stretch	Rfree/a
F11d	1+d(u,X)
F12d	d(u,Y)
F13d	d(u,Z)
F21d	d(v,X)
F22d	1+d(v,Y)
F23d	d(v,Z)
F31d	d(w,X)
F32d	d(w,Y)
F33d	1+d(w,Z)
C11d	F11d*F11d+F21d*F21d+F31d*F31d
C12d	F11d*F12d+F21d*F22d+F31d*F32d
C13d	F11d*F13d+F21d*F23d+F31d*F33d
C22d	F12d*F12d+F22d*F22d+F32d*F32d
C23d	F12d*F13d+F22d*F23d+F32d*F33d
C33d	F13d*F13d+F23d*F23d+F33d*F33d
qd	(C11d+C22d+C33d)/3
pd	sqrt(max(1e-24,((C11d-qd)^2+(C22d-qd)^2+(C33d-qd)^2+2*(C12d^2+C13d^2+C23d^2))/6))
rd	min(1,max(-1,((C11d-qd)*(C22d-qd)*(C33d-qd)+2*C12d*C13d*C23d-(C11d-qd)*C23d^2-(C22d-qd)*C13d^2-(C33d-qd)*C12d^2)/(2*pd^3)))
stretchMax	sqrt(max(0,qd+2*pd*cos(acos(rd)/3)))