Rotate an image without cropping in OpenCV in C++

Just try the code below, the idea is simple:

1. You need to create a blank image with the maximum size you're expecting while rotating at any angle. Here you should use Pythagoras as mentioned in the above comments.

2. Now copy the source image to the newly created image and pass it to warpAffine. Here you should use the centre of newly created image for rotation.

3. After warpAffine if you need to crop exact image for this translate four corners of source image in enlarged image using rotation matrix as described here

4. Find minimum x and minimum y for top corner, and maximum x and maximum y for bottom corner from the above result to crop image.

This is the code:

int theta = 0;
Mat src,frame, frameRotated;
src = imread("rotate.png",1);
cout<<endl<<endl<<"Press '+' to rotate anti-clockwise and '-' for clockwise 's' to save" <<endl<<endl;

int diagonal = (int)sqrt(src.cols*src.cols+src.rows*src.rows);
int newWidth = diagonal;
int newHeight =diagonal;

int offsetX = (newWidth - src.cols) / 2;
int offsetY = (newHeight - src.rows) / 2;
Mat targetMat(newWidth, newHeight, src.type());
Point2f src_center(targetMat.cols/2.0F, targetMat.rows/2.0F);


while(1){
src.copyTo(frame);
double radians = theta * M_PI / 180.0;
double sin = abs(std::sin(radians));
double cos = abs(std::cos(radians));

frame.copyTo(targetMat.rowRange(offsetY, offsetY + frame.rows).colRange(offsetX, offsetX + frame.cols));
Mat rot_mat = getRotationMatrix2D(src_center, theta, 1.0);
warpAffine(targetMat, frameRotated, rot_mat, targetMat.size());
 //Calculate bounding rect and for exact image
 //Reference:- https://stackoverflow.com/questions/19830477/find-the-bounding-rectangle-of-rotated-rectangle/19830964?noredirect=1#19830964
    Rect bound_Rect(frame.cols,frame.rows,0,0);

    int x1 = offsetX;
    int x2 = offsetX+frame.cols;
    int x3 = offsetX;
    int x4 = offsetX+frame.cols;

    int y1 = offsetY;
    int y2 = offsetY;
    int y3 = offsetY+frame.rows;
    int y4 = offsetY+frame.rows;

    Mat co_Ordinate = (Mat_<double>(3,4) << x1, x2, x3, x4,
                                            y1, y2, y3, y4,
                                            1,  1,  1,  1 );
    Mat RotCo_Ordinate = rot_mat * co_Ordinate;

    for(int i=0;i<4;i++){
       if(RotCo_Ordinate.at<double>(0,i)<bound_Rect.x)
         bound_Rect.x=(int)RotCo_Ordinate.at<double>(0,i); //access smallest 
       if(RotCo_Ordinate.at<double>(1,i)<bound_Rect.y)
        bound_Rect.y=RotCo_Ordinate.at<double>(1,i); //access smallest y
     }

     for(int i=0;i<4;i++){
       if(RotCo_Ordinate.at<double>(0,i)>bound_Rect.width)
         bound_Rect.width=(int)RotCo_Ordinate.at<double>(0,i); //access largest x
       if(RotCo_Ordinate.at<double>(1,i)>bound_Rect.height)
        bound_Rect.height=RotCo_Ordinate.at<double>(1,i); //access largest y
     }

    bound_Rect.width=bound_Rect.width-bound_Rect.x;
    bound_Rect.height=bound_Rect.height-bound_Rect.y;

    Mat cropedResult;
    Mat ROI = frameRotated(bound_Rect);
    ROI.copyTo(cropedResult);

    imshow("Result", cropedResult);
    imshow("frame", frame);
    imshow("rotated frame", frameRotated);
    char k=waitKey();
    if(k=='+') theta+=10;
    if(k=='-') theta-=10;
    if(k=='s') imwrite("rotated.jpg",cropedResult);
    if(k==27) break;

}

Cropped Image

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;
}

Thanks Robula! Actually, you do not need to compute sine and cosine twice.

import cv2

def rotate_image(mat, angle):
  # angle in degrees

  height, width = mat.shape[:2]
  image_center = (width/2, height/2)

  rotation_mat = cv2.getRotationMatrix2D(image_center, angle, 1.)

  abs_cos = abs(rotation_mat[0,0])
  abs_sin = abs(rotation_mat[0,1])

  bound_w = int(height * abs_sin + width * abs_cos)
  bound_h = int(height * abs_cos + width * abs_sin)

  rotation_mat[0, 2] += bound_w/2 - image_center[0]
  rotation_mat[1, 2] += bound_h/2 - image_center[1]

  rotated_mat = cv2.warpAffine(mat, rotation_mat, (bound_w, bound_h))
  return rotated_mat