - Advanced Graphics
Future Work
Object clustering and lazy evaluation should be investigated for the modification of object-object links and the creation of new links.
Incorporating new techniques for visibility preprocessing or meshing along lines of radiance discontinuities may speed up computations.
Questions?
Thank you for your attention
Utrecht University - Advanced Graphics