实现图形学中画直线，园和圆弧以及直线裁剪，区域填充等功能 1．　　对于画直线，只需知道起点和终点即可。在工作区间中按下鼠标（即选择起点），拖动至另一处松开（终点），即得到直线。 2．　　画圆需要圆心坐标、半径、及起始角和终止角，至于实验要求的画1/4圆弧，可设起始角=0，终止角=45，若要画整个圆弧起始角和终止角分别为0和360即可。 3．　　椭圆需要的参数更多，有椭圆中心坐标、长轴、短轴、长轴和X轴正向夹角、及起始离心角和终止离心角，要画整个椭圆还是部分椭圆只需控制起始和终止离心角。与画圆弧类似，具体说明源程序中也有。 4．　　直线裁减实现的功能是：给定任一条直线（由两点控制），对任一矩形，裁减直线落在矩形内部的一段，但是本程序并没有真正把该部分裁掉，只是用红色把它显示出来，也就是说，红色部分就是被矩形裁减掉的部分。-The realization of graphics drawing a straight line, park and circular arc and straight line cutting, filled with features such as region 1 to draw a straight line, only need to know the starting point and end point can be. Press the mouse in the working interval (i.e. choosing starting point), drag to another release (the end), that is to be a straight line. The 2 circle center coordinates and radius, need, start angle and end angle, as far as the experimental requirements of painting 1/4 arc, can set the starting angle end angle =0, =45, to draw the entire arc starting angle and termination angle are respectively 0 and 360 can be. More parameters need 3 elliptic, with ellipse center coordinates, long axis, short axis, long axis and the positive X axis angle, and the initial eccentric angle and termination of centrifugal angle, to draw the whole or part of the ellipse ellipse only need to control the initiation and termination of eccentric angle. Similar to an arc, a specific descr ipt