IK (Inverse Kinematics) and FK (Forward Kinematics)
Forward Kinematics (FK)
Forwards Kinematics or FK as it is more widely known is a way of posing or animating a skeleton each part / bone at a time, for example, in the case of an arm using FK, to animate it reaching forward, you would need to rotate the arm at the shoulder joint, and then you would need to rotate the joint directly below that at the elbow. It is quite an intuitive way to animate as it provides a good visual way of posing the character just by rotating the joints into position.
Inverse Kinematics (IK)
Inverse Kinematics or IK as it is more widely known is a way of posing or animating a skeleton by using a goal based system, IK, in my opinion would be best described as imagine you have no control of you own arm, and someone else instead moves it for you, the easiest way for this person to move you arm into the required position would be to grab your arm at the wrist and then move it into position, the whole rest of the arm would follow accordingly, this is how IK works. In the case of an arm, you would have shoulder and forearms joints, you would then create an IK chain from the top to the bottom, and then when you have done that, you would select the IK handle that would be created at the bottom of the forearm joint (wrist area) and the just move that, the whole arm would then move along accordingly.
Maya provides three different types of IK Solver, these are (default) ikRPSolver (Rotate Plane), ikSCSolver (Single Chain) and also the IK Spline Handle. These are in turn described as : –
ikRPSolver
The Rotate Plane IK Solver is the default IK Solver and is probably used the most when setting up characters, it is most useful when used in conjunction with chains in joints such as an arm (where the elbow will need to be rotated into place) or a leg (where the knee will need to be rotated into place). It essentially has the ability to control the twisting direction of the chain that it is controlling.
ikSCSolver
The Single Chain IK Solver is used when no twisting action is required of the joint chain, as in a characters foot for example.
IK Spline Handle
The IK Spline Handle is actually quite a powerful handle, it is best used when you would have a complex series of joints, for example a characters spine or a character who has a tail, in both cases you would require a solver that would simplify the process of animating a lot of joints together in the same area at the same time. When a Spline Handle is used between a chain of joints, a Spline is created along the chain, and in turn the CV’s (Control Vertices) are what would be selected and manipulated for animating with, generally the CV’s would in turn be constrained to Cluster Deformers for ease of use when manipulating for animation.
Constraints
As the Maya Online Help states ‘Constraints enable you to constrain the position, orientation, or scale of an object to other objects. Further, with constraints you can impose specific limits on objects and automate animation processes.’ Constraints are used quite a lot in animation and character setup, and it’s worthwhile getting to know how they all work, so you can best utilise them within your scenes. Maya offers a number of different types of constraints, these are : –
Point constraints – Point constraints constrain an object’s position to the position of one or more objects. For example, constraining a button to a characters shirt.
Aim constraints – Aim constraints constrain an object’s orientation so that it always aims at other objects. For example, constraining eyes on a character so that they always look in a particular direction.
Orient constraints – An orient constraint causes an object to follow the orientation of one or more objects. For example, a crowd scene may require all characters to look in a particular direction at the same time, you can orient constrain all the heads to one main character to set this up.
Scale constraints – A scale constraint causes an object to follow the scaling of one or more objects. For example, constraining two characters together so that when grows (scale Y), the other grows too.
Geometry constraints – A geometry constraint restricts an object to a NURBS surface, NURBS curve, or polygonal surface (mesh). For example, constraining a drinks can to a characters hand.
Normal constraints – Normal constraints constrain an object’s orientation so that it aligns with the normal vectors of a NURBS or polygonal surface (mesh). For example, constraining a spider to a character to get it to walk over the surface correctly.
Tangent constraints – Tangent constraints constrain an object’s orientation so that the object always points in the direction a curve. For example constraining a fly character to a curve so that it animates along the curve and keeps the direction that the curves holds, i.e. if it goes up, the fly character would be facing up.
Pole vector constraints – A pole vector constraint constrains an IK rotate plane handle’s pole vector. For example,. controlling the position of a characters knee of elbow when animating (more information later).
Taken from http://area.autodesk.com/tutorials/character_rigging_in_maya
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