FABRIK: A fast, iterative solver for the Inverse Kinematics problem

Abstract

Inverse Kinematics is defined as the problem of determining a set of appropriate joint configurations for which the end effectors move to desired positions as smoothly, rapidly, and as accurately as possible. However, many of the currently available methods suffer from high computational cost and production of unrealistic poses. In this paper, a novel heuristic method, called Forward And Backward Reaching Inverse Kinematics (FABRIK), is described and compared with some of the most popular existing methods regarding reliability, computational cost and conversion criteria. FABRIK avoids the use of rotational angles or matrices, and instead finds each joint position via locating a point on a line. Thus, it converges in few iterations, has low computational cost and produces visually realistic poses. Constraints can easily be incorporated within FABRIK and multiple chains with multiple end effectors are also supported.

Introduction

This paper addresses the problem of manipulating articulated figures in an interactive and intuitive fashion for the design and control of their posture. This problem finds its application in the areas of robotics, computer animation, ergonomics and gaming. In computer graphics, articulated figures are a convenient model for humans, animals or other virtual creatures from films and video games. Inverse Kinematics (IK) has also been used in rehabilitation medicine in order to observe asymmetries or abnormalities. The most popular method for animating such models is motion-capture; however, despite the availability of highly sophisticated techniques and expensive tools, many problems appear when dealing with complex figures. Most virtual character models are very complex; they are made up of many joints giving a system with a large number of degrees of freedom, thus, it is often difficult to produce a realistic character animation.

Inverse Kinematics is a method for computing the posture via estimating each individual degree of freedom in order to satisfy a given task that meets user constraints; it plays an important role in the computer animation and simulation of articulated figures. This paper presents a new heuristic iterative method, Forward And Backward Reaching Inverse Kinematics (FABRIK), for solving the IK problem in different scenarios. FABRIK uses a forward and backward iterative approach, finding each joint position via locating a point on line. FABRIK has been utilised in highly complex systems with single and multiple targets, with and without joint restrictions. It can easily handle end effector orientations and support, to the best of our knowledge, all chain classes. A reliable method for incorporating constraints is also presented and utilised within FABRIK. The proposed method retains all the advantages of FABRIK, producing visually smooth movements without oscillations and discontinuities. Several experiments have been implemented for comparison purposes between the most popular manipulator solvers, including multiple end effectors with multiple tasks, and highly constrained joints. The proposed algorithm is very efficient both in simple and complex problems resulting in similar or even better poses than highly sophisticated methods, requiring less processing time and fewer iterations to reach the target. Another important advantage of the proposed methodology is the simplicity of the algorithm, which enables easy configuration to any IK problem.