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since you already know the angle, you could easily use getRotationMatrix2D:

double angle = 31.0;
double scale = 1.0;
Point2f center(img.cols/2, img.rows/2);
Mat M = getRotationMatrix2D(center, angle, scale);

if you want to derive it from (at least 3) pairs of Points via getAffineTransform, use vector<Point2f>, which can be casted to a cv::Mat:

vector<Point2f> a,b;
a.push_back(keypoints_a[i].pt); // extract the point from the keypoints;
b.push_back(keypoints_b[j].pt);  
...
Mat M = getAffineTransform(Mat(a), Mat(b));

finally, rotate it:

Mat rotated;
warpAffine(img, rotated, M, Size());

good luck !

since you already know the angle, you could easily use getRotationMatrix2D:

double angle = 31.0;
double scale = 1.0;
Point2f center(img.cols/2, img.rows/2);
Mat M = getRotationMatrix2D(center, angle, scale);

if you want to derive it from (at least 3) pairs of Points via getAffineTransform, use vector<Point2f>, which can be casted to a cv::Mat:

vector<Point2f> a,b;
a.push_back(keypoints_a[i].pt); // extract the point from the keypoints;
b.push_back(keypoints_b[j].pt);  // you will need **exactly** 3 points in a and 3 points in b
...
Mat M = getAffineTransform(Mat(a), Mat(b));

finally, rotate it:

Mat rotated;
warpAffine(img, rotated, M, Size());

good luck !

since you already know the angle, you could easily use getRotationMatrix2D:

double angle = 31.0;
double scale = 1.0;
Point2f center(img.cols/2, img.rows/2);
Mat M = getRotationMatrix2D(center, angle, scale);

if you want to derive it from (at least (exactly 3) pairs of Points via getAffineTransform, use vector<Point2f>, which can be casted to a cv::Mat:

vector<Point2f> a,b;
a.push_back(keypoints_a[i].pt); // extract the point from the keypoints;
b.push_back(keypoints_b[j].pt); // you will need **exactly** 3 points in a and 3 points in b
...
Mat M = getAffineTransform(Mat(a), Mat(b));

finally, rotate it:

Mat rotated;
warpAffine(img, rotated, M, Size());

good luck !