Voxel Based 6DOF Haptic Rendering Improvements

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Voxel Based 6-DOF Haptic Rendering Improvements

• In Lee
• inism_at_postech.ac.kr
• VR Lab, POSTECH

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Outline

• Voxel-Based 6-DOF Haptic Rendering
• Distance Fields
• Geometrical Awareness
• Temporal Coherence
• Implementation
• Results
• Summary
• References

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Voxel-Based6DOF Haptic Rendering

• Voxel
• The extension of the pixel to 3D
• It contains position, color, stiffness, etc.

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Voxel-Based6DOF Haptic Rendering

• Neighbor voxel
• A voxel that share a vertex, edge, or face with
  the subject voxel
• each voxel has 26 neighbor voxels

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Voxel-Based6DOF Haptic Rendering

• Voxel types
• Each voxel is allocated 2 bits of memory that
  designate it as a free space, interior, surface,
  or proximity voxel

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Voxel-Based6DOF Haptic Rendering

• Voxmap
• A model to represent static objects in
  environments
• It consists of all voxels of static objects

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Voxel-Based6DOF Haptic Rendering

• Point Shell
• A model to represent a dynamic object
• It consists of surface points and associated
  inward surface normals

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Voxel-Based6DOF Haptic Rendering

• Tangent plane
• Dynamically constructed plane which pass through
  the (surface) voxels center and has the same
  normal as the penetrated points normal
• Penetration depth
• It is calculated as the distance d from the point
  to a tangent plane
• If the point has not penetrated below tangent
  plane, then d is zero

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Voxel-Based6DOF Haptic Rendering

• Tangent plane force model
• where, kff is a stiffness and d is
  a penetration depth

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Voxel-Based6DOF Haptic Rendering

• Exact surface interpenetration avoiding
• Extend surface voxels of the static object to the
  second layer of free-space voxels
• It is called the force layer

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Voxel-Based6DOF Haptic Rendering

• Voxel tree
• The depth of a tree is limited to three
• Each non-leaf node has 23N children nodes
• Empirically, the memory usage of the tree is
  minimized when N3
• The root node is called hyperchunk
• The middle level node is called chunk

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Voxel-Based6DOF Haptic Rendering

• 6-DOF Spring-Damper System
• wherekT spring translational stiffnessbT
  spring translational viscosity v objects
  relative linear velocitykR spring rotational
  stiffnessbR spring rotational viscosity ?
  equivalent-axis angle? objects relative
  angular velocity

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Voxel-Based6DOF Haptic Rendering

• Virtual Stiffness Consideration
• To guarantee that haptic stability, the force sum
  is replaced by some average-like method below

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Distance Fields

• Distance field
• An extension of the voxel grouping
• Each voxel is allocated 4 bits of memory
• While available values are 015, we use 010 for
  certain reasons

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Distance Fields

• Collision offsetting
• An extension of the exact surface
  interpenetration avoidance
• The force layer is moved to more distant
  free-space layers
• Although this idea can be extended to the
  pointshell, but not recommended

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Geometrical Awareness

• Main Idea
• Voxels may inherit properties of their parent
  polyhedral objects
• By some geometrical property of the polyhedron,
  we only need to test whether vertex-surface and
  edge-edge contacts are occurred to detect
  collisions

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Geometrical Awareness

• Applying the geometrical awareness
• In voxelization time, each voxel is labeled as
  vertex, edge, or surface according to its
  underlying polyhedral geometry
• Then, each voxel is allocated with 16 bits of
  memory it is an extension of the distance field
• 12bits are used to contain minimum distances from
  vertex, edge, and surface 4 bits for each
• Remaining 4bit are unused

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Geometrical Awareness

• Applying the geometrical awareness

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Temporal Coherence

• Main Idea
• If we know a points position and max. speed, we
  can predict how many frames will elapse before
  contact can possibly occur
• It allow us to reduce the frequency of point
  sampling and use more points contained to the
  pointshell
• Therefore, more accuracy could be attainable

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Temporal Coherence

• MaxTravel
• It is defined as the max. distance that any point
  may travel in a haptic framewherenCapacity
  is the of tests that can be performed per
  haptic frame nMandatory is the of tests for
  points already in contactni is the of points
  in free space at i voxels from contacts is the
  voxel size

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Temporal Coherence

• Distance-to-contact queue
• A mechanism for tracking and updating the status
  of points
• All points that currently have the same
  distance-to-contact value belong to the same
  queue
• Each queue has its length equal to the number of
  points in a pointshell
• Each point is represented in one bit

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Temporal Coherence

• Hierarchical temporal coherence
• All points that came from the same chunk of the
  voxel tree are assigned to contiguous bit
  addresses in the distance-to-contact queues
• Whenever a chunks whole points is known to lie
  in free space, all points of the chunk is removed
  from their queues, and the chunks centroid is
  will be tracked

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Temporal Coherence

• Point drifting
• The point drift is defined as the exact distance
  between a points current position and its
  previously stored position
• The voxel lookup is skipped until the point has
  possibly drifted as much as two queues away from
  contact

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Temporal Coherence

• Dynamic pre-fetching
• To overcome the memory limitation, a pre-fetching
  technique is needed
• Points are loaded whenever its parent chunk
  sufficiently close to the surface
• To guarantee that a certain data is loaded to
  memory before needed, the haptic process have to
  be suspended until pre-fetching is done

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Implementation

• Separate Architecture
• Haptic controller
• Visual interface

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Implementation

• Haptic controller
• 2.8GHz Xeon dual processor, 2GB RAM
• Windows XP
• Haptic device
• PHANToM Premium 1.5
• Graphics application
• FlyThru

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Results

• Voxelization and loading time measurements

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Results

• Performance Improvement
• The number of usable points is increased about
  200-fold due to algorithmic enhancements
• It equivalent to a 14-fold improvement in
  accuracy

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Results

• Other Findings
• VPS could simulate mating-surface scenarios
  without using kinematic constraints
• Voxelization time is inversely proportional to
  the square of the voxel size
• Artificial viscosity caused by MaxTravel was
  evident whenever objects were in contact or
  nearly so

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Summary

• We discussed the VPS haptic rendering method
• We also discussed some techniques for improving
  VPS

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References

• W. A. McNeely, K. D. Puterbaugh, and J. J. Troy,
  Six Degree-of-Freedom Haptic Rendering Using
  Voxel Sampling, Proc. ACM SIGGRAPH, pp.401-408,
  Aug.1999.
• W. A. McNeely, K. D. Puterbaugh, and J. J. Troy,
  Voxel-Based 6-DOF Haptic Rendering
  Improvements, Haptics-e, vol. 3, 2006.