We model these properties for a single hair by using a mass-spring
system. Similar to [Rosenblum et al. 1991; Petrovic et al. 2005;
Selle et al. 2008; McAdams et al. 2009], we sequentially connect
each hair particle with a linear spring, controlling stretch (Section
3.1). Our method differs from prior work with the formulations
of two additional springs. To control bend along the curl, we propose
a novel bending spring by computing frames along a smoothed
representation of the curve (Section 3.2). We also introduce a spring
limiting the longitudinal stretch of curls (Section 3.3). These three
springs comprise our per hair force model.