We
represent a single hair by a piece wise linear curve and calculate a
material frame for each point. Our force model must satisfy three
properties to create our desired visual looks. First, we desire relatively
inextensible hair rods, allowing artistic control over stretch.
Second, we want a hair model that holds an arbitrary curl’s shape
while allowing it to flex with motion. Last, we want our curls to
reasonably maintain their initial shape during motion, avoiding curl
unwinding.