Inverse Map for Bilinear Elements

Sid

Hello all,

I want to compute (\xi,\eta) for given (x,y). I know there is a simple map from the former to the latter: the famous bilinear map. But can I do the inverse for general quadrilateral? How is this usually done?

( I need to interpolate some function value within a quadrilateral using bilinear interpolation. So, I want to compute a function value at some point (x0,y0) in the PHYSICAL plane. I don't know where this point is mapped in the \xi-\eta plane..... So I cannot use N1=(1-\xi)*(1-\eta), etc)

Thanks