# Solve poisson equation with Neumann b.c. (matlab or octave)

I'm trying to reconstruct an image given its Laplacian, which results in a Poisson equation and I'm using Neumann boundary conditions (derivative at boundary = 0).

What I have is the laplacian (f, left-hand side), and the image borders (boundaries).

How can I solve this in Matlab or Octave? I found the poisolv() and assempde() functions but I don't quite understand how they work.