c++ - OpenCV Mat rotation gets wrong result -

i want rotate image 90 degrees. code following:

int main(int argc, const char * argv[]) {     mat img = imread("/users/chuanliu/desktop/src4/p00.jpg");      resize(img, img, size(1024, 683));     imwrite("/users/chuanliu/desktop/resize.jpg", img);     mat dst;     mat rot_mat = getrotationmatrix2d(point(img.cols / 2.0, img.rows / 2.0), 90, 1);     warpaffine(img, dst, rot_mat, size(img.rows, img.cols));      imwrite("/users/chuanliu/desktop/roatation.jpg", dst);      return 0; } 

but result following:
before rotation:

after rotation:

it seems center of rotation has sth wrong. don't think set wrong center. there can tell me wrong?

the centre specified in terms of source image's dimensions point(img.cols / 2.0, img.rows / 2.0) not rotating image swapping width , height in output size when calling warpaffine:

size(img.rows, img.cols) 

so looks might need specify centre in terms of output image coordinates; eg. point(rows/2, cols/2).

update:

nope, that's not solution. there simple , efficient method rotating image 90 degrees: using cv::transpose() function:

int main() {     cv::mat img = cv::imread("5syfi.jpg");     cv::mat img_rotated;      cv::transpose(img, img_rotated);        cv::imwrite("out.jpg", img_rotated);      return 0; } 

using combination of cv::transpose() (to rotate) , cv::flip() (to mirror vertically , horizontally) can perform rotations 90, 180 , 270 degrees.

using warpaffine() more flexible, more expensive (ie. slower) calculate. if need rotate multiple of 90deg, use cv::transpose. if need rotate arbitrary angle, use warpaffine/warpperspective functions. @micka's answer gives great example of how that.