Rotate area to align the major axis horizontally with opencv - c++

Question

Please can someone here who can help me with this. I'm trying to rotate a segmented region of an image to align the major axis horizontally.
I have a segmented region in the center of the image following the steps used herein. Move area of an image to the center using OpenCV
I read this OPENCV: PCA application error in image_proc, but it does not help me solve my problem.
I have this
I want this

Answer 1

Slightly different than how Miki suggested, I used findNonZero, minAreaRect, and WarpAffine.
You can either use 270 or 90 on the getRotationMatrix2D to align the major axis with the horizontal.
#include "stdafx.h"
#include <opencv/cxcore.h>
#include <opencv2\core\mat.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <iostream>
#include <opencv/cxcore.h>
#include <opencv/highgui.h>
#include <opencv/cv.h>
#include <opencv2/opencv.hpp>
#include <opencv2/core/core.hpp>
#include <opencv2/videoio/videoio.hpp>
using namespace cv;
using namespace std;
int main() {
//getting the image
Mat image = imread("C:/this/is/a/path/to/an/image.png");
//create new image that looks exactly like old image
Mat rot_image = image.clone();
rot_image = Scalar(0);
//showing the image
namedWindow("Image", CV_WINDOW_NORMAL| CV_WINDOW_KEEPRATIO | CV_GUI_EXPANDED);
namedWindow("Rotated Image", CV_WINDOW_NORMAL| CV_WINDOW_KEEPRATIO | CV_GUI_EXPANDED);
imshow("Image", image);
waitKey(0);
imshow("Rotated Image", rot_image);
waitKey(0);
//convert image
Mat img_bw;
inRange(image, Scalar(1,1,1), Scalar(255,255,255), img_bw);
imshow("Rotated Image", img_bw);
waitKey(0);
//find coordinates
Mat nonZeroCoordinates;
findNonZero(img_bw, nonZeroCoordinates);
RotatedRect rect = minAreaRect(nonZeroCoordinates);
rect.center = Point(image.cols/2, image.rows/2);
//get the Rotation Matrix
Mat M = getRotationMatrix2D(rect.center, 270, 1.0);
// perform the affine transformation
warpAffine(image, rot_image, M, image.size(), INTER_CUBIC);
//displaying the image
imshow("Rotated Image", rot_image);
waitKey(0);
//saving the new image
imwrite("C:/this/is/a/path/to/a/rotatedImage.png", rot_image);
}
That code turns this:
to this:
You can take the rect.center line out if you're sure your object is already going to be in the center.