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copied!<pre><code>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; </code></pre> <p><strong>Update : for OpenCV2.0</strong></p> <h3>1. choose one type to represent the element<br /></h3> <p>Mat (or CvMat) has 3 dimensions: row, col, channel.<br /> We can access one element (or pixel) in the matrix by specifying the row and col.<br /></p> <p><code>CV_32FC2</code> means the element is 32bit floating point value with 2 channels.<br /> So elem in above code is one acceptable representation of <code>CV_32FC2</code>.<br /></p> <p>You can use other representations you like. For example :<br /></p> <pre><code>typedef struct elem_ { float val[2]; } elem; typedef struct elem_ { float x;float y; } elem; </code></pre> <p>OpenCV2.0 adds some new types to represent the element in the matrix,like :<br /></p> <pre><code>template<typename _Tp, int cn> class CV_EXPORTS Vec // cxcore.hpp (208) </code></pre> <p>So we can use <code>Vec<float,2></code> to represent <code>CV_32FC2</code>, or use :<br /></p> <pre><code>typedef Vec<float, 2> Vec2f; // cxcore.hpp (254) </code></pre> <p>See the source code to get more type that can represent your element.<br /> Here we use <code>Vec2f</code></p> <h3>2. access the element</h3> <p>The easiest and efficiant way to access the element in the Mat class is Mat::at.<br /> It has 4 overloads : <pre><code>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; </code></pre> <h3>3. interact with old interface</h3> <p>Mat provides 2 conversion functions: <br /></p> <pre><code>// 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; </code></pre>

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