def canvas_rect(dw, dh):
        """Resizes a canvas dimensions so that it better fits on client browser.""" 
        ar = dw / dh 
        h = 400 if ar > 3 else 500
        w_min = 300
        w_max = 1000
        w = int(ar * h)
        if w > w_max: w = w_max
        if w < w_min: w = w_min
        return w, h