“…These constraints offer the advantage of efficient processing, allowing for interactive manipulation of the free form geometry. Non-linear constraints that are commonly considered are the area [Elb01,HSB05], the volume [RSB95], second order differential constraints such as convexity [KS95,PE98], curve constraints [CW92, GL96, PGL + 02], and first and second order fairing functionals such as the bending energy of a thin plate [CG91,GC95,FRSW87,HS92,BH94,Hah98]. Satisfying non-linear constraints, however, requires intense computational effort, so that their use for interactive shape manipulation is generally very limited.…”