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Skeletal animation, sometimes referred to as rigging, is a technique in computer animation
Computer animation

Computer animation is the art of creating moving images with the use of computers. It is a subfield of computer graphics and animation....
, particularly in the animation of vertebrates, in which a character is represented in two parts: a surface representation used to draw the character (called the skin) and a hierarchical set of bones used for animation only (called the skeleton).

This technique is used by constructing a series of 'bones'.






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Skeletal animation, sometimes referred to as rigging, is a technique in computer animation
Computer animation

Computer animation is the art of creating moving images with the use of computers. It is a subfield of computer graphics and animation....
, particularly in the animation of vertebrates, in which a character is represented in two parts: a surface representation used to draw the character (called the skin) and a hierarchical set of bones used for animation only (called the skeleton).

This technique is used by constructing a series of 'bones'. Each bone has a three dimensional transformation (which includes its position, scale and orientation), and an optional parent bone. The bones therefore form a hierarchy
Hierarchy

A 'hierarchy' is an arrangement of items The word derives from the Greek language , from ?e?????? , "president of sacred rites, high-priest" and that from , "sacred" + , "to lead, to rule"....
. The full transform of a child node is the product of its parent transform and its own transform. So moving a thigh-bone will move the lower leg too. As the character is animated, the bones change their transformation over time, under the influence of some animation controller.

Each bone in the skeleton is associated with some portion of the character's visual representation. In the most common case of a polygonal mesh character, the bone is associated with a group of vertices
Vertex (geometry)

In geometry, a vertex is a special kind of point which describes the corners or intersections of geometric shapes. Vertices are commonly used in computer graphics to define the corners of surfaces in 3d models, where each such point is given as a vector....
; for example, in a model of a human being, the 'thigh' bone would be associated with the vertices making up the polygons in the model's thigh. Portions of the character's skin can normally be associated with multiple bones, each one having a scaling factors called vertex weights, or blend weights. The movement of skin near the joints of two bones, can therefore be influenced by both bones.

For a polygonal mesh, each vertex can have a blend weight for each bone. To calculate the final position of the vertex, each bone transformation is applied to the vertex position, scaled by its corresponding weight. This algorithm is called matrix palette skinning, because the set of bone transformations (stored as transform matrices
Matrix (mathematics)

In mathematics, a matrix is a rectangular array of numbers, as shown at the right. In addition to a number of elementary, entrywise operations such as matrix addition a key notion is matrix multiplication....
) form a palette for the skin vertex to choose from.

Strengths


  • Bone represent set of vertices (or some other objects, which represent for example a leg).
    • Animator controls fewer characteristics of the model
      • Animator can focus on the large scale motion.
    • Bones are independently movable.


An animation can be defined by simple movements of the bones, instead of vertex by vertex (in the case of a polygonal mesh).

Weaknesses

  • Bone represents set of vertices (or some other object).
    • Does not provide realistic muscle
      MUSCLE

      MUSCLE is public domain, multiple sequence alignment software for protein and nucleotide sequences.MUSCLE is integrated into UGENE bioinformatics tool as a plugin....
       movement and skin motion
    • Possible solutions to this problem:
      • Special muscle controllers attached to the bones
      • Consultation with physiology
        Physiology

        Physiology is the study of the mechanical, physical, and biochemical functions of living organisms. Physiology has traditionally been divided between plant physiology and animal and all living things physiology but the principles of physiology are universal, no matter what particular organism is being studied....
         experts (increase accuracy of musculoskeletal realism with more thorough virtual anatomy simulations)


Applications


Skeletal animation is the standard way to animate characters or mechanical objects for a prolonged period of time (usually over 100 frames). It is commonly used by video game artists
Game artist

A game artist is an artist who creates art for one or more types of games. Game artists are responsible for all of the aspects of game development that call for visual art....
 and in the movie industry, and can also be applied to mechanical objects and any other object made up of rigid elements and joints.

Performance capture (or motion capture
Motion capture

Motion capture, motion tracking, or mocap are terms used to describe the process of recording motion and translating that movement onto a digital model....
) can speed up development time of skeletal animation, as well as increasing the level of realism.

For motion that is too dangerous for performance capture, there are computer simulation
Computer simulation

A computer simulation, a computer model or a computational model is a computer program, or network of computers, that attempts to simulation an abstract model of a particular system....
s that automatically calculate physics of motion and resistance with skeletal frames. Virtual anatomy properties such as weight of limbs, muscle reaction, bone strength and joint constraints may be added for realistic bouncing, buckling, fracture and tumbling effects known as virtual stunt
Stunt

A stunt is an unusual and difficult physical feat, or any act requiring a special skill, performed for artistic purposes in TV, theatre, or film....
s. Virtual stunts are controversial due to their potential to replace stunt performer
Stunt performer

A stunt performer is someone who performs dangerous stunts, often as a career.These stunts are sometimes rigged so that they look dangerous while still having safety mechanisms, but often they are as dangerous as they appear to be....
s. However, there are other applications of virtual anatomy simulations such as military
Military

A military is an organization authorized by its nation to use force, usually including use of weapons, in defending its country by combating actual or Threat of force ....
  and emergency
Emergency

An emergency is a situation which poses an immediate risk to health, life, property or Natural environment. Most emergencies require urgent intervention to prevent a worsening of the situation, although in some situations, mitigation may not be possible and agencies may only be able to offer palliative care for the aftermath....
 response. Virtual soldiers, rescue workers, patients, passengers and pedestrians can be used for training, virtual engineering and virtual testing of equipment. Virtual anatomy technology may be combined with artificial intelligence
Artificial intelligence

Artificial intelligence is the intelligence of machines and the branch of computer science which aims to create it. Major AI textbooks define the field as "the study and design of intelligent agents,"...
 for further enhancement of animation and simulation technology.

External links




See also

  • Computer animation
    Computer animation

    Computer animation is the art of creating moving images with the use of computers. It is a subfield of computer graphics and animation....
  • 3D computer graphics
    3D computer graphics

    3D computer graphics are graphics that use a Cartesian coordinate system#Three-dimensional coordinate system representation of geometric data that is stored in the computer for the purposes of performing calculations and rendering 2D images....
  • Forward kinematic animation
    Forward kinematic animation

    Forward kinematic animation is a method in 3D computer graphics for animating models.The essential concept of forward kinematics animation is that the positions of particular parts of the model at a specified time are calculated from the position and orientation of the object, together with any information on the joints of an articulated mo...
  • Inverse kinematics
    Inverse kinematics

    Inverse kinematics is the process of determining the parameters of a kinematic pair flexible object in order to achieve a desired pose. Inverse kinematics is a type of motion planning....
  • Inverse kinematic animation
  • Per-vertex animation