[eigen] Eigen::Matrix and cv::Mat conversions

[lfrfly@xxxxxxxxxxx]

I want to store the vertices of a convex polygon in an 
Eigen::Matrix<double, 7,2>, and then find the area of the polygon using 
the method of determinants. In other words, I need to make a series of 
mat.block<2,2>(n,0).det() calls. However, for this to work, the 
vertices must be in order (clockwise or ccw doesn't matter). So I 
figured I'd use a convex hull algorithm to reorder them.

Well, I already have a dependency on OpenCV, so I thought I'd use their 
convexHull() method. However, to do this I must either reinterpret or 
copy the contents of the Eigen::Matrix into a cv::Mat_<cv::Vec2f>(7,1). 
The internal format of this type is straightforward----I can just write 
floats in rowmajor order using std::transform() and it will be fine.

The tricky part is figuring how what is the best way to get at that 
data in the Eigen::Matrix. Curiously, upon examining the internals from 
data() in the debugger, the data appears to be in column-major order 
even though I didn't specify that (is it the default for some reason?). 
I know I can use the rowwise() method, but it isn't obvious to me if 
it's possible to get any kind of iterator from that, or if there's some 
other way to do the copy cleanly.

Of course I could simply loop over the thing, but I strive for more 
elegant solutions when possible.