I am modeling fracture propagation using a 2-D dynamic unstructured grid. As the fracture propagates over time, the elements move accordingly. For a given time step, I would like to interpolate the data of my unstructured grid $z_i=f(x_i,y_i)$ (where $(x_i,y_i)$ are the nodes of the unstructured triangles) to obtain the value of the function $z$ on the Cartesian grid (where $x$ and $y$ regularly spaced). My grid size is about $100 \times 100$ nodes. There are many ways of doing this interpolation scheme but I do not know enough about the topic to decide which scheme is robust and simple enough. I am using Fortran 90 (do not ask why...). Is there any open-source algorithm available? I do not want to reinvent the wheel!