Exploded Views for Volume Data

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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 object
• Frequently focus objects are occluded by other
  structures
• Reduce opacity, cutaways information is still
  removed, particularly for large focus objects
• Alternative approach split up context structures
  and displace them
• Rely 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
  manipulation
• Our contribution fully interactive approach,
  automated part layout, view-dependent explosions,
  high-quality rendering

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Overview (1)
FocusSelection
PartDefinition
Layout Generation
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)
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Overview (2)
FocusSelection
PartDefinition
Layout Generation
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)
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Overview (3)
FocusSelection
PartDefinition
Layout Generation
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

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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

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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)

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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

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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)
unconstrained explosion
parts connected by slider joint, left part is
static
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Overview (4)
FocusSelection
PartDefinition
Layout Generation
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

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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Performance
GPU 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

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Thank you for your attention! Questions?
http//www.volumeshop.org