Eigen: assign value to map

Question

I'm using opencv with Eigen.

Here is a sample on how to link an opencv Mat to an Eigen Matrix: OpenCV CV::Mat and Eigen::Matrix

The key is to use Eigen's Map type, which can point to the memory allocated by opencv.

According to the Eigen docs, the Map is supposed to be transparent, virtually all operations that work on matrices should work on Map, too. https://eigen.tuxfamily.org/dox/group__TutorialMapClass.html

Here is a some code from the link above. m2map points to the memory of a matrix called m2:

m2map(3) = 7;   // this will change m2, since they share the same array

But for me, even the simplest assignment fails:

Eigen::Map<Eigen::Matrix<float, Eigen::Dynamic, Eigen::Dynamic, Eigen::RowMajor>> eigenHeatmap(heatmap.ptr<float>(), heatmap.rows, heatmap.cols);

eigenHeatmap=0.f;

produces:

/.../Code.cpp:69:25: error: no match for ‘operator=’ (operand types are ‘Eigen::Map<Eigen::Matrix<float, -1, -1, 1> >’ and ‘float’)
             eigenHeatmap=0.f;

The whole error message is rather long: https://pastebin.com/i3AWs6C7

I'm using eigen3.3.3, opencv3.2.0 and g++ 5.4.0

score 3 · Answer 1 · answered Apr 12 '17 at 05:12

3

You should try eigenHeatmap.setZero(); or eigenHeatmap.setConstant(0.f); instead.

answered Apr 12 '17 at 05:12

Avi Ginsburg

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score 3 · Answer 2 · answered Apr 12 '17 at 07:26

Alternative to Avi's answer, if you are doing lots of element-wise operations, use Array instead of Matrix, i.e.

Eigen::Map<Eigen::Array<float, Eigen::Dynamic, Eigen::Dynamic, Eigen::RowMajor>> eigenHeatmap(heatmap.ptr<float>(), heatmap.rows, heatmap.cols);

eigenHeatmap=0.f;

And if you use that a lot, make some typedefs first:

typedef Eigen::Array<float, Eigen::Dynamic, Eigen::Dynamic, Eigen::RowMajor> ArrayXXfR;
typedef Eigen::Map<ArrayXXfR> ArrayMap;
ArrayMap eigenHeatmap(heatmap.ptr<float>(), heatmap.rows, heatmap.cols);

Addendum: For more details about the Array class, read the corresponding tutorial here.

Eigen: assign value to map

2 Answers2