Rotate an image without cropping in OpenCV in C++

Question

I'd like to rotate an image, but I can't obtain the rotated image without cropping

My original image:

Now I use this code:

#include <opencv2/core/core.hpp> #include <opencv2/highgui/highgui.hpp> #include <opencv2/imgproc/imgproc.hpp>  // Compile with g++ code.cpp -lopencv_core -lopencv_highgui -lopencv_imgproc  int main() {     cv::Mat src = cv::imread("im.png", CV_LOAD_IMAGE_UNCHANGED);     cv::Mat dst;      cv::Point2f pc(src.cols/2., src.rows/2.);     cv::Mat r = cv::getRotationMatrix2D(pc, -45, 1.0);      cv::warpAffine(src, dst, r, src.size()); // what size I should use?      cv::imwrite("rotated_im.png", dst);      return 0; } 

And obtain the following image:

But I'd like to obtain this:

Answer 1

My answer is inspired by the following posts / blog entries:

• Rotate cv::Mat using cv::warpAffine offsets destination image
• http://john.freml.in/opencv-rotation

Main ideas:

• Adjusting the rotation matrix by adding a translation to the new image center
• Using cv::RotatedRect to rely on existing opencv functionality as much as possible

Code tested with opencv 3.4.1:

#include "opencv2/opencv.hpp"  int main() {     cv::Mat src = cv::imread("im.png", CV_LOAD_IMAGE_UNCHANGED);     double angle = -45;      // get rotation matrix for rotating the image around its center in pixel coordinates     cv::Point2f center((src.cols-1)/2.0, (src.rows-1)/2.0);     cv::Mat rot = cv::getRotationMatrix2D(center, angle, 1.0);     // determine bounding rectangle, center not relevant     cv::Rect2f bbox = cv::RotatedRect(cv::Point2f(), src.size(), angle).boundingRect2f();     // adjust transformation matrix     rot.at<double>(0,2) += bbox.width/2.0 - src.cols/2.0;     rot.at<double>(1,2) += bbox.height/2.0 - src.rows/2.0;      cv::Mat dst;     cv::warpAffine(src, dst, rot, bbox.size());     cv::imwrite("rotated_im.png", dst);      return 0; }