void zoom(float delta, float mouse_x, float mouse_y)
{
    zoom += delta;
    view.w = window.w/zoom;
    view.h = window.h/zoom;
    // view.x = ???
    // view.y = ???
}

view.x = mouse_x - view.w/2;
view.y = mouse_y - view.h/2;

Vect2f mouse_true(mouse_position.x/zoom + view.x, mouse_position.y/zoom + view.y);
Vect2f mouse_relative(window_size.x/mouse_pos.x, window_size.y/mouse_pos.y);
view.x = mouse_true.x - view.w/mouse_relative.x;
view.y = mouse_true.y - view.h/mouse_relative.y;

var a = (mouse.x + camera.x) / zoom;

// now increase the zoom e.g.: like that:
zoom = zoom + 1;

var newPosition = a * zoom - mouse.x;

camera.setX(newPosition);
screen.setWidth(originalWidth * zoom);