The only thing, transformPoint returns me coordinates like: (-0.68; 0.4), (-0.685, 0.477). I think they are in [-1;-1].
Yes, sorry I forgot that. You need to convert these normalized coordinates to viewport coordinates, so this is a linear conversion from (-1, -1, 1, 1) to (0, 0, width, height).
x = (x + 1) * width / 2;
y = (y + 1) * height / 2;
Maybe you'll also need to invert the Y axis (
y = height - y), I don't remember.
Sorry that you need this ugly hack to solve your problem.