Transcript L18_ModelingSkin - Computer Science and Engineering

Modeling Skin
Separate object(s) per limb (and joints)
Continuous Skin
Rigidly transform vertices
Transform vertices between bisectors
Blend using multiple bones
Pose space deformations
Wrap skin around model of internal structure
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Separate objects
Overlapping gives appearance of
continuous surface - somewhat
Add element at joint to smooth
between limbs
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Continuous skin
Design character skin
Define underlying skeleton from skin
Bind skin vertex to skeleton
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Rigidly bind vertex to closest bone
Skin vertex
Closest bone
Associate vertex with closest ‘bone’
Rigidly transform vertex as bone transforms
Rick Parent - CIS682
Rigidly transform vertices
Problems with
overlap and
stretching
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Bind vertex to bone - bisectors
Locate vertex relative to closest bone
Normal of bisecting plane
p1  p0 p 2  p1
n

p1  p0 p 2  p1
Skin vertex
v
s
t
d
p2
p0
Distance from vertex to bone
(v  p1)  ( p2  p1)
d
p2  p1
p1
Joint bisectors
Line parallel to bone through vertex
s – distance from vertex to first bisector
t – distance between bisectors
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Map vertices to relative location
Use d, s, t to
reconstruct location
of vertex relative to
bone
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Bind vertex to multiple bones
Locate vertex relative to closest bone
Skin vertex
20%
80%
User ‘paints’ which bones affect what vertices
Relative distance used to compute weights
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Blend transformation of vertices
v    i Ci v
Vertices around joint are
affected by both bones
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But twists can collapse vertices
So add addition joints or use alternatives to linear blending
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Pose space deformations
User ‘sculpts’ skin in various poses
For each vertex solve weights to use
with Radial Basis Functions for
scattered data interpolation
See: www.cs.wisc.edu/graphics/Course/cs-838-2001/Students/tx/p2.htm
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Use anatomy to model deep structure
What artists do
Surface geometry affected by what’s happening underneath
Skin wraps bones, muscles, tendons, subcutaneous tissue
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Warp skin based on model of
internal structure
Skeletal articulation modifies shape of muscle
Muscle shape modifies skin shape
Add model of tendons, fatty tissue
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Reference material
Artistic Anatomy
Anatomy related to surface appearance
versus Medical Anatomy
Well-developed literature
Forensic Science
Facial Reconstruction
Plastic Surgery
Surface Appearance Reconstruction
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Layered: Bone-Muscle-Skin
Muscle &Tendons
Fatty Tissue
Bone
Skin
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Modeling Layered
FFD muslces
John Chadwick, ABD
CSE OSU ‘90
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Modeling - Thalmann, EPFL
Implicit surfaces
From http://ligwww.epfl.ch/
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Modeling
Embed polyhedra in implicit surface
Implicits used for approx. collision detect & respond
Polyhedra used for display
Use M.-P. Cani technique to distort
implicits based on collisions
Karan Singh, PhD
CSE OSU ‘95
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Human Figure
Ferdi Scheepers, PhD
CSE OSU ‘97
Use artistic anatomy to design muscles
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Human Figure
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Human Figure
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Figures - Wilhelms, UCSB
http://www.cse.ucsc.edu/~wilhelms/fauna/Monkeys/5monk_parts.gif
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Use Visible Human dataset
Skeletal Muscle Modeling Using a NURBSBased Finite Element Method
www.digital-humans.org/Report2004/Documents/13-MuscleStressAndStrain.htm
http://graphics.stanford.edu/~fedkiw/
Rick Parent - CIS682