The above answers are too complex and hog your CPU. Your question was not arbitrary rotation, but ‘Rotate Opencv Matrix by 90, 180, 270 degrees’.
UPDATE 30 JUN 2017:
This functionality is supported by OpenCV, but not documented: https://github.com/opencv/opencv/blob/master/modules/core/include/opencv2/core.hpp#L1041
void rotate(InputArray src, OutputArray dst, int rotateCode);
with
enum RotateFlags {
ROTATE_90_CLOCKWISE = 0, //Rotate 90 degrees clockwise
ROTATE_180 = 1, //Rotate 180 degrees clockwise
ROTATE_90_COUNTERCLOCKWISE = 2, //Rotate 270 degrees clockwise
};
Original Answer & Arbitrary degree rotation:
You can also do this by using flip and transpose operation, ie for 90CW:
transpose(matSRC, matROT);
flip(matROT, matROT,1); //transpose+flip(1)=CW
etc. Figure out the other commands yourself (thinking=learning) by introducing yourself with the transpose and flip operation form the Docs.
void rot90(cv::Mat &matImage, int rotflag){
//1=CW, 2=CCW, 3=180
if (rotflag == 1){
transpose(matImage, matImage);
flip(matImage, matImage,1); //transpose+flip(1)=CW
} else if (rotflag == 2) {
transpose(matImage, matImage);
flip(matImage, matImage,0); //transpose+flip(0)=CCW
} else if (rotflag ==3){
flip(matImage, matImage,-1); //flip(-1)=180
} else if (rotflag != 0){ //if not 0,1,2,3:
cout << "Unknown rotation flag(" << rotflag << ")" << endl;
}
}
So you call it like this, and note the matrix is passed by reference.
cv::Mat matImage;
//Load in sensible data
rot90(matImage,3); //Rotate it
//Note if you want to keep an original unrotated version of
// your matrix as well, just do this
cv::Mat matImage;
//Load in sensible data
cv::Mat matRotated = matImage.clone();
rot90(matImage,3); //Rotate it
Rotate by arbitrary degrees
While I’m at it, here is how to rotate by an arbitrary degree, which i expect to be 50x more expensive. Note that rotation in this manner will include black padding, and edges will be rotated to oustide of the image’s original size.
void rotate(cv::Mat& src, double angle, cv::Mat& dst){
cv::Point2f ptCp(src.cols*0.5, src.rows*0.5);
cv::Mat M = cv::getRotationMatrix2D(ptCp, angle, 1.0);
cv::warpAffine(src, dst, M, src.size(), cv::INTER_CUBIC); //Nearest is too rough,
}
Calling this for a rotation of 10.5 degrees then is obviously:
cv::Mat matImage, matRotated;
//Load in data
rotate(matImage, 10.5, matRotated);
I find it remarkable that these kind of extremely basic functions are not part of OpenCV, while OpenCV does have native things like face detection (that’s not really maintained with questionable performance). Remarkable.
Cheers