Skeletal animation
Encyclopedia
Skeletal animation is a technique in computer animation
in which a character is represented in two parts: a surface representation used to draw the character (called skin or mesh) and a hierarchical set of interconnected bones (called the skeleton or rig) used to animate (pose and keyframe) the mesh. While this technique is often used to animate humans or more generally for organic modeling, it only serves to make the animation process more intuitive and the same technique can be used to control the deformation of any object — a spoon, a building, or a galaxy.
This technique is used in virtually all animation systems where simplified user interfaces allows animators to control often complex algorithms and a huge amount of geometry; most notably through inverse kinematics
and other "goal-oriented" techniques. In principle, however, the intention of the technique is never to imitate real anatomy or physical processes, but only to control the deformation of the mesh data.
. 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. A rig is generally composed of both forward kinematics
and inverse kinematics
parts that may interact with each other. Skeletal animation is referring to the forward kinematics
part of the rig, where a complete set of bones configurations identifies a unique pose.
Each bone in the skeleton is associated with some portion of the character's visual representation. Skinning is the process of creating this association. In the most common case of a polygonal mesh character, the bone is associated with a group of vertices
; 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. In most state-of-the-art graphical engines, the skinning process is done on the GPU thanks to a shader program
.
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
) form a palette for the skin vertex to choose from.
An animation can be defined by simple movements of the bones, instead of vertex by vertex (in the case of a polygonal mesh).
Weaknesses
Below a few relevant links:
Automatic Rigging and Animation of 3D Characters
Mixamo Online Animation Service - Automatic Rigging
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
) 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
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
s. Virtual stunts are controversial due to their potential to replace stunt performer
s. However, there are other applications of virtual anatomy simulations such as military
and emergency
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
for further enhancement of animation and simulation technology.
Computer animation
Computer animation is the process used for generating animated images by using computer graphics. The more general term computer generated imagery encompasses both static scenes and dynamic images, while computer animation only refers to moving images....
in which a character is represented in two parts: a surface representation used to draw the character (called skin or mesh) and a hierarchical set of interconnected bones (called the skeleton or rig) used to animate (pose and keyframe) the mesh. While this technique is often used to animate humans or more generally for organic modeling, it only serves to make the animation process more intuitive and the same technique can be used to control the deformation of any object — a spoon, a building, or a galaxy.
This technique is used in virtually all animation systems where simplified user interfaces allows animators to control often complex algorithms and a huge amount of geometry; most notably through inverse kinematics
Inverse kinematics
Inverse kinematics is a subdomain of kinematics, which is of particular interest in robotics and computer animation. In contrast to forward kinematics, which calculates the position of a body after a series of motions, inverse kinematics calculates the motions necessary to achieve a desired...
and other "goal-oriented" techniques. In principle, however, the intention of the technique is never to imitate real anatomy or physical processes, but only to control the deformation of the mesh data.
Technique
This technique is used by constructing a series of 'bones,' sometimes referred to as rigging. Each bone has a three dimensional transformation (which includes its position, scale and orientation), and an optional parent bone. The bones therefore form a hierarchyHierarchy
A hierarchy is an arrangement of items in which the items are represented as being "above," "below," or "at the same level as" one another...
. 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. A rig is generally composed of both forward kinematics
Forward kinematics
Forward kinematics is computation of the position and orientation of robot's end effector as a function of its joint angles. It is widely used in robotics, computer games, and animation. The reverse process is known as inverse kinematics....
and inverse kinematics
Inverse kinematics
Inverse kinematics is a subdomain of kinematics, which is of particular interest in robotics and computer animation. In contrast to forward kinematics, which calculates the position of a body after a series of motions, inverse kinematics calculates the motions necessary to achieve a desired...
parts that may interact with each other. Skeletal animation is referring to the forward kinematics
Forward kinematics
Forward kinematics is computation of the position and orientation of robot's end effector as a function of its joint angles. It is widely used in robotics, computer games, and animation. The reverse process is known as inverse kinematics....
part of the rig, where a complete set of bones configurations identifies a unique pose.
Each bone in the skeleton is associated with some portion of the character's visual representation. Skinning is the process of creating this association. 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 that describes the corners or intersections of geometric shapes.-Of an angle:...
; 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. In most state-of-the-art graphical engines, the skinning process is done on the GPU thanks to a shader program
Shader (realtime, logical)
A shader is essentially a computer program executed on a special environment. This article specifically covers realtime shaders which are shaders meant to execute on consumer-level GPUs....
.
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, symbols, or expressions. The individual items in a matrix are called its elements or entries. An example of a matrix with six elements isMatrices of the same size can be added or subtracted element by element...
) form a palette for the skin vertex to choose from.
Benefits and drawbacks
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.
- Animator controls fewer characteristics of the model
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 muscleMuscleMuscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
movement and skin motion - Possible solutions to this problem:
- Special muscle controllers attached to the bones
- Consultation with physiologyPhysiologyPhysiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...
experts (increase accuracy of musculoskeletal realism with more thorough virtual anatomy simulations)
- Does not provide realistic muscle
Automatic Rigging
Several solutions for the automatic or semi-automatic rigging of animated characters (FK or IK) have been developed in the last years, some of them leveraging machine learning techniques.Below a few relevant links:
Automatic Rigging and Animation of 3D Characters
Mixamo Online Animation Service - Automatic Rigging
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 artistsGame 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 movement and translating that movement on to a digital model. It is used in military, entertainment, sports, and medical applications, and for validation of computer vision and robotics...
) 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 simulate 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 cinema...
s. Virtual stunts are controversial due to their potential to replace stunt performer
Stunt performer
A stuntman, or daredevil 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 greater society to use lethal force, usually including use of weapons, in defending its country by combating actual or perceived threats. The military may have additional functions of use to its greater society, such as advancing a political agenda e.g...
and emergency
Emergency
An emergency is a situation that poses an immediate risk to health, life, property or 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...
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 that aims to create it. AI textbooks define the field as "the study and design of intelligent agents" where an intelligent agent is a system that perceives its environment and takes actions that maximize its...
for further enhancement of animation and simulation technology.
See also
- Computer animationComputer animationComputer animation is the process used for generating animated images by using computer graphics. The more general term computer generated imagery encompasses both static scenes and dynamic images, while computer animation only refers to moving images....
- 3D computer graphics3D computer graphics3D computer graphics are graphics that use a three-dimensional representation of geometric data that is stored in the computer for the purposes of performing calculations and rendering 2D images...
- Forward kinematic animationForward kinematic animationForward kinematic animation is a method in 3D computer graphics for animating models.The essential concept of forward kinematic 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...
- Inverse kinematicsInverse kinematicsInverse kinematics is a subdomain of kinematics, which is of particular interest in robotics and computer animation. In contrast to forward kinematics, which calculates the position of a body after a series of motions, inverse kinematics calculates the motions necessary to achieve a desired...
- Morph target animationMorph target animationMorph target animation, per-vertex animation, shape interpolation, or blend shapes is a method of 3D computer animation used together with techniques such as skeletal animation. In a morph target animation, a "deformed" version of a mesh is stored as a series of vertex positions...
- Interactive skeleton-driven simulationInteractive skeleton-driven simulationInteractive skeleton-driven simulation is a scientific computer simulation technique used to approximate realistic physical deformations of dynamic bodies in real-time. It involves using elastic dynamics and mathematical optimizations to decide the body-shapes during motion and interaction with...