@Obliv So, what you're trying to say is that $E,L\subset\mathbb{R}^2$ and you have a function $t : \mathbb{R}^2\to\mathbb{R}^2, (x,y)\mapsto (u(x,y),v(x,y))$ such that $t(E) = L$? I don't know what "f transforms into a function T(u(x,y),v(x,y))" is supposed to mean, though.