Exploded Views for Volume Data Stefan Bruckner, M. Eduard Gröller Institute of Computer Graphics and Algorithms Vienna University of Technology.

Exploded Views for Volume Data

Stefan Bruckner, M. Eduard Gröller

Institute of Computer Graphics and Algorithms

Vienna University of Technology

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Motivation (1)

Volumetric data sets: information about interior and exterior of an objectFrequently focus objects are occluded by other structuresReduce opacity, cutaways: information is still removed, particularly for large focus objectsAlternative approach: split up context structures and displace themRely on human perception to “put pieces back together”

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Motivation (2)

Plastinated anatomic sculpture (G. von Hagens, “Bodyworlds”)

Interactive exploded-view illustration

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Related Work

[McGuffin et al. 2003] Interactive deformation widgets for browsing segmented volume data[Islam et al. 2004] Modeling splitting operations for volumetric data sets[Cornea et al. 2005] Curve-skeleton for partitioning volumes[Cornea et al. 2006] Feature-aligned volume manipulationOur contribution: fully interactive approach, automated part layout, view-dependent explosions, high-quality rendering

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Overview (1)

FocusSelection

PartDefinition

LayoutGeneration

Rendering

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Focus Selection (1)

Approximate specification of focus object in dataset

Via transfer function, segmentation, volume painting

Degree-of-interest function specifies importance of each voxel [Viola et al. 2004]

Can be refined after an initial exploded view has been generated

All voxels with nonzero degree-of-interest are called selection, rest is background

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Focus Selection (2)

selection

background

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Overview (2)

FocusSelection

PartDefinition

LayoutGeneration

Rendering

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Part Definition (1)

Partition of the background object into several non-overlapping regions

Could be done automatically (curve skeleton, symmetry detection)

Simple interactive approach: user can split volume along arbitrary planes

Different tools: axis splitter, depth splitter, line splitter

Splitting can be refined/modified once the view is exploded

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Part Definition (2)

part

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Overview (3)

FocusSelection

PartDefinition

LayoutGeneration

Rendering

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Layout Generation (1)

Displacing each part manually is cumbersome and time-consuming

Would have to be adjusted whenever the viewpoint changes

Several potentially conflicting layout requirements

We use a three-dimensional force-directed layout approach for part arrangement

Different forces represent our layout requirements

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Layout Generation (2)

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Explosion Force (1)

Part arrangement should depend on the shape of the selection object

The explosion force moves the parts away from the selection object

A set of points (explosion points) within the selection object is generated

Each point exhibits a repulsive force on all parts

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Explosion Force (2)

For each part Pi

For each explosion point ej

Find point p on Pi closest to ej

Apply Fe to Pi at point p with r = p - ej

reec r

reF

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Viewing Force (1)

Occlusions of the selection object should be prevented for every viewpoint

The explosion force does not take the viewpoint into account

The viewing force moves parts away from the line of sight

Modeled after the graph distortion viewing technique by [Carpendale et al. 1996]

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Viewing Force (2)

For each part Pi

For each explosion point ej

Find point p along ray to ej closest to the center of Pi

Apply Fv to center of Pi with r = center(Pi) - p

vv

c rF

r r

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Spacing Force (1)

Parts should move apart in order to prevent clustering

Spacing force causes parts to repel each other

Each part exhibits a spacing force on all other parts

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Spacing Force (2)

For each part Pi

For each part Pj

Apply Fs to Pi with r = center(Pi) - center(Pj)

2ssc r

rrF

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Return Force (1)

All previous forces move parts away from their original position

To reach an equilibrium, we need a force which works opposite to the other ones

The return force pulls parts back to their initial location

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Return Force (2)

For each part Pi

For each vertex vj of Pi

Apply Fr to Pi with r being the vector from the current position of vj to its original location

lnr rcFr

rr

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Degree-of-Explosion

For ease of use, we have one global degree-of-explosion parameter

The degree-of-explosion controls how “exploded” the view should be

This parameters scales the normalized explosion, viewing, and spacing forces

For additional control, the contribution of each force to the total force can be adjusted

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Constraints (1)

Add additional joints to constrain part movement, e.g. based on semantics

Different joints: hinge joint, slider joint, ball joint, etc.

Adjust mass of parts: heavier parts will move less

Special case: infinite mass means part will not be displaced (useful for “breakaway” views)

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Constraints (2)

unconstrainedexplosion

parts connected by slider joint, left part is static

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Overview (4)

FocusSelection

PartDefinition

LayoutGeneration

Rendering

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Rendering

Background object divided into multiple non-intersecting parts

Each of the parts has its own transformation matrix

Selection object may intersect one or several parts

Additional bounding geometry (e.g. octree) for background and selection (used for empty-space skipping)

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Algorithm

Process parts in visibility order (GPU-based visibility sort is performed)

Use Z-Buffer to compute intersections between part geometry and bounding geometry

Store the resulting entry & exit points + depth for background and selection in 2 sets of off-screen buffers

Use buffers in fragment program to perform raycasting for background and selection

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Example

selection entry and

exit buffers

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Example

selection entry and

exit buffers

background entry and

exit buffers

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Example

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Raycasting

Do raycasting for selection and background for one part in a single fragment program

Two intervals per ray, one for selection and one for background

selection

background

Need to sample both volumes and do multi-volume compositing

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Performance

Expected bad performance due to dynamic branching in fragment program

Does not seem to be the case, most probably due to coherency in branching patterns

Some performance penalties due to usage of OpenGL FBOs – might disappear in the future

Comparison with reference raycaster – same shading and compositing routines, but no handling of exploded views

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PerformanceGPU: GeForce 6800 GT Image: 512 x 512 Volume: 256 x 256 x 166

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Demonstration

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Conclusions and Future Work

Automated approach for interactive generation of exploded view illustrations

Uses a flexible and extensible force-based layout approach

Fast high-quality GPU-based renderer using raycasting

Future: automatic part generation (curve skeleton, symmetry detection)

Pre-generated explosion setups, use template matching to adjust to different datasets

Thank you for your attention! Questions?

http://www.volumeshop.org