Initializing a matrix

Question

I wrote the following code:

int _tmain(int argc, _TCHAR* argv[])
{

int vals[]={1,2,3,4,5,6,7,8,9};

CvMat mat = cvMat(3,3,CV_8UC1,vals);

for(int i=0;i<mat.rows;i++)
{
    int* ptr = (int*) (mat.data.ptr + i* mat.step);

    for(int j=0;j<mat.cols;j++)
    {
                printf("%d\t",*ptr++);

    }
    printf("\n");
}

return 0;
}

The output I got is:

1              2               3
512            768             1024
196608         262144          327680

The matrix is not initialized properly. The pointer ptr points to the beginning of each row and incrementing it gives the element in corresponding column. Is my assumption correct? Is there any mistake with the cvMat constructor used or the access method of elements?

Answer 1

Why dont you use the cv::Matclass?

It has some handy functions which handle memory better.

for declaring and initializing cv::Mat in your case the code will look like this:

int main()
{
   int vals[] = {1,2,3,4,5,6,7,8,9};

    Mat mat = cv::Mat(3, 3, CV_8UC1, vals).clone();

    for(int i = 0; i < mat.rows; i++)
    {
        for(int j = 0; j < mat.cols; j++)
        {
             cout << mat.at<uchar>(j,i) << " ";
        }
        cout << endl;
    }
    return 0;
}

Answer 2

Wrong. mat.data.ptr is for unsigned chars from http://www.cs.iit.edu/~agam/cs512/lect-notes/opencv-intro/opencv-intro.html

uchar*  ptr;     // data pointer for an unsigned char matrix

you should use

mat.data.i

for integers

You initialize matrix properly but printing it wrong because of using the wrong pointer of data structure.

CvMat                      // 2D array
  |-- int   type;          // elements type (uchar,short,int,float,double) and flags
  |-- int   step;          // full row length in bytes
  |-- int   rows, cols;    // dimensions
  |-- int   height, width; // alternative dimensions reference
  |-- union data;
      |-- uchar*  ptr;     // data pointer for an unsigned char matrix
      |-- short*  s;       // data pointer for a short matrix
      |-- int*    i;       // data pointer for an integer matrix
      |-- float*  fl;      // data pointer for a float matrix
      |-- double* db;      // data pointer for a double matrix

See? Did this help?