computer animation of vertebrates by martin dobšík brno university of technology, czech republic...
TRANSCRIPT
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Computer Animation of
Vertebrates
by Martin Dobšík
Brno University of Technology, Czech Republic
e-mail: [email protected]: http://www.fee.vutbr.cz/~dobsik
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Contents
Computer animation (CA) of human body and animals
• Main application areas
• Creating digital actors and animals
CA of soft tissues
• Types of animation models
• Geometric deformation model• Geometric/physically based model (layers)• Biomechanically based model• Deep overview of Anatomically based model of J. Wilhelms• CA of Vertebrates at Brno University of Technology - DCSE
We will focus on visual aspects.Not on artificial intelligence or artificial life!
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Computer Animation of Human
Body and Animals
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Main Application Areas
• Biomechanical and Biomedical Applications
- The Virtual Cadaver
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Cadaver dissections are widely used in medicine for four centuries. There are many problems with them:
· expensive
· difficult to obtain· quickly perishable
Virtual Cadaver can be very useful tool in medical educational practice
Visible Human Project (1995) – major advance in this field
[Maurell98]
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Main Application Areas
- Surgery Simulation
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Can provide efficient, safe, realistic and relatively economical method for training clinicians in various surgical tasks.
Example: Minimal invasive surgery
Ch. Kuhn, U. Kühnapfel, H.-G. Krumm - 1996
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Main Application Areas
- Orthopaedics
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New ways if investigating musculoskeletal systems. It allows (e.g.):• estimation of the force contribution of each muscle component during motion• experimentation of modifications of the musculoskeletal topology• comprehension of the complex motion coordination strategies
Example: Musculoskeletel simulation for orthopaedic rehablitationS.L. Delp, J.P. Loan, 1995
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Main Application Areas
- Bioengineering (e.g.: simulation of proesthesis behavior)
- Augmented reality in medicine (to see directly inside patient)
- Telemedicine (e.g.: remote surgical oeration)
- Ergonomic studies
- and many others.
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Main Application Areas
• Virtual Reality and Interactive applications (games)
- Teleconferencing
- Military - fight simulations, soldiers training in dangerous situations, etc.
- Architecture
- Design
- Games
- and many others
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Main Application Areas 9
Networked collaborative virtual environment system, for tennis playingMolet, Aubel, Capin, Carion, Lee, Noser, Padzic, Sanier,
Magnenant-Thalmann, Thalmann - 1999 ([Molet99])University of Geneva, Swiss Federal Institute of Techology
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Main Application Areas
- Textile industry - dressing virtual humans
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Interactive clothing system ([Volino97])Pandzic, Capin, Magnenant-Thalmann, Thalmann
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Main Application Areas
• Computer Animation for Films
- Actors in dangerous situations
- Changes to actors body (coloured hair, mising leg, ...)
- Cartoon films
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Beth Hofer - Caracter facial animation at PDI ([Terzopoulos97])
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Main Application Areas 12
Geri‘s GamePixar Animation studios
Virtual MarilynThalmann and Thalmann 1993
- Films with synthetic actors
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Creating Digital Actors and Animals 13
Three important stages:
- Modeling - specification of geometry to create shapes, that visualize features
- Animation - creates illusion of motion, by iterating through the process of modeling and rendering at discrete time intervals
- Rendering - adding lights textures and other optical features to the model and displaying
The rest ofthis lecture
First attempts to CA of human body worked only with simplified version of skeleton which was modeled via lines in 3D, rendered as a lines in 2D and the animation techniques were mainly derived from robotics (forward and inverse kinematics)
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Creating Digital Actors and Animals 14
Different application areas require different modeling, rendering and animation techniques:
• Biomechanical and biomedical apps: physical and medical accuracy is important, visual appearance is not critical, motion is created via physical simulation
• VR and Interactive apps: speed of the application is important - balance between speed and visual appearance, animation via key-framing or motion capturing
• Digital actors in films: highest photorealistic detail is necessary, mostly traditional key-frame animation is used
Current research in all the three areas tries to incorporate as much of physical, anatomical and biomechanical knowledge as
possible
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Creating Digital Actors and Animals 15
MODELING:
Common structure of models used in CA was derived from traditional animation:
Cartoons - first draw stick figure (skeleton), then rounded forms representing flash and finally outline representing skin
Clay Animation - plasticene is wrapped around metal armature
Thus we use similar layered approach:
- first define articulated skeleton:
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Creating Digital Actors and Animals 16
- than define some representation of internal organs (geometry, behavior):
- finally add a representation of surface - skin:
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Creating Digital Actors and Animals 17
ANIMATION:
To specify motion of whole human body in terms of simple polygonal model is almost impossible!
Solution:
describe motion of skeleton - then find corresponding skin deformations automatically (most of current animation systems)
Various animation methods differ mainly in the way, how they solve the problem of soft tissue and skin deformation.
Some researchers attempts to create a physically based model of muscle and then define motion of whole body in terms of muscle activities, resulting in skeleton motion and skin deformations (Chen and Zeltzer 1992, Ng-Thow-Hing 1994)
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Creating Digital Actors and Animals 18
Specifying skeleton motion using forward and inverse kinematics
Kinematics -- the study of motion without regard to the forces that cause it.
Forward (FK):
drawing graphics
),( fP Inverse (IK):
specify fewer degrees of freedom (DOF)
more intuitive control
automatic calculation of desired joint
angles
)(, 1 Pf
P
?
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Computer Animation of
Soft Tissues
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Types of Animation Models
Large number of systems for CA of vertebrates were developed. Every one uses little different deformation model:
• Geometric deformation models - work only with geometry
• Physically based - animation of some part of the model is solved via simulation
• Models that incorporate biomechanical knowledge
• Models that incorporate anatomical knowledge
Most of them work with layered models
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JLD Operators (Thalmann, Lperriére, Thalmann, 1988)
Joint-Dependent Local Deformation Operators ([Thalmann88])
• Purely geometric model
• Stick skeleton with constraints at joint angles
• Skin made of triangle mesh
• Geometric operators are used to create smooth deformation of skin at joints. For each joint one special operator.
• Special operators are used to simulate muscle inflation according to flexion angles at joints
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JLD Operators (Thalmann, Lperriére, Thalmann, 1988) 22
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The CRITTER System (Chadwick, Haumann, Parent, 1989) 23
First system which uses layered approach ([Chadwick89]).
Authors used four layers:
1) Motion specification (behavior layer). Forward and inverse kinematics, procedural animation
2) Motion foundation, articulated armature (skeleton layer). Common articulated structure - links connected at joints
3) Shape transition, squash and stretch (muscle and fatty tissue layer). This layer is represented as a set of Free Form Deformation Blocks (FFD) attached to the skeleton. Mass is added to the control points of FFD blocks and these points are connected by springs. Allows to simulate dynamic behavior of muscle and fatty tissue.
4) Surface description, surface appearance and geometry (skin, clothing and fur layer). Many type of surface description (polygonal, B-Splines, etc.).
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The CRITTER System (Chadwick, Haumann, Parent, 1989) 24
Abstract muscle deformation: Pair of adjoining FFDs
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The CRITTER System (Chadwick, Haumann, Parent, 1989) 25
FFD blocks straddling the joint connecting two links.
(a) joint angle is 0; (b) angle is below threshold; (c) angle is grater then user definable threshold
FFD blocks straddling the joint connecting two links.
(a) joint angle is 0; (b) angle is below threshold; (c) angle is grater then user definable threshold
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The CRITTER System (Chadwick, Haumann, Parent, 1989) 26
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The LEMAN System (Turner, Thalmann, 1993) 27
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The LEMAN System (Turner, Thalmann, 1993) 28
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The LEMAN System (Turner, Thalmann, 1993)
Results
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Study of Biomechanics (Chen, Zeltzer 1992, [Chen92]) 30
• Geometry obtained from VHP
• Model of muscle active force due to
neural excitation
• model of muscle passive forces
• Muscle is discretized into four 20 node isoparametric bricks
• muscle reaction is simulated by Finite Element Method:
- find the equation of motion:
168 equations
- integrating these equations yields resulting motion
Based on biomechanical studies of prof. Zajac
)(tTKuuCuM
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Study of Biomechanics (Chen, Zeltzer 1992) 31
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Study of Biomechanics (Chen, Zeltzer 1992) 32
Relaxed muscle deforms due to gravity
Active muscle pulled taut
Shortening of biceps upon activation. Inverse kinematics was applied to forearm
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Unrealistic Deformations 33
However, if we do not incorporate accurate anatomical knowledge to the model, the deformation tends to appear unrealistic, even if the model is very complex
Huang, Thalmans, Boulic, Mas, 1994
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Anatomically Based Modeling (Wilhelms 1997)
References:
[Wilhelms97a] Wilhelms, J. and Gelder, A. V.: Anatomically Based Modeling, In SIGGRAPH 97 conference proceedings, ACM SIGGRAPH, Addison Wesley, August 1997.
[Wilhelms97b] Wilhelms, J. and Gelder, A. V.: Animals with Anatomy, IEEE Computer Graphics and Applications, 17(3):22-30, May 1997.
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•skeleton - rigid segments connected by joints
•four type of materials:
- bones
-muscles (attached to bones)
-generalized tissue (to give shapes in regions without muscles)
- skin (attached to underlying tissues with anchors)
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Anatomically Based Modeling (Wilhelms 1997)
• Joints have 3 revolute degree of freedom (constrained by max & min angle)
• Skeleton and generalized tissues - triangle meshes or ellipsoids they change position during motion but not the shape
Muscles - terminology
• Proximal - closer to body center; Distal - more distant to body center
• Origin - the place(s) on one or more bones where the muscle originates (is connected via tendons to bone)
• Insertion - location on one or more bones where the muscle inserts via tendons to bone (more distal to the body center then origin)
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The diameter and shape of muscle changes according to the relative position of origin and an insertion.
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Anatomically Based Modeling (Wilhelms 1997) 36
Default muscle shape
2 origins and 2 insertions
User adjusted muscle shapes (Side and front view in three
different levels of contraction)
Muscle Shape
is discretized generalized deformed cylinder whose axis is a curve from midpoint of origins to midpoint of insertions
7 muscle sections, 8 elliptical slices
slices form radial polygons; connecting polygon points axially gives the muscle shape
no explicit tendons
slice coordinate frames
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Anatomically Based Modeling (Wilhelms 1997)
Muscle Positioning - parameters controlled by the user
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• 2 origin locations• 2 insertion locations• x,y scale of each muscle slice• x,y,z translation of each slice
from default location• x,y,z rotation from default state
• pivot on/off• which slice acts as a pivot• origin of the pivot coordinate frame• orientation of the pivot coordinate
framepivots (muscles bends
around pivot)
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Anatomically Based Modeling (Wilhelms 1997)
Muscle Animation
Muscle rest length - distance from the midpoint of origins through the pivot, to midpoint of insertions; muscle in resting position
Muscle present length - recalculates on every joint angle change
Width and thickness of internal slices (not insertion and origin) is scaled by:
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lengthpresent
lengthrest
_
_
Thus assuring approximately non-varying volume of muscle
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Anatomically Based Modeling (Wilhelms 1997)
Skin
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blurred voxelized modelskin reconstructed using
marching cubes alg.
skeleton, muscles and soft tissues voxelized model
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Anatomically Based Modeling (Wilhelms 1997)
Anchoring Skin
Anchor of skin vertex is the nearest underlying point
Virtual anchor is the initial skin position relative to underlying component
Anchoring - find the nearest point on underlying component and store the coordinates of vertex in local coordinate frame of that component.
Vertex is associated to the muscle section between two slices, four vertices in each slice are selected.
Parametric trilinear transformation which maps a unit cube into the warped cell between two muscle slices is defined. It maps points in parameter space (t,u,v) into physical space (x,y,z).
These transformations are defined on adjacent cubes and maps each shared corner to the same point thus assuring C0 continuity.
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Anatomically Based Modeling (Wilhelms 1997)
Anchoring Skin
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Anatomically Based Modeling (Wilhelms 1997)
The Elastic Skin Model
• Edges of the triangle meshed skin are considered to be a springs (stiffness and rest length)
• Spring stiffnesses ke of edge between vertex v and vj is:
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221),(
len
aavvk je
where len …… is the length of the edge a1 , a2 … are areas of triangles sharing the edge
all computed once in the rest position
• Spring stiffnesses ka between the skin vertex and its virtual anchor is:
i
iaa
aCvk
3)(
where Ca …… is user definable constant (usually 0.1)
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Anatomically Based Modeling (Wilhelms 1997)
The Elastic Skin Model
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Anatomically Based Modeling (Wilhelms 1997)
Relaxation
• The process of equilibrium finding commences from positions of virtual anchors and is iterative
- wj = vj - v …present length of edge
- length_excess = wj - rest_length
- length_excess is multiplied by ke to give scalar value of elastic force
- net_elastic_force is sum of forces from all adjacent edges of vertex v and of edge to to its virtual anchor
- elastic_relaxation_vector = net_elastic_force / sum of all stiffness coefficients contributing to vertex v
- All skin vertices are translated by their elastic_relaxation_vectors
- The process iterates until the maximum relaxation vector is below user specified threshold or max. num. of iterations occurs.
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Anatomically Based Modeling (Wilhelms 1997)
Anchors, virtual anchors and relaxation - monkey shoulder under arm
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Skin vertices coincide with virtual anchors
• brown - skin triangle mesh
• yellow - skin vertices connected to their muscle anchor
Skin vertices after relaxation
• red lines - connects skin vertices to their virtual anchors
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Anatomically Based Modeling (Wilhelms 1997)
Elastic Model - results
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Anatomically Based Modeling (Wilhelms 1997)
Results
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Anatomically Based Modeling (Wilhelms 1997)
approx. 150,000 triangles, 30 relaxations = 25 seconds to recompute on SGI with 4 processors 150MHz R4400
Result: good compromise between visual appearance and speed
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CA at our Department
Running projects:
• Biomechanically Based Computer Animation of Human Hand - Dobšík M. (PhD thesis)
• Capturing the Motion of Human Figure using Single Camera View - Fědor M. (PhD thesis)
• Artificial Life in Virtual Reality - Marušinec J. (PhD thesis)
• and many other projects of students at master’s degree.
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CA at our Department
Obtaining Human Body Model from VHD Data
We decided for two strategies concurrently:
1) Reconstruction of 3D polygonal models from 2D slices
Two stages:
• find a contour of organ (e.g.: muscle, skin, ...)
• reconstruct 3D model from sequence of 2D slices
2) Direct polygonization of voxel model
Marching cubes method
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CA at our Department 51
VHDVo xe l
Visua liza tio nPrc hlík M .
C o nto urDe te c tio n
Hla d ík K.
PO LYG O N ALM O DEL
Surfa c e s fro mC o nto urs
Ve le šík J .
Se g m e nta tio nJ irko vský M .
Dire c tPo lyg o niza tio n
He ro u t A .
a ll VHD d a ta
se le c te d d a ta
2D slic e s
2D c o nto urs
3D v
oxe
ls a
fter
seg
me
nta
tion
3D vo xe ls
3D p o lyg o na l m o d e l
Cooperation of MSc students
Obtaining Human Body Model from VHD Data
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Volume Vizualization by M. Prchlík MSc Student
We selected a subvolume containing right hand of Visible Female (2.5GB of image data)
RayCasting is used for vizualization of RBG colored volume data (Visible female - Voxel cube 0.33mm x 0.33mm x 0.33mm). [Jurzykowski99]
Complete selected volume
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CA at our Department 53
Volume Vizualization
approx. 13 sec to display complete volume (water remove only) on SunEnterprise 450, 400MHz, 4GB of memory
Detail of hand(650 slices - 0.8GB)
Wrist (650 slices - 0.8GB)
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CA at our Department 54
Reconstruction of 3D Models from 2D contours - Velešík J. [Velesik00]
Some results:
Concave surfacesBranching and LinkingBranching and Linking
Multiple branchingTermination of branchPolygonization of branch at saddle (concave surface)
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CA at our Department 55
Skin texturing by Pavel Jurzykowski MSc student [Jurzykowski99]
Based on work:[Wu95] Y. Wu, N. Magnenat-Thalmann, D. Thalmann (1995), "A dynamic wrinkle model
in facial animation and skin ageing", J. Visual. Comp. Anim., 6, 195–205.
Approximation of microstructure of human skin
using Bump mapping
+ =
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Skin texturing by Pavel Jurzykowski MSc student
Macrostructure of skin is represented by Bezier cubics
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Skin texturing by Pavel Jurzykowski MSc student
Limbs (here finger) are divided into segments. Each segment is textured independently. Texture is created dynamically. Continuity is maintained between segments.
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References #1 58
[Chadwick89] Chadwick, J., E., Haumann, F., R. and Parent, E., R.: Layered Construction for Deformable Animated Characters. In SIGGRAPH’89 conference proceedings, ACM SIGGRAPH, 1989, pages 243-252.
[Chen92] Chan, D., T. and Zeltzer, D.: Pump It Up: Computer Animation of Biomechanically Based Model of Muscle Using the Finite Element Method. In SIGGRAPH’92 conference proceedings, ACM SIGGRAPH, 1992, pages 89-98.
[Hing98] Ng-Thow-Hing, V.: Mini-tutorial on Creating Digital Humans and Animals with Computer Graphics, CITO '98 Inaugural Researcher Retreat, University of Toronto, 1998.
[Jurzykovski99] Jurzykowski, P.: Vizualizace a animace lidské kůže. Master’s Thesis, Brno University of Technology, Czech Republic, 1999.
[Maurell98] Maurell, W. : 3D Modeling of the Human Upper Limb including the Biomechanics of Joints, Muscles and Soft Tissues, Ph.D. Thesis, Laboratoire d'Infographie - Ecole Polytechnique Federale de Lausanne, 1998.
[Molet99] Molet, T., Aubel, A., Çapin, T., Carion, S., Lee, E., Magnenat-Thalmann, N., Noser, H., Pandzic, I., Sannier, G., Thalmann, D.: ANYONE FOR TENNIS? Presence, Vol. 8, No. 2, MIT press, April 1999, pp.140-156.
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References #2 59
[Prchlik00] Prchlík, M.: Vizualisation of Voxel Models. Master’s Thesis, Brno University of Technology, Czech Republic, 1999.
[Terzopoulos97] Terzopoulos, D., Mones-Hattal, B., Hofer, B., Parke, F., Sweetland, D. and Waters, K.: Facial animation (panel): past, present and future. In SIGGRAPH’97 conference proceedings, ACM SIGGRAPH, Addison Wesley, August 1997, Pages 434 - 436.
[Thalmann88] Magnenat-Thalmann, A., Laperriere, R., Thalmann, D.: Joint-Dependent Local Deformations for Hand Animation and Object Grasping, Proc. Graphics Interface'88, Edmonton,1988.
[Turner93] Turner, R. and Thalmann, D.: The Elastic Surface Layer Model for Animated Character Construction. In Computer Graphics International, 1993.
[Velesik00] Velešík, J.: Reconstruction of 3D Models using 2D image slices. Master’s Thesis, Brno University of Technology, Czech Republic, 1999.
[Volino97] Volino, P. and Nadia Magnenant-Thalmann: Developing Simulation Techniques for an Interactive Clothing System. Published in Proc. VSMM `97, IEEE Computer Society, 1997, pp. 109-118.
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References #3 60
[Wilhelms97a] Wilhelms, J. and Gelder, A. V.: Anatomically Based Modeling, In SIGGRAPH’97 conference proceedings, ACM SIGGRAPH, Addison Wesley, August 1997.
[Wilhelms97b] Wilhelms, J. and Gelder, A. V.: Animals with Anatomy, IEEE Computer Graphics and Applications, 17(3):22-30, May 1997.
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THE END