使用鼠标缩放二维多边形

我开发了一个基于Python的绘图程序Whyteboard( https://launchpad.net/whyteboard

我正在开发允许用户旋转和缩放他们绘制的多边形的功能。我的问题是:

我有一个问题:我需要弄清楚如何计算一个调整大小的“比例”来应用于多边形。例如,(在按住鼠标的情况下),用户将鼠标移离形状应该是一个“增长”动作,而将鼠标移向形状应该会使其收缩。

def rescale(self, x, y):
    """ 
    x and y are the current mouse positions. the center and "original" mouse 
    coords are calculated below
    """
    if not self.center:
        a = sum([x for x, y in self.points]) / len(self.points)
        b = sum([y for x, y in self.points]) / len(self.points)
        self.center = (a, b)
    if not self.orig_click:  # where the user first clicked on
        self.orig_click = (x, y)
    if not self.original_points:  # the points before applying any scaling
        self.original_points = list(self.points)


    orig_click = self.orig_click
    original_distance = math.sqrt((orig_click[0] - self.center[0]) ** 2 + (orig_click[1] - self.center[1]) ** 2)

    current_distance = (math.sqrt((x - self.center[0]) ** 2 + 
                       (y - self.center[1]) ** 2))
    self.scale_factor = current_distance / original_distance        

    for count, point in enumerate(self.original_points): 
        dist = (point[0] - self.center[0], point[1] - self.center[1]) 
        self.points[count] = (self.scale_factor * dist[0] + self.center[0], self.scale_factor * dist[1] + self.center[1]) 

目前,这段代码似乎可以快速地将多边形缩小到零,并且再多的鼠标移动也不会使它变大。有时它会做相反的事情,并迅速成长;但不要退缩。