  We extend the approach of [Smith et al. 2019] to derive analytical
expressions for the eigenvalues and eigenmatrices of an isotropic membrane
energy density function $\psi : \mathbb{R}^{3x2} \to \mathbb{R}$. Clamping the
eigenvalue expressions to be positive for each quadrature point of a finite
element membrane simulation guarantees a positive semi-definite Hessian for the
full discrete membrane energy, enabling an efficient Newton-type simulation.
