Stretchable and Twistable Bones for Skeletal Shape Deformation

晃晃

Stretchable and Twistable Bones for Skeletal Shape Deformation

Alec Jacobson Olga Sorkine

New York University & ETH Zurich







Abstract

Skeleton-based linear blend skinning (LBS) remains the most popu-

lar method for real-time character deformation and animation. The

key to its success is its simple implementation and fast***cution.

However, in addition to the well-studied elbow-collapse and candy-

wrapper artifacts, the space of deformations possible with LBS is

inherently limited. In particular, blending with only a scalar weight

function per bone prohibits properly handling stretching, where

bones change length, and twisting, where the shape rotates along

the length of the bone. We present a simple modification of the LBS

formulation that enables stretching and twisting without changing

the existing skeleton rig or bone weights. Our method needs only an

extra scalar weight function per bone, which can be painted man-

ually or computed automatically. The resulting formulation sig-

nificantly enriches the space of possible deformations while only

increasing storage and computation costs by constant factors.

CR Categories: I.3.7 [Computer Graphics]: Three-Dimensional

Graphics and Realism—Animation

Keywords: shape deformation, articulated character animation,

linear blend skinning

1 Introduction

Skinning and skeletal deformation remain standard for character

animation because the associated deformation metaphor is directly

intuitive for many situations: most characters are creatures or hu-

mans who ought to behave as if a skeleton was moving underneath

their skin. The motion capture pipeline, for example, explicitly

relies on this metaphor to build a subspace representation of hu-

man motion [Anguelov et al. 2005]. At the cost of performance,

some applications demand physical accuracy, ensuring preservation

of volume or simulating muscles [Teran et al. 2005]. Other applica-

tions, such as video games, crowd simulation and interactive anima-

tion editing, cannot afford to compromise real-time performance, so

they trade accuracy for speed and often adopt the simplest and most

efficient implementation of skeletal deformation.

The long-standing standard real-time skeletal deformation method

is linear blend skinning (LBS), also known as skeletal subspace de-

formation or enveloping [Magnenat-Thalmann et al. 1988; Lewis

et al. 2000]. In a typical workflow, a trained rigging artist manually

cons***cts and fits a skeleton of rigid bones within the target shape.

The skeleton is bound to the shape by assigning a set of correspon-

dence weights for each bone, a process which can be tedious and

labor-intensive. To deform the shape, animators assign transforma-

tions to each skeleton bone, either directly or with the assistance of

an inverse kinematics engine or motion capture data. These trans-

formations are propagated to the shape by blending them linearly

as matrix operations according to the bone weights.









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