IMA Haptics Workshop - David Johnson - Univ. of Utah
Haptic Rendering of NURBS Models
David Johnson
Virtual Prototyping ProjectSchool of Computing
University of Utah
www.cs.utah.edu/~dejohnso
IMA Haptics Workshop - David Johnson - Univ. of Utah
Sample Design Pipeline
Clay model is modified
IMA Haptics Workshop - David Johnson - Univ. of Utah
Motivation -Virtual Prototyping
• Replace physical prototypes with digital ones.
• Allow human interaction with virtual models.
• Test– aesthetics– assembility– ergonomics
IMA Haptics Workshop - David Johnson - Univ. of Utah
Approach
• Add sense of touch to virtual prototyping system.
• Avoid conflicting cues between visual and haptic senses.
• Leverage innate human talents at manipulation.
IMA Haptics Workshop - David Johnson - Univ. of Utah
Computing Haptic Forces
• Approximate complicated surface deformation with a wall model
• Force proportional to penetration depth
IMA Haptics Workshop - David Johnson - Univ. of Utah
Local vs Global Closest Point
Why do we want to track the local closest pt?
Global closest point
Localclosest point
Desiredpenetration
depth
Previous position
IMA Haptics Workshop - David Johnson - Univ. of Utah
Background - Surface Rep
• Choice of surface representation– Intermediate– Polygonal– Implicit– Voxel– NURBS
IMA Haptics Workshop - David Johnson - Univ. of Utah
NURBS Representation
• Same as used in CAD– avoid translation
• Compact storage• Exact (assembly)• Thompson ‘97,
Johnson ‘98, ‘99, Stewart and Buttulo ‘97
IMA Haptics Workshop - David Johnson - Univ. of Utah
NURBS Surfaces
Control mesh
Control point
IMA Haptics Workshop - David Johnson - Univ. of Utah
NURBS Background• Piecewise-polynomials of two variables
• Control mesh weights the basis functions
• Refinement adds DOFs to surface
IMA Haptics Workshop - David Johnson - Univ. of Utah
Trimmed NURBS Models• Defined in parametric domain
• Directed closed polygon
• Loops
• Edges
• Segments
Parametric Domain Surfaces
IMA Haptics Workshop - David Johnson - Univ. of Utah
Multi-stage process
• Three stages– Proximity– Local Tracking– Contact
• Helps manage overall scene complexity
IMA Haptics Workshop - David Johnson - Univ. of Utah
Global Minimum Distance Framework
• Use a hierarchical refinement method (LUB-tree)– completely stable– reasonable speed (20-100 Hz)
• Experimenting with local methods – 1000 Hz for point to trimmed model– Some stability concerns
IMA Haptics Workshop - David Johnson - Univ. of Utah
LUB-Tree Framework• LUB-Tree
– Bounding hierarchy– Establish upper bound on minimum
distance– Measure lower bound for current bounding
volumes– Prune away impossible areas– Refine
IMA Haptics Workshop - David Johnson - Univ. of Utah
LUB-Tree: Example• Bounding volumes around model
IMA Haptics Workshop - David Johnson - Univ. of Utah
LUB-Tree: Example
• Establish upper bound on minimum distance
IMA Haptics Workshop - David Johnson - Univ. of Utah
LUB-Tree: Example• Measure lower bound for current
bounding volumes
IMA Haptics Workshop - David Johnson - Univ. of Utah
LUB-Tree: Example
• Prune regions further away than the upper bound
IMA Haptics Workshop - David Johnson - Univ. of Utah
LUB-Tree: Example
• Refine remaining regions
• Repeat
IMA Haptics Workshop - David Johnson - Univ. of Utah
LUB-Tree: NURBS
• Apply polygonal style method to NURBS models
IMA Haptics Workshop - David Johnson - Univ. of Utah
LUB-Tree: NURBS
• Refinement– Group remaining polynomial pieces– Extract– Refine
IMA Haptics Workshop - David Johnson - Univ. of Utah
Time-critical Properties• Upper and lower bounds found quickly
IMA Haptics Workshop - David Johnson - Univ. of Utah
Local Tracking
• Initialized by global method
• Local methods– faster– give the desired behavior
IMA Haptics Workshop - David Johnson - Univ. of Utah
Direct Parametric Tracking (DPT)
Initial State Movement Project Compute parametric change and new surface position
IMA Haptics Workshop - David Johnson - Univ. of Utah
DPT derivation for a curve
• B-spline curve
• Velocity curve relates movement along curve to change in parameter
or
IMA Haptics Workshop - David Johnson - Univ. of Utah
DPT derivation (cont.)
• Use linear approximation
• Tangents are efficiently evaluated
IMA Haptics Workshop - David Johnson - Univ. of Utah
DPT derivation (cont.)
• All together
• 1000’s of Hz
• Pretty reliable
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IMA Haptics Workshop - David Johnson - Univ. of Utah
0)),((
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vSuS
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• Multidimensional Newton’s method
FJ x
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Higher-order tracking
• Orthogonal to tangents
IMA Haptics Workshop - David Johnson - Univ. of Utah
Haptic “Philosophy”
• Coherence can save you!
• Perform one iteration of Newton’s method at higher rates rather than more iterations at lower rates.
IMA Haptics Workshop - David Johnson - Univ. of Utah
Improved stability
DPT method Higher-order method
IMA Haptics Workshop - David Johnson - Univ. of Utah
Dealing with Trimming Loops• Brute force approach not feasible
• Grid approach– Boundary on
bounding box– Walk algorithm
IMA Haptics Workshop - David Johnson - Univ. of Utah
Grid Data• Goblet
– 3 surfaces– 254 segs– 13 max– 3 mean– 89% empty
Gear 22 surfaces 1256 segs 15 max 4 mean 92% empty
Crank Shaft 73 surfaces 412 segs 36 max 4 mean 89% empty
IMA Haptics Workshop - David Johnson - Univ. of Utah
Current Projects
• Develop systems of equations for two models– minimum distance– penetration depth
Nelson, D, Johnson, D., and Cohen, E., "Haptic Rendering of Surface-to-Surface Sculpted Model Interaction," in Proc. 8th Annual Symp. on Haptic Interfaces for Virtual Environment and Teleoperator Systems, (Nashville, TN), ASME, November 1999.
IMA Haptics Workshop - David Johnson - Univ. of Utah
Current Projects
• Apply concepts from NURBS local tracking back to polygonal models
• Encapsulate directional information in data structure
• Prune based on orientation, not spatial bounds
eyenormal cone
view cone
IMA Haptics Workshop - David Johnson - Univ. of Utah
Local minimum distance
Sphere Holes3 Small bunny Large bunny
# triangles 32,700 11,800 10,800 69,500
Time (ms) 0.06 0.05 0.1 0.2
IMA Haptics Workshop - David Johnson - Univ. of Utah
Local minimum distance• Also for model-model distance
IMA Haptics Workshop - David Johnson - Univ. of Utah
Acknowledgements
• Tom Thompson, John Hollerbach, and Donald Nelson provided some material for slides.
• Sponsored by NSF
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