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Abstract
Skin movement modeling is a crucial element for realistic animation of both human and creature
characters. In this paper, we are primarily concerned with a special skin movement effect, skin sliding.
Although physical properties, such as skin elasticity, are usually regarded important in physical
simulation, it often leads to an increasing computational load. A complete physical solution will also
limit the animator’s ability to fine-tune the visual effects. In this paper we present a physically based skin
sliding modeling method borrowing an idea from structural mechanics called bar networks. The
advantage of using a bar network system in simulating the physical properties of skin surfaces is that
only a set of sparse linear equations need to be solved, making the method much faster than a simulation
based technique. As only one additional step is added to the animation pipeline to restore the skin elastic
property, this algorithm is compatible with all existing skin deformation method such as smooth skinning,
muscle base skin deformation, cluster and Free Form Deformations, and it therefore does not disturb the
existing production pipeline.

Introduction
Realistic skin movement has been paid a
great deal of research efforts over the recent years. It
is crucial for convincing character animation. In this
seminars I am concentrating mainly on the modeling
of the skin sliding phenomenon, which, comparing
with other skinning related deformation effects, has
received relatively little attention in computer
animation. Human and animal skin exhibits complex
physical characteristics, making it more dynamic than
kinematic. Skin becomes an extremity to the multiple
layers of underlying anatomical structures. The
movement of skin during body motion is highly
dependent on the above said underlying anatomical
structures thereby making it essential to delve into
the physical aspects of the skin-sliding phenomenon.
This is where physics based simulation approaches
come into play. Incorporating physical properties of
anatomic structures can potentially improve realism.
Physics can be used at the muscle level, bones, fat or
elastic skin layer. Some work on skin sliding has
been carried out in this category using spring-mass
systems to page link the skin surface with the underlying
anatomical structure. Spring networks usually do not
consider the elasticity of the skin itself.
One possible way to simulate that would be
to replace the edges of the skin mesh also with
springs. Though this would general a good effect of
skin slide, the large number of springs on the skin
model would increase the computation significantly.
Skin sliding can be essentially thought of as a three
dimensional problem with two-dimensional
attributes. With relation to the surface, the movement
of the skin is parallel to the surface, with little or no
perpendicular motion to the surface.

In this paper we present a novel method to
model skin slide. Bar-nets are useful in form finding.
By considering the skin as a sheet and reducing it to a
2D problem, it becomes very easy to take into
consideration the physical properties of skin
elasticity. In addition, our method can be easily
integrated into the animation pipeline without any
change to the traditional methods of skinning, giving
the animator the freedom to control and design
actively during the skinning phase. Attacking skin
sliding as a 2D problem makes our method very
efficient and fast The parameters of the barnets can
be adjusted to achieve the effect of different skin
elasticity

Old School Methods
The very first animated characters were 2D
sprites. Just like traditional animation or flip books.
When we moved to 3D, our first animated characters
were “jointed.” Each limb or part of a limb is a
separate rigid object. The Problem with that is that
the Interpenetration at joints and lack of accuracy.

Physics Based Animation
Create a model based on the physics of a
situation, and solve equations for what happens. They
are physically meaningful and have a capacity to
generate more realistic simulation. It has been
applied to Rigid Objects, Cloth, Water, Smoke,
Explosion etc.
Spring Mass Systems
Model objects as systems of springs and masses. The
springs exert forces, and you control them by
changing their rest length. We propose a physically-
based approach based on anatomical knowledge for
real-time animation. A reasonable but simple,
physical model for muscles. The skin is modeled by a
mass-spring system with nonlinear springs which
have a stress-strain relationship to simulate the elastic
dynamics of real skin. Muscles are modeled as forces
deforming the spring mesh. Based on the action units
(AUs), various expressions and deformations can be
generated by a combination of contractions of a set of
muscles.
Skeletons And Skins
Modern approach is called “skinning.”
Here the Skin is a 3D model made out of
triangles. Skeleton is invisible and only the skin
is seen by the player. Each vertex of each
triangle is attached to one or more bones. We use
weights to define bones’ influences. Weights at a
joint must always add up to 1.Each vertex of
each triangle is attached to one or more bones.
We use weights to define bones’ influences.
Weights at a joint must always add up to 1.
Skeletons have two kinds of poses:
– Bind Pose: The skeleton’s pose
when the skin was first attached.
– Current Pose: Any other pose of
the skeleton; usually a frame of an
animation.
The bind pose is like a “home base” for the
character’s skeleton. If you drew the mesh
without its skeleton, it would appear in its bind
pose.

Smooth Skinning
Skin deformation, or skinning, has time and
again proven to be an indefatigable part of character
animation. In the current scenario of computer
animation, where realism is paramount, efficient and
visually believable techniques of skin deformation
are essential. An intuitive attempt to deform a
character was involving a skeleton into skin
deformation. This approach treats the skin as a shell
that moves by an explicit function of the skeleton.
Vertices of the skin are deformed by a weighted
combination of the joint transformations of the
character’s skeleton. Collectively, such methods are
known as the Smooth Skinning.
During smooth skinning, you bind a model's
deformable objects to a skeleton. After smooth
skinning, the deformable objects are called smooth
skin objects (or skin objects, or skin). The points
(CVs, vertices, or lattice points) of the deformable
objects are then referred to as smooth skin points, or
skin points.
If you want to change the results of smooth
skinning to create unique skeletal deformation
effects, you can edit or paint the weights of smooth
skinning at the point level (the CV, vertex, or lattice
point level). Additionally, to add further deformation
effects to smooth skin, you can use Maya's deformers
and smooth skin influence objects.
Joints closer to a smooth skin point will
have a greater influence than joints far from the skin
point. The joint closest to a smooth skin point will
have the greatest influence. Which joints have the
next greatest influence can depend on whether you
want Maya to consider the skeleton's hierarchy
during binding or to ignore the skeleton's hierarchy
during binding

Physics Based Approaches
The joint-based system is popular owing to
its interactivity and use of minimal animation data.
There are basically two main approaches to modeling
skin deformations, namely, physics-based approaches
(also known as anatomy-based approach) and
example-based skinning. Physics-based methods are
based on the anatomy, elastic mechanics, or
biomechanics of skin deformation originating from
the movements of muscles and tendons. They are
physically meaningful and have a capacity to
generate more realistic simulation.
The example-based approach forms a
suitable alternative where computational expenses are to be minimized. An artist models certain key poses
of the characters. New poses are interpolated from
these key poses. However because each example skin
shape was modeling separately, it is impossible to
realize a smooth sliding effect using example based
method.
Because of the heavy computation involved
in the simulation of skin sliding effect, no
commercial animation software provides this
function until 2009. Maya 2009 in its muscle package
gives the animator an option to paint the sliding
weight in the muscle simulation. However what it
does is only to push out the skin surface wherever
there is collision between muscle and skin surface
mesh.

Conclusion
An efficient skin sliding technique has been
developed for realistic animation of virtual human
and animal characters. By considering the bind pose
skin surface as the elastic target, we are able to
simulate skin elasticity with good results. With our
method, the animator can still use their familiar
deformation method to achieve the result they
expected. Our method basically adds an additional
step after the deformation to restore the skin elasticity
in order to achieve the skin sliding effect. Our skin
sliding simulation is compatible with any kinds of
skin deformation method, irrespective of the type of
skinning methods. Only an extra step is added to the
deformed shape to restore the elasticity of the skin
surface. And since the overall shape remains the
same even after the skin sliding, the animator’s
creative work is well protected

References
1] H.J. Schek, The force density method for form
finding and computation of general networks,
Computer Methods in Applied Mechanics and
Engineering, vol. 3, 1974, pp. 115–134.
[2] J.J. Zhang, , X. Yang, , and Y. Zhao, Bar-net
driven skinning for character animation, Computer
Animation and Virtual Worlds, vol. 18, 2007, pp.
437-446.
[3] N. Magnenat-Thalmann, R. Laperrire, and D.
Thalmann, Jointdependent local deformations for
hand animation and object grasping, In Proceedings
on Graphics interface, vol. 88, 1988, pp. 26-33.