Editing Skeletal animation (section)

== Technique ==
As described in an instructional article by Josh Petty:<ref>{{cite web |url=https://conceptartempire.com/what-is-rigging/ |title=What is 3D Rigging for Animation & Character Design? |last=Petty |first=Josh |work=Concept Art Empire |date=26 July 2018 |access-date=29 November 2018}}</ref>
{{Quote|Rigging is making our characters able to move. The process of rigging is we take that digital sculpture, and we start building the skeleton, the muscles, and we attach the skin to the character, and we also create a set of animation controls, which our animators use to push and pull the body around.}}

This technique constructs a series of bones (which need not correspond to any real-world anatomical feature), sometimes also referred to as rigging in the noun sense. Each bone has a three-dimensional transformation from the default [[bind pose]] (which includes its position, scale and orientation), and an optional parent bone. The bones therefore form a [[hierarchy]]. 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 bone configurations identifies a unique pose.

Each bone in the skeleton is associated with some portion of the character's visual representation (the [[Polygon mesh|mesh]]) in a process called  skinning. In the most common case of a [[Polygon mesh|polygonal mesh]] character, the bone is associated with a group of [[Vertex (geometry)|vertices]]; for example, in a model of a human being, the bone for the thigh 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 [[Weight map|vertex weights]], or [[Weight map|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]] by a [[Shader|shader program]].

For a polygonal mesh, each vertex can have a blend weight for each bone. To calculate the final position of the vertex, a [[transformation matrix]] is created for each bone which, when applied to the vertex, first puts the vertex in bone space then puts it back into mesh space.  After applying a matrix to the vertex, it is scaled by its corresponding weight. This [[algorithm]] is called matrix-palette skinning or linear-blend skinning,<ref>{{Cite web |url= https://www.skinning.org/direct-methods.pdf |title=Direct Skinning Methods and Deformation Primitives |last=Kavan |first=Ladislav |work=Skinning.org |publisher=University of Pennsylvania }}</ref> because the set of bone transformations (stored as transform [[matrix (mathematics)|matrices]]) form a palette for the skin vertex to choose from.

=== Benefits and drawbacks ===
==== Strengths ====
* A bone represents a set of vertices (or some other object which represents something, such as a leg),
** The animator needs to control fewer characteristics of the model,
*** The 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 ====
* A bone can only represent a set of vertices (or some other precisely defined object), and is not more abstract or conceptual.
** Does not provide realistic [[muscle]] movement and skin motion. Possible solutions to this problem:
*** Special muscle controllers attached to the bones.
*** Consultation with [[physiology]] experts, to increase accuracy of [[musculoskeletal]] realism with more thorough virtual anatomy simulations.