OpenCV Perspective corrention and cropping - c++

Question

I'm currently trying to correct the perspective of a random taken image showing a rectangle.
The perspective correction is working fine, but i want to crop the image to the target, too. Si I've tried to transform the given contour of my target by the perspective matrix (cv::Mat) and crop it with the results.
My method is currently crashing at the marked line with the following error.
OpenCV Error: Assertion failed (mtype == type0 || (CV_MAT_CN(mtype) == CV_MAT_CN(type0) && ((1 << type0) & fixedDepthMask) != 0)) in create, file /Volumes/build-storage/build/master_iOS-mac/opencv/modules/core/src/matrix.cpp, line 2430
libc++abi.dylib: terminating with uncaught exception of type cv::Exception: /Volumes/build-storage/build/master_iOS-mac/opencv/modules/core/src/matrix.cpp:2430: error: (-215) mtype == type0 || (CV_MAT_CN(mtype) == CV_MAT_CN(type0) && ((1 << type0) & fixedDepthMask) != 0) in function create
Code
cv::Mat correctMat(cv::Mat mat, std::vector<cv::Point> contour) {
double minObjectSize = 100.0;
if (contour.size() == 4) {
cv::Rect rect = cv::boundingRect(contour);
if (rect.height < minObjectSize || rect.width < minObjectSize) {
NSLog(#"Objects size was too small: %d * %d", rect.width, rect.height);
}
else {
std::vector<Point2f> quad_pts;
std::vector<Point2f> squre_pts;
quad_pts.push_back(Point2f(contour[0].x, contour[0].y));
quad_pts.push_back(Point2f(contour[1].x, contour[1].y));
quad_pts.push_back(Point2f(contour[3].x, contour[3].y));
quad_pts.push_back(Point2f(contour[2].x, contour[2].y));
squre_pts.push_back(Point2f(rect.x, rect.y));
squre_pts.push_back(Point2f(rect.x, rect.y + rect.height));
squre_pts.push_back(Point2f(rect.x + rect.width, rect.y));
squre_pts.push_back(Point2f(rect.x + rect.width, rect.y + rect.height));
Mat transmtx = getPerspectiveTransform(quad_pts, squre_pts);
Mat transformed = Mat::zeros(mat.rows, mat.cols, CV_8UC3);
cv::line(mat, quad_pts[0], quad_pts[1], Scalar(0,0,255), 5, CV_AA, 0);
cv::line(mat, quad_pts[1], quad_pts[2], Scalar(0,0,255), 5, CV_AA, 0);
cv::line(mat, quad_pts[2], quad_pts[3], Scalar(0,0,255), 5, CV_AA, 0);
cv::line(mat, quad_pts[3], quad_pts[0], Scalar(0,0,255), 5, CV_AA, 0);
warpPerspective(mat, transformed, transmtx, mat.size());
std::vector<cv::Point2f> transformedPoints;
cv::Matx23f matrix = *transmtx.clone().ptr();
cv::transform(quad_pts, transformedPoints, matrix);
cv::Mat cropped = transformed(cv::boundingRect(transformedPoints));
fixColorOfMat(cropped);
return cropped;
}
}
return mat;
}
I do not really know what the error message is telling me so i hope somebody here could help me solving this crash.
Next Problem
I've changed the code a bit, the transform action on the point vector works now, but i'm unabled to crop the Mat by these transformed points.
std::vector<cv::Point2f> transformedPoints;
cv::Matx23f matrix = *transmtx.clone().ptr();
cv::transform(quad_pts, transformedPoints, matrix);
// Crash
cv::Mat cropped = transformed(cv::boundingRect(transformedPoints));
The error message says
OpenCV Error: Assertion failed (0 <= roi.x && 0 <= roi.width && roi.x + roi.width <= m.cols && 0 <= roi.y && 0 <= roi.height && roi.y + roi.height <= m.rows) in Mat, file /Volumes/build-storage/build/master_iOS-mac/opencv/modules/core/src/matrix.cpp, line 522
libc++abi.dylib: terminating with uncaught exception of type cv::Exception: /Volumes/build-storage/build/master_iOS-mac/opencv/modules/core/src/matrix.cpp:522: error: (-215) 0 <= roi.x && 0 <= roi.width && roi.x + roi.width <= m.cols && 0 <= roi.y && 0 <= roi.height && roi.y + roi.height <= m.rows in function Mat

Answer 1

Hmm, I used memcpy() convert vector to Mat directly. Maybe you can try that on the transform part.
memcpy(Mat.data, vector.data(), vector.size()*sizeof(Point));
As long as you got the mask matrix of contour, I think you can use bitwise() to corp your target result.
Btw, I cannot explain opencv's error statement, as I have never understood it.