Implicit Representations of the Human Intestines for Surgery Simulations L. France A. Angelidis P....
-
Upload
brianne-phillips -
Category
Documents
-
view
212 -
download
0
Transcript of Implicit Representations of the Human Intestines for Surgery Simulations L. France A. Angelidis P....
Implicit Representations of the Human Intestines for Surgery Simulations
Implicit Representations of the Human Intestines for Surgery Simulations
L. France A. Angelidis P. Meseure M-P. Cani J. Lenoir F. Faure C. Chaillou
LIFL, Lille, FranceiMAGIS-GRAVIR, Grenoble, France
2
Context
Laparoscopic Surgery
3
Context
Laparoscopic Surgery
Medical Simulator XitactTM
original
virtual
4
Context
Surgical technique: Clearing stage, by pulling & folding
the intestines
Challenges for the simulation: Large displacements Numerous contacts and self-collisions
5
Outline
3-component Model for the Intestines Mechanical Model Collision/Self-collision Model Skinning Model
New Skinning Methods, with Implicit Surfaces Using Point-Skeletons Generated by a Convolution Surface
Conclusion
6
Model of Intestines
Mechanical Model Motion computation
Collision Model Interaction computation
Skinning Model Object representation
Mechanical Mechanical ModelModel
Collision Collision ModelModel
Geometrical Geometrical ModelModel
7
Mechanical + Collision Models
Mechanical Model• Intestines’ axis: A cubic Catmull-Rom spline
• Motion computed by dynamic resolution of Lagrange equations applied to splines
Collision/Self-collision Model• Approximation of all objects by spheres for collision • If collision, computation of a penalty force proportional
to the penetration
n
kkk sbtqtsP
0
)()(),(q2
q0q1
q3q4
q5
qi: control pointsbi: basis functions
Self-collision
Neighbor spheres
8
Skinning Model: Previous Method
Parametric surface [VRIC’02]• A generalized cylinder with a spline skeleton
associated to a circular section of varying radius
qi
qj
ks-ds
bs-dsts-ds
ks
bs ts
sss
dssss
tbk
ktb
9
Skinning Model: New methods
Basis: Implicit surfaces Definition
• Examples:
Advantages• Straightforward detection of object’s interior/exterior • Simplification of the collision detection between objects
ePfPS )(f: field functione: iso-valueP: points
10
Distance surfacesEvaluation of the field function f for any point P
• From its distance to its closest point on the skeleton
Method using Point-Skeletons (1)
PS
x
x
s
f(S,P)
11
Method using Point-Skeletons (2)
Application to the IntestinesGeometric Model:
• Implicit surface generated by discrete point-skeletons positioned along the spline curve
Animation: • Variation of the skeleton shape according to the movement of
the spline points• To avoid topology changes during the simulation: Adaptive
positioning of spheres along the curve at regular intervals
Visualization• Use of a marching cubes algorithm (real-time
implementation)
x xx x
12
Method using Point-Skeletons (3)
Results: Video
13
Results: Blending of several skeletons contributions = sum of their
field valuesSuppression of surface folds at the joint of skeletons => Continuous shape for the intestines model
Potential creation of bulgesDifficulty to provide a constant radius: Variation of the
number of skeletons => Fluctuations of the geometry Avoidance of blending between non-consecutive parts
=> Requirement of blending graphBlending control not at a sufficient rate due to marching
cube method
Method using Point-Skeletons (4)
14
Method generated by a Convolution Surface (1)
Definition:
• Shape = set of connected convolution segment-skeletons
For a single convolution segment-skeleton:• Field value at a point P = sum of the contribution of all
the point-skeletons along the segment • Closed-form solution of this integral for various point-
skeleton kernel functions • Fastest solution:
),(
sinsin)(
221
HPdPf
P
H
d(P,H)
x
15
Method generated by a Convolution Surface (2)
Display of the surface at interactive rate• Use of seed-based method which takes benefit from the
temporal coherence • Surface rendered at different levels of detail by adapting
the discretization of the surface triangulation
Unwanted blending managed by local convolution
Di
Dj
Pi
ui
Pjuj
16
Method generated by a Convolution Surface (3)
Results: Video
17
Method generated by a Convolution Surface (4)
ResultsAvoidance of bulges on the surface that coats the segments Possible changes of the number of segments at each time
step without creating jumps on the implicit surface geometry
Visual rendering of the intestines satisfying: No blending and no bulges created
Computation time still too slow if a fine discretization of the surface object is wanted
18
Conclusion
Two implicit solutions to improve the Skinning of the Intestines
• Using Point-Skeletons + Generated by a Convolution Surface
Advantages• Good visual results for the movements and deformations of
the intestines• Adaptive implicit surfaces based on convolution
=> Animation possibly displayed at different levels of detail
• Simulation at interactive rate of the intestines in the abdominal cavity
19
Future Work
Improvement of: Dynamic adaptation of the discretization of
the skeleton • According to the varying curvature
Addition of contact surfaces • To better handle contact
More precise detection of self-collisions • By taking into account the information provided by the
implicit surface