OpenCV : Determine orientation of ellipse/RotatedRect in FitEllipse()

In my other post, I examined the RotatedRect of minAreaRect() function. But later i found out that the ellipse/RotatedRect of minFitEllipse() function doesn’t work the same way as that of minAreaRect(). (Looks like the two developers didn’t talk to each other before developing these functions !)

So, i again examined the RotatedRect extraacted from ellipse of the minFitEllipse() function and here are the results:

1. The angle property is the angle between vertical and the edge of 0th and 1st vertices of the rectangle.
2. If the angle is less than 90 degrees the 0th vertex if the left most (or left-bottom if angle is 0 degrees) vertex. 1st, 2nd, 3rd vertices follow in clockwise manner.
3. If the angle is greater than or equal to 90 degrees (but < 180), 0th vertex is the top most vertex (or top left vertex if angle == 90). 1st, 2nd, 3rd vertices follow in clockwise manner.
4. Height is always greater than (or equal to) Width.

 

This is a code snippet i used to get and draw RotatedRect of fitellipse() function.

RotatedRect box = fitEllipse(contour);
Point2f vtx[4];
box.points(vtx);
for( int j = 0; j < 4; j++ )
    line(drawing, vtx[j], vtx[(j+1)%4], Scalar(0,255,0),2);

 

Below are test cases for various test objects (in white). Green rectangle is the RotatedRect of fitEllipse() function.

Cases :  0 <= Angle < 90

 

Cases :  90 <=Angle < 180