CODEWITHSUNDEEP
c++opencvmatrixeigen
lhk
lhk

Reputation: 30086

Eigen: assign value to map

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

Upvotes: 1

Views: 1522

Answers (2)

chtz
chtz

Reputation: 18817

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.

Upvotes: 3

Avi Ginsburg
Avi Ginsburg

Reputation: 10596

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

Upvotes: 3

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