看ObjectARX开发帮助遇到问题，谁来解释一下

看ObjectARX开发帮助遇到问题，谁来解释一下
在这个函数Acad::ErrorStatus getBulgeAt(unsigned int index,double& bulge) 中bulge是什么意思？帮助中的解释是这样的：The bulge factor is used to indicate how much of an arc segment is present at this vertex. The bulge factor is the tangent of one fourth the included angle for an arc segment, made negative if the arc goes clockwise from the start point to the endpoint.看不懂。The bulge factor is the tangent of one fourth the included angle for an arc segment,这句怎么理解？请大家指教。