功能是计算线段与坐标轴的夹角，与中学时学的一样，计算线段AB返回结果是那个α，单位为弧度。

;根据两点坐标返回线段与坐标轴的夹角(弧度)
;Point1到point2线段（线段AB）
;
Function Cal2PointsAngle ,point1,point2

    dx = point2[0]- point1[0]
    dy = point2[1]- point1[1]

    IF dx EQ 0 THEN BEGIN ; x轴
        IF dy GT 0 THEN result = !PI/2 ELSE $
        result = 3*!PI/2

    ENDIF ELSE BEGIN
        IF dy EQ 0 THEN BEGIN ;x轴
            IF dx GT 0 THEN result = 0 ELSE $
            result = !PI
        ENDIF ElSE BEGIN
            result = ATAN(dy/dx)
            IF result GT 0 THEN IF dx LT 0 THEN result = result +!Pi
            IF result LT 0 THEN IF dx LT 0 THEN result = result +!Pi ELSE $
            result = result + 2*!PI
        ENDELSE
    END

    return,result MOD (2*!PI)
END