How to store all the pixels within a RotatedRect to another matrix?

Question

I am going to process a region of pixels defined by RotatedRect in OpenCV. Although I know the rectangle center, size, and angle, I am not sure how to store all the x and y of this region to another matrix. I have checked some other posts, some suggest to rotate the image, but this will crop part of the image. Can you please help me out?

Answer 1

Try this (not sure I undestand the problem perfectly):

#include "opencv2/opencv.hpp"
#include <vector>
using namespace std;
using namespace cv;
//----------------------------------------------------------
//
//----------------------------------------------------------
void getQuadrangleSubPix_8u32f_CnR( const uchar* src, size_t src_step, Size src_size,
                                   float* dst, size_t dst_step, Size win_size,
                                   const double *matrix, int cn )
{
    int x, y, k;
    double A11 = matrix[0], A12 = matrix[1], A13 = matrix[2];
    double A21 = matrix[3], A22 = matrix[4], A23 = matrix[5];

    src_step /= sizeof(src[0]);
    dst_step /= sizeof(dst[0]);

    for( y = 0; y < win_size.height; y++, dst += dst_step )
    {
        double xs = A12*y + A13;
        double ys = A22*y + A23;
        double xe = A11*(win_size.width-1) + A12*y + A13;
        double ye = A21*(win_size.width-1) + A22*y + A23;

        if( (unsigned)(cvFloor(xs)-1) < (unsigned)(src_size.width - 3) &&
            (unsigned)(cvFloor(ys)-1) < (unsigned)(src_size.height - 3) &&
            (unsigned)(cvFloor(xe)-1) < (unsigned)(src_size.width - 3) &&
            (unsigned)(cvFloor(ye)-1) < (unsigned)(src_size.height - 3))
        {
            for( x = 0; x < win_size.width; x++ )
            {
                int ixs = cvFloor( xs );
                int iys = cvFloor( ys );
                const uchar *ptr = src + src_step*iys;
                float a = (float)(xs - ixs), b = (float)(ys - iys), a1 = 1.f - a, b1 = 1.f - b;
                float w00 = a1*b1, w01 = a*b1, w10 = a1*b, w11 = a*b;
                xs += A11;
                ys += A21;

                if( cn == 1 )
                {
                    ptr += ixs;
                    dst[x] = ptr[0]*w00 + ptr[1]*w01 + ptr[src_step]*w10 + ptr[src_step+1]*w11;
                }
                else if( cn == 3 )
                {
                    ptr += ixs*3;
                    float t0 = ptr[0]*w00 + ptr[3]*w01 + ptr[src_step]*w10 + ptr[src_step+3]*w11;
                    float t1 = ptr[1]*w00 + ptr[4]*w01 + ptr[src_step+1]*w10 + ptr[src_step+4]*w11;
                    float t2 = ptr[2]*w00 + ptr[5]*w01 + ptr[src_step+2]*w10 + ptr[src_step+5]*w11;

                    dst[x*3] = t0;
                    dst[x*3+1] = t1;
                    dst[x*3+2] = t2;
                }
                else
                {
                    ptr += ixs*cn;
                    for( k = 0; k < cn; k++ )
                        dst[x*cn+k] = ptr[k]*w00 + ptr[k+cn]*w01 +
                        ptr[src_step+k]*w10 + ptr[src_step+k+cn]*w11;
                }
            }
        }
        else
        {
            for( x = 0; x < win_size.width; x++ )
            {
                int ixs = cvFloor( xs ), iys = cvFloor( ys );
                float a = (float)(xs - ixs), b = (float)(ys - iys), a1 = 1.f - a, b1 = 1.f - b;
                float w00 = a1*b1, w01 = a*b1, w10 = a1*b, w11 = a*b;
                const uchar *ptr0, *ptr1;
                xs += A11; ys += A21;

                if( (unsigned)iys < (unsigned)(src_size.height-1) )
                    ptr0 = src + src_step*iys, ptr1 = ptr0 + src_step;
                else
                    ptr0 = ptr1 = src + (iys < 0 ? 0 : src_size.height-1)*src_step;

                if( (unsigned)ixs < (unsigned)(src_size.width-1) )
                {
                    ptr0 += ixs*cn; ptr1 += ixs*cn;
                    for( k = 0; k < cn; k++ )
                        dst[x*cn + k] = ptr0[k]*w00 + ptr0[k+cn]*w01 + ptr1[k]*w10 + ptr1[k+cn]*w11;
                }
                else
                {
                    ixs = ixs < 0 ? 0 : src_size.width - 1;
                    ptr0 += ixs*cn; ptr1 += ixs*cn;
                    for( k = 0; k < cn; k++ )
                        dst[x*cn + k] = ptr0[k]*b1 + ptr1[k]*b;
                }
            }
        }
    }
}

//----------------------------------------------------------
// 
//----------------------------------------------------------
void myGetQuadrangleSubPix(const Mat& src, Mat& dst,Mat& m )
{
    CV_Assert( src.channels() == dst.channels() );

    cv::Size win_size = dst.size();
    double matrix[6];
    cv::Mat M(2, 3, CV_64F, matrix);
    m.convertTo(M, CV_64F);
    double dx = (win_size.width - 1)*0.5;
    double dy = (win_size.height - 1)*0.5;
    matrix[2] -= matrix[0]*dx + matrix[1]*dy;
    matrix[5] -= matrix[3]*dx + matrix[4]*dy;

    if( src.depth() == CV_8U && dst.depth() == CV_32F )
        getQuadrangleSubPix_8u32f_CnR( src.data, src.step, src.size(),
        (float*)dst.data, dst.step, dst.size(),
        matrix, src.channels());
    else
    {
        CV_Assert( src.depth() == dst.depth() );
        cv::warpAffine(src, dst, M, dst.size(),
            cv::INTER_LINEAR + cv::WARP_INVERSE_MAP,
            cv::BORDER_REPLICATE);
    }
}
//----------------------------------------------------------
// 
//----------------------------------------------------------
void getRotRectImg(cv::RotatedRect rr,Mat &img,Mat& dst)
{
    Mat m(2,3,CV_64FC1);
    float ang=rr.angle*CV_PI/180.0;
    m.at<double>(0,0)=cos(ang);
    m.at<double>(1,0)=sin(ang);
    m.at<double>(0,1)=-sin(ang);
    m.at<double>(1,1)=cos(ang);
    m.at<double>(0,2)=rr.center.x;
    m.at<double>(1,2)=rr.center.y;
    myGetQuadrangleSubPix(img,dst,m);
}

//----------------------------------------------------------
// 
//----------------------------------------------------------
int main(int argc, char* argv[])
{
    Mat img=imread("D:\\ImagesForTest\\lena.jpg");
    img.convertTo(img,CV_32FC3,1.0/255.0);

    cv::RotatedRect rr(cv::Point2f(200,200),Size(50,50),-30);

    // rotated rectangle
    Point2f rect_points[4];
    rr.points( rect_points );

    for( int j = 0; j < 4; j++ )
    {
        line( img, rect_points[j], rect_points[(j+1)%4], Scalar(0,1,0), 1, CV_AA );
    }

    imshow("colImg",img);
    Mat dst(rr.size,CV_32FC3);
    getRotRectImg(rr,img,dst);
    imshow("rotImg",dst);
    cv::waitKey(0);
    cv::destroyAllWindows();
    return 0;
}

The result:

Answer 2

Implementation with OpenCV warpAffine.

Mat getAffineTransformForRotatedRect(RotatedRect rr) {
    float angle = rr.angle * M_PI / 180.0;
    // angle += M_PI; // you may want rotate it upsidedown
    float sinA = sin(angle), cosA = cos(angle);
    float data[6] = {
         cosA, sinA, rr.size.width/2.0f - cosA * rr.center.x - sinA * rr.center.y,
        -sinA, cosA, rr.size.height/2.0f - cosA * rr.center.y + sinA * rr.center.x};
    Mat rot_mat(2, 3, CV_32FC1, data);
    return rot_mat.clone();
}

Mat getRotatedRectImg(const cv::Mat &mat, RotatedRect rr) {
    Mat M, result;
    M = getAffineTransformForRotatedRect(rr);

    warpAffine(mat, result, M, rr.size, INTER_CUBIC);

    return result;
}