opencv中RotatedRect的angle这个成员变量总是诡异的不同寻常（http://stackoverflow.com/questions/15956124/minarearect-angles-unsure-about-the-angle-returned），官网也没有任何解释。
下面是获取RotatedRect的绝对角度（0-180）的函数：
static double calcLineDegree(const Point2f& firstPt, const Point2f& secondPt)
{
double curLineAngle = 0.0f;
if (secondPt.x - firstPt.x != )
{
curLineAngle = atan(static_cast<double>(firstPt.y - secondPt.y) / static_cast<double>(secondPt.x - firstPt.x));
if (curLineAngle < )
{
curLineAngle += CV_PI;
}
}
else
{
curLineAngle = CV_PI / 2.0f; //90度
}
return curLineAngle*180.0f/CV_PI;
}
static double getRcDegree(const RotatedRect box)
{
double degree = 0.0f;
Point2f vertVect[];
box.points(vertVect);
//line 1
const double firstLineLen = (vertVect[].x - vertVect[].x)*(vertVect[].x - vertVect[].x) +
(vertVect[].y - vertVect[].y)*(vertVect[].y - vertVect[].y);
//line 2
const double secondLineLen = (vertVect[].x - vertVect[].x)*(vertVect[].x - vertVect[].x) +
(vertVect[].y - vertVect[].y)*(vertVect[].y - vertVect[].y);
if (firstLineLen > secondLineLen)
{
degree = calcLineDegree(vertVect[], vertVect[]);
}
else
{
degree = calcLineDegree(vertVect[], vertVect[]);
}
return degree;
}