Hierarchical Radiosity for Dynamic Environments

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

Utrecht University - Advanced Graphics

Previous Work


• 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


Taxonomy of LinksThe links are classified into three

categories

Object-Object Object-Environment Environment-Environment

• Updated• Deleted• Created• Or remain unchanged

Links can be:


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


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


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


Creation is of hard

complexity

Mesh Optimization


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


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


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


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


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.


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.


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


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


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.


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.


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


Creating the Volume


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


• 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)


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.


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


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



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


Date post:	24-Feb-2016
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Hierarchical Radiosity for Dynamic Environments

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