Inverse kinematics

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Inverse kinematics is the process of determining the parameters of a jointed flexible object (a kinematic chain) in order to achieve a desired pose. Inverse kinematics is a type of motion planning. Inverse kinematics are also relevant to game programming and 3D animation, where a common use is making sure game characters connect physically to the world, such as feet landing firmly on top of terrain.

An articulated figure consists of a set of rigid segments connected with joints. Varying angles of the joints yields an indefinite number of configurations. The solution to the forward kinematic animation problem, given these angles, is the pose of the figure. The solution to the more difficult inverse kinematics problem is to find the joint angles given the desired configuration of the figure (i.e., end effector). In the general case there is no analytic solution for the inverse kinematics problem. However, inverse kinematics may be solved via nonlinear programming techniques. Certain special kinematic chains—those with a spherical wrist—permit kinematic decoupling. This treats the end effector's orientation and position independently and permits an efficient closed-form solution.

Inverse kinematics is a tool utilized frequently by 3D artists. It is often easier for an artist to express the desired spatial appearance rather than manipulate joint angles directly. For example, inverse kinematics allows an artist to move the hand of a 3D human model to a desired position and orientation and have an algorithm select the proper angles of the wrist, elbow, and shoulder joints.

For example, when one wants to reach for a door handle, their brain must make the necessary calculations to position his limbs and torso such that the hand locates near the door. The main objective is to move the hand but the many complex articulations of several joints must occur to get the hand to the desired location. Similarly with many technological applications, inverse kinematic mathematical calculations must be performed to articulate limbs in the correct ways to meet desired goals. One example where inverse kinematic calculations are often essential is robotics, where an operator wants to position a tool using a robot arm but certainly doesn't want to manipulate each robot joint individually. Other applications include computer animation where animators may want to operate a computer generated character, but find it impossibly difficult to animate individual joints. The solution is to model the virtual joints of the puppet and allow the animator to move the hands, feet and torso, and the computer automatically generates the required limb positions to accomplish this using inverse kinematics.

Key to the successful implementation of inverse kinematics is animation within constraints: computer characters' limbs must behave within reasonable anthropomorphic limits. Similarly, robotic devices have physical constraints such as the environment they operate in, the limitations of the articulations their joints are capable of, and the finite physical loads and speeds at which they are able to operate.

Other applications of inverse kinematic algorithms include interactive manipulation, animation control and collision avoidance.

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