cv::getPerspectiveTransform throws Error after cv::eigen2cv - c++

Question

My Code:
cv::Mat
getPerspectiveTransform(Eigen::MatrixXd quadrangle, Eigen::MatrixXd warpedQuadrangle) {
cv::Mat transMat;
cv::Mat quad(4,2,CV_32FC1);
cv::Mat warpedQuad(4,2,CV_32FC1);
cv::eigen2cv(quadrangle,quad);
cv::eigen2cv(warpedQuadrangle,warpedQuad);
std::cout << "[ ] quadrangle in cv::Mat " << quad << std::endl;
transMat = cv::getPerspectiveTransform(quad,warpedQuad);
return transMat;
}
Error:
C++ exception with description "OpenCV(4.6.0) /home/ci/opencv/modules/imgproc/src/imgwarp.cpp:3392: error: (-215:Assertion failed) src.checkVector(2, CV_32F) == 4 && dst.checkVector(2, CV_32F) == 4 in function 'getPerspectiveTransform'
Suspected Issue:
eigen2cv is converting my CV_32FC1 to CV_64F. getPerspectiveTransform is expecting CV32F as its input.
What should be the ideal solution to this?

Answer 1

eigen2cv changes the layout of output matrix according to type of the input matrix, as can be seen from its source. So, e.g., if your Eigen matrices use 64-bit floats, output Mat's will have CV_64F depth. In this case, the simplest solution is to convert output matrices to CV_32F using Mat::convertTo (documentation):
Mat quadF, warpedQuadF;
quad.convertTo(quadF, CV_32F);
warpedQuad.convertTo(warpedQuadF, CV_32F);
transMat = cv::getPerspectiveTransform(quadF, warpedQuadF);
Since matrices are pretty small, this conversion is unlikely to be a performance issue, but it's possible to avoid it by rewriting 2 overloads of getPerspectiveTransform function (source1 and source2) to work with 64-bit floats. Second overload just delegates to the first, and the first works with double's internally anyway, so it's pretty trivial:
cv::Mat getPerspectiveTransform64(const Point2d src[], const Point2d dst[], int solveMethod)
{
CV_INSTRUMENT_REGION();
Mat M(3, 3, CV_64F), X(8, 1, CV_64F, M.ptr());
double a[8][8], b[8];
Mat A(8, 8, CV_64F, a), B(8, 1, CV_64F, b);
for( int i = 0; i < 4; ++i )
{
a[i][0] = a[i+4][3] = src[i].x;
a[i][1] = a[i+4][4] = src[i].y;
a[i][2] = a[i+4][5] = 1;
a[i][3] = a[i][4] = a[i][5] = a[i+4][0] = a[i+4][1] = a[i+4][2] = 0;
a[i][6] = -src[i].x*dst[i].x;
a[i][7] = -src[i].y*dst[i].x;
a[i+4][6] = -src[i].x*dst[i].y;
a[i+4][7] = -src[i].y*dst[i].y;
b[i] = dst[i].x;
b[i+4] = dst[i].y;
}
solve(A, B, X, solveMethod);
M.ptr<double>()[8] = 1.;
return M;
}
cv::Mat getPerspectiveTransform64(InputArray _src, InputArray _dst, int solveMethod)
{
Mat src = _src.getMat(), dst = _dst.getMat();
CV_Assert(src.checkVector(2, CV_64F) == 4 && dst.checkVector(2, CV_64F) == 4);
return getPerspectiveTransform64((const Point2d*)src.data, (const Point2d*)dst.data, solveMethod);
}
This can now be used directly without additional conversion:
transMat = getPerspectiveTransform64(quad, warpedQuad);