Rotate an image without cropping in OpenCV in C++

c++, opencv

Solution

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;
}

Problem

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:

Original source

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