直线段图元类，其基本属性有直线段的ID号，颜色，线型，线宽，起点和终点等，这些属性是可读可写的。为实现一定的绘图目的，可能还需要知道直线段的中点坐标，长度和方位角等。还能实现平移、旋转等操作-Pixel line segment, whose basic properties are straight-line segment ID number, color, line, line width, starting point and destination and so on, these attributes can be written is readable. Graphics to achieve a certain purpose, you may also need to know the coordinates of the midpoint of line segments, such as the length and azimuth. Can achieve translation, rotation and other operations