Question

I’m trying to learn how to use OpenCV’s new C++ interface.

How do I access elements of a multi channel matrix? For example:

Mat myMat(size(3, 3), CV_32FC2);

for (int i = 0; i < 3; ++i)
{
    for (int j = 0; j < 3; ++j)
    {
        //myMat_at_(i,j) = (i,j);
    }
}

What is the easiest way to do this? Something like cvSet2D of the old interface.
What is the most efficient way? Similar to using direct pointers in the old interface.

Answer 1

typedef struct elem_ {
        float f1;
        float f2;
} elem;
elem data[9] = { 0.0f };
CvMat mat = cvMat(3, 3, CV_32FC2, data );

float f1 = CV_MAT_ELEM(mat, elem, row, col).f1;
float f2 = CV_MAT_ELEM(mat, elem, row, col).f2;

CV_MAT_ELEM(mat, elem, row, col).f1 = 1212.0f;
CV_MAT_ELEM(mat, elem, row, col).f2 = 326.0f;

Update : for OpenCV2.0

1. choose one type to represent the element

Mat (or CvMat) has 3 dimensions: row, col, channel.
We can access one element (or pixel) in the matrix by specifying the row and col.

CV_32FC2 means the element is 32bit floating point value with 2 channels.
So elem in above code is one acceptable representation of CV_32FC2.

You can use other representations you like. For example :

typedef struct elem_ { float val[2];    } elem;
typedef struct elem_ { float x;float y; } elem;

OpenCV2.0 adds some new types to represent the element in the matrix,like :

template<typename _Tp, int cn> class CV_EXPORTS Vec // cxcore.hpp (208)

So we can use Vec<float,2> to represent CV_32FC2, or use :

typedef Vec<float, 2> Vec2f; // cxcore.hpp (254)

See the source code to get more type that can represent your element.
Here we use Vec2f

2. access the element

The easiest and efficiant way to access the element in the Mat class is Mat::at.
It has 4 overloads :

template<typename _Tp> _Tp& at(int y, int x);                // cxcore.hpp (868)
template<typename _Tp> const _Tp& at(int y, int x) const;    // cxcore.hpp (870)
template<typename _Tp> _Tp& at(Point pt);                    // cxcore.hpp (869)
template<typename _Tp> const _Tp& at(Point pt) const;        // cxcore.hpp (871)
// defineded in cxmat.hpp (454-468)

// we can access the element like this :
Mat m( Size(3,3) , CV_32FC2 );
Vec2f& elem = m.at<Vec2f>( row , col ); // or m.at<Vec2f>( Point(col,row) );
elem[0] = 1212.0f;
elem[1] = 326.0f;
float c1 = m.at<Vec2f>( row , col )[0]; // or m.at<Vec2f>( Point(col,row) );
float c2 = m.at<Vec2f>( row , col )[1];
m.at<Vec2f>( row, col )[0] = 1986.0f;
m.at<Vec2f>( row, col )[1] = 326.0f;

3. interact with old interface

Mat provides 2 conversion functions:

// converts header to CvMat; no data is copied     // cxcore.hpp (829)
operator CvMat() const;                            // defined in cxmat.hpp
// converts header to IplImage; no data is copied
operator IplImage() const;

// we can interact a Mat object with old interface :
Mat new_matrix( ... );
CvMat old_matrix = new_matrix;  // be careful about its lifetime
CV_MAT_ELEM(old_mat, elem, row, col).f1 = 1212.0f;