How to create empty Mat in OpenCV?

Question

How to create empty Mat in OpenCV? After creation I want to use push_back method to push rows in Mat.

Something like:

Mat M(0,3,CV_32FC1);

or only option is:

Mat M;
M.converTo(M,CV_32FC1);

?

Answer 1

You can create an empty matrix simply using:

Mat m;

If you already know its type, you can do:

Mat1f m; // Empty matrix of float

If you know its size:

Mat1f m(rows, cols);  // rows, cols are int
or 
Mat1f m(size);  // size is cv::Size

And you can also add the default value:

Mat1f m(2, 3, 4.1f);
//
// 4.1 4.1 4.1
// 4.1 4.1 4.1

If you want to add values to an empty matrix with push_back, you can do as already suggested by @berak:

Mat1f m;
m.push_back(Mat1f(1, 3, 3.5f));   // The  first push back defines type and width of the matrix
m.push_back(Mat1f(1, 3, 9.1f));
m.push_back(Mat1f(1, 3, 2.7f));

// m
// 3.5 3.5 3.5
// 9.1 9.1 9.1
// 2.7 2.7 2.7 

If you need to push_back data contained in vector<>, you should take care to put values in a matrix and transpose it.

vector<float> v1 = {1.1f, 2.2f, 3.3f, 4.4f, 5.5f};
vector<float> v2 = {1.2f, 2.3f, 3.4f, 4.5f, 5.6f};

Mat1f m1(Mat1f(v1).t());
Mat1f m2(Mat1f(v2).t());

Mat1f m;
m.push_back(m1);
m.push_back(m2);

// m
// 1.1 2.2 3.3 4.4 5.5
// 1.2 2.3 3.4 4.5 5.6

Answer 2

just start with an empty Mat. the 1st push_back will determine type and size.

Mat big;
big.push_back(Mat(1,5,CV_64F,3.5));
big.push_back(Mat(1,5,CV_64F,9.1));
big.push_back(Mat(1,5,CV_64F,2.7));

[3.5, 3.5, 3.5, 3.5, 3.5;
 9.1, 9.1, 9.1, 9.1, 9.1;
 2.7, 2.7, 2.7, 2.7, 2.7]