Inverse Kinematics – Cyclic Coordinate Descent (CCD)

Abstract

This article examines the popular inverse kinematic (IK) method known as cyclic coordinate descent (CCD) and its viability for creating and controlling highly articulated characters (e.g., humans and insects). The reason CCD is so popular is that it is a computationally fast, algorithmically simple, and straight-forward technique for generating IK solutions that can run at interactive frame rates. Whereas it can be relatively clear-cut to construct an IK system using CCD, we address a number of engineering solutions necessary to make the CCD technique a viable and practical method for character-based environments, such as games. We discuss implementation details, limitations (e.g., angle limits, performance tips, convergence problems, oscillation issues, and comfort factors), and their applicability to articulated configurations. Whereas a plain implementation may focus only on a single-linked chained IK problem and disregard multiple connected hierarchical goals (e.g., articulated characters), we examine both cases. We also examine why naïve constructions of the CCD algorithm can be incorrect even, though they converge on a solution. Furthermore, we discuss how the CCD algorithm can be fine-tuned to produce more natural lifelike character poses that can be used to generate realistic motions. Hence, after reading this article, the reader should have the knowledge to design and create an effective and flexible CCD implementation for real-time environments, such as games, while understanding and appreciating the limitations and hazards in a practical situation.