computer animation of vertebrates by martin dobšík brno university of technology, czech republic...

Computer Animation of

Vertebrates

by Martin Dobšík

Brno University of Technology, Czech Republic

e-mail: [email protected]: http://www.fee.vutbr.cz/~dobsik

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!

2

Computer Animation of Human

Body and Animals

Main Application Areas

• Biomechanical and Biomedical Applications

- The Virtual Cadaver

4

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]


- Surgery Simulation

5

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


- Orthopaedics

6

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


- 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.

7


• Virtual Reality and Interactive applications (games)

- Teleconferencing

- Military - fight simulations, soldiers training in dangerous situations, etc.

- Architecture

- Design

- Games

- and many others

8

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


- Textile industry - dressing virtual humans

10

Interactive clothing system ([Volino97])Pandzic, Capin, Magnenant-Thalmann, Thalmann


• Computer Animation for Films

- Actors in dangerous situations

- Changes to actors body (coloured hair, mising leg, ...)

- Cartoon films

11

Beth Hofer - Caracter facial animation at PDI ([Terzopoulos97])

Main Application Areas 12

Geri‘s GamePixar Animation studios

Virtual MarilynThalmann and Thalmann 1993

- Films with synthetic actors

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)


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


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:


- than define some representation of internal organs (geometry, behavior):

- finally add a representation of surface - skin:


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)


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

?

Computer Animation of

Soft Tissues

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

20

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

21

JLD Operators (Thalmann, Lperriére, Thalmann, 1988) 22

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.).


Abstract muscle deformation: Pair of adjoining FFDs


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

The LEMAN System (Turner, Thalmann, 1993) 27

The LEMAN System (Turner, Thalmann, 1993) 28

The LEMAN System (Turner, Thalmann, 1993)

Results

29

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

Study of Biomechanics (Chen, Zeltzer 1992) 31

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

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

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.

34

•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)


• 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)

35

The diameter and shape of muscle changes according to the relative position of origin and an insertion.

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


Muscle Positioning - parameters controlled by the user

37

• 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)


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:

38

lengthpresent

lengthrest

_

_

Thus assuring approximately non-varying volume of muscle


Skin

39

blurred voxelized modelskin reconstructed using

marching cubes alg.

skeleton, muscles and soft tissues voxelized model


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.

40


Anchoring Skin

41


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:

42

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)


The Elastic Skin Model

43


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.

44


Anchors, virtual anchors and relaxation - monkey shoulder under arm

45

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


Elastic Model - results

46


Results

47


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

48

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.

49

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

50

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


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


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)


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)


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

+ =


Skin texturing by Pavel Jurzykowski MSc student

Macrostructure of skin is represented by Bezier cubics


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.

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.

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.

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.

THE END

computer animation of vertebrates by martin dobšík brno university of technology, czech republic...

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