Muscular Facial Models

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Muscular Facial Models
Adriana Davidescu 16.Mai 2002
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• This presentation is based on two articles
• Kähler, K.,Haber, J. and Seidel, H.- P. (2001).
  Geometry-Based Muscle Modeling for Facial
  Animation, Proceedings Graphics Interface 2001,
  pp. 37-46
• Terzopoulos, D. and Waters, K. (1990).
  Physically-Based Facial Modeling, Analysis and
  Animation, Journal of Visualisation and Computer
  Animation 1(2), pp.73-80.

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Introduction 1

• Artificial face modeling has recently been based
  on an anatomical simulation of the human face.
• First of all, a high fidelity geometric model
  captures the facial shape of a person. It is
  achieved through a range scanner.

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Introduction 2

• The muscles lie underneath the skin and have a
  great influence on the shape and appearance of
  the skin.
• The next problem of artificial facial modeling is
  to adapt the muscle geometry to the already
  created facial geometric model.

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Types of Artificial Animated Models

• Parametric Models
• manipulates directly the geometry of the
• surface of the skin.
• The MPEG-4 standard specifies a set of 68
• facial animation parameters (FAPs), which can
• be adapted to any suitable head model.
• Shortcomingsthe range of possible facial
  expressions is
• limited.

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Types of Muscle-Based Facial Animation

• Physics-based models
• represent the elastic properties of the skin
  using a mass-spring system.
• Comparing these models with the geometrical
• modelling,the deformation of the skin is realised
• automatically through the facial tissue,which is
• due primarily to the volume preservation system.

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The Strucure of the Skin

• The skin has a layered structure
• epidermis
• a superficial layer of dead cells
• dermis
• the mechanical properties of the skin are due
  mostly to this layer.
• it consists of collagen (72), elastin (4) and
  gelatinous ground substance (20).
• subcutaneous fatty tissue
• it allows to slide freely over the muscle layer

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Head Model Components

• Input data
• a generic skull
• an arbitrary triangle mesh, obtained from a
  range scanner ( for real human faces or synthetic
• models ).
• The mass-spring will be implemented between the
  skull and the skin mesh.

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Head Model Components 2

• Two (three) layers
• skin / tissue ( a mass-spring system)
• muscle layer attached to the skull, inserted into
  skin
• bone structure fixed skull, rotating jaw
• Eyes, teeth, tongue as separate components

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The Trilayer Model of Skin Tissue

• The skin mesh consists of polygons .
• Subcutaneous fatty tissue has the elastic
  properties of the dermis and epidermis.
• It allows skin to slide freely.
• Springs connect the surface nodes to bone layer
  or to an underlying muscle. They imitate the
  non-linear properties of skin, i.e. they become
  stiffer under high strain.

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The Trilayer Model of Skin Tissue

• The muscle layer consist of fibres which are
  working in unison.
• 268 muscles can be activated to create facial
  expressions.
• There are about 55,000 possible facial
  expressions.These are classified in turn in 6
  primary expressions that communicate emotions
• anger, disgust, fear, happiness, sadness and
  surprise.

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The Mass - Spring System
skin mesh
muscle layer
skull
mirrored muscles/skull attachments
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Basic Muscle Model

• Muscles are built from individual fibres which
  consist in turn of piecewise linear segments.
• To each segment is added a quadric shape
  (ellipsoid).
• The ellipsoid form preserves the volume and the
  elasticity of the muscular fibers.

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Types of Muscles

• linear
• sheet (may consist of piecewise combined linear
  muscle fibres)
• curved
• sphincter (the muscles of the mouth)

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Terzopoulos and Waters Head Model

• The mass-system contains 3 layers
• cutaneous layer
• subcutaneous fatty tissue
• muscle layer
• The jaw rotation is specified through a
  menu-driven interface.

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Terzopoulos and Waters Head Model 2

• The authors provide for the first time a 3D model
  of human face.
• The model of the mass-system is a deformable
  lattice constructed from point masses connected
  through springs.

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Kaehler, Haber and Seidel Head Model

• There are only 2 layers
• skin tissue (epidermis and dermis)
• muscle layer
• They make a distinction between the fixed part of
  the skull and the rotating jaw.
• The authors design a muscle editing tool which
  allows a very quickly generation of animated
  faces.

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Contraction

• it is simulated as a flow over the skull
• achieved by moving the control points
• contraction value c 0 (relaxed muscle)
• c 1 (contracted muscle)

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Bulging

• Bulging is achieved in three phases
• (for linear muscles)
• scaling of the height and length of each muscle
  segment (relaxed and contracted variant)
• defining the central segment with the maximum of
  contraction
• multiply each linear segment si with a quadratic
  function that vanishes over the first and last
  segment and has the maximum value of 1.0 over the
  central segment.

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Muscle Shaping 1

• Muscles are sketched by hand from another face.
  They are automatically adapted to local geometry.
• Input Parameters
• skin thickness
• minimum
• maximum

thickness of the muscle layer
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Muscle Shaping 2

• Editing Parameters
• the number of muscle fibers/segments per fiber
• geometry of each segment (width, height and
  length ).
• position of the muscle fiber control points
  (i.e.the origin and the end).

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Muscle Shaping 3

• Steps of building a muscle
• shaping the grids
• refining the grids
• creating the muscle
• attaching the muscle

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Muscle Shaping 3

• Shaping the grids
• every grid is sketched row by row
• the 4 corner points are defined
• ( for a sphincter muscle is specified the centre
  of contraction )
• adapted to the local geometry
• displaced in a prescribed distance below the skin.

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Muscle Shaping 3

• Refining the grid
• calculating the distances from each cell to the
  skin surface.
• adjusting the maximum and minimum bounds of cell
  thickness.

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Muscle Shaping 4

• Creating a muscle
• (creating sheet of muscle fibers)
• one muscle fiber is inserted longitudinally into
  each strip of grid cells
• each ellipsoid is adapted to the sorrounding
  cell they are slightly enlarged over rhe cell
  boundaries.

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lateral view
refined grid
top view
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Muscle Shaping 5

• Attaching the muscle
• a).To skin
• assign skin nodes to muscle
• springs connect skin nodes with muscles
• creating mirror springs
• b).To the skull and jaw
• attach lower muscle
• control points to jaw

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Conclusions

• The physically-based models are applied on human
  heads and animal models
• its aim one parametrization for all faces
• by reediting it can be adapted
• it provides a rough muscle layout
• easy to build and use.