Curve skeleton skinning for human and creature characters

Abstract

The skeleton driven skinning technique is still the most popular method for animating deformable human and creature characters. Albeit an industry de facto due to its computational performance and intuitiveness, it suffers from problems like collapsing elbow and candy wrapper joint. To remedy these problems, one needs to formulate the non-linear relationship between the skeleton and the skin shape of a character properly, which however proves mathematically very challenging. Placing additional joints where the skin bends increases the sampling rate and is an ad hoc way of approximating this non-linear relationship. In this paper, we propose a method that is able to accommodate the inherent non-linear relationships between the movement of the skeleton and the skin shape. We use the so-called curve skeletons along with the joint-based skeletons to animate the skin shape. Since the deformation follows the tangent of the curve skeleton and also due to higher sampling rates received from the curve points, collapsing skin and other undesirable skin deformation problems are avoided. The curve skeleton retains the advantages of the current skeleton driven skinning. It is easy to use and allows full control over the animation process. As a further enhancement, it is also fairly simple to build realistic muscle and fat bulge effect. A practical implementation in the form of a Maya plug-in is created to demonstrate the viability of the technique.

Differential blending for expressive sketch-based posing

“…Although invented in the 80s, skeleton-based skinning [Magnenat-Thalmann et al 1988] remains the most popular method for animating articulated characters in games and films, due to its simplicity, efficiency, and intuitiveness. However, skinning methods have many problems, and over the years a large set of techniques have been developed to address various shortcomings of the original method [Alexa 2002;Kavan anď Zára 2005;Yang et al 2006;Forstmann and Ohya 2006;Forstmann et al 2007;Kavan et al 2008;Jacobson and Sorkine 2011] ranging from alleviating deformation artifacts such as collapsing elbows and candy-wrapper joints to adding robust support for more complex deformations such as bone twisting and bone stretching.…”

“…more artistic control, more bones and joints can be added to the skeleton, but this results in additional effort and increased complexity. A solution to this limitation is a skeleton with stretchable/twistable [Jacobson and Sorkine 2011] or curved bones [Yang et al 2006;Forstmann and Ohya 2006;Forstmann et al 2007]. Even though curved bones can capture the expressiveness desired by the artist, the current skinning methods that deform the model to conform with the new pose often fail in the presence of large rotations leading to visual artifacts as illustrated in Figure 1.…”

“…A new set of skinning weights needs to be computed for each added bone, making the skinning process tedious. A more viable alternative is using curved bone based skeletons [Yang et al 2006;Forstmann et al 2007]. In these structures, the bones represent a continuous set of transformations.…”

Skinning with dual quaternions

“…Some researchers propose combatting skinning artifacts by implementing a different rigging method, e.g., based on sweep surfaces [Hyun et al 2005] or auxiliary curved skeletons [Yang et al 2006;Forstmann and Ohya 2006]. In some cases, this also allows advanced effects to be animated, such as muscle bulging.…”

As-Rigid-As-Possible Character Deformation Using Point Handles

“…It is also possible to reduce the number of LBS artifacts by replacing rigid bones with other types of handles. Works such as [11,12] use curve skeletons to fix jointcollapse. Curved skeletons need new rigging controls that are inconsistent with the existing rigging pipeline [13].…”