Why does assertion fail here

Question

Why does the assertion fail here when i create a CvMat *? It does not happen with an image i load in cv::Mat using a pointer.

    struct RGB { unsig         


        

Answer 1

Because sizeof(RGB) != sizeof(float), which is what you filled the matrix with here:

 CvMat* mat = cvCreateMat(300,300,CV_32FC1); 

CV_32FC1 means 1 component, 32-bit floating point. You probably want CV_8UC3. See here or another OpenCV reference.

Answer 2

You can skip the entire IplImage misery if you use cv::Mat img = cv::loadImage("blah.jpg"); Also it is better to use row ptr for going through all the pixels.
It knows the jumps, so you don't have to worry!

From the refman:

If you need to process a whole row of a 2D array, the most efficient way is to get the pointer to the row first, and then just use the plain C operator []

Be aware that if you are loading bigger images which have "jumps" in their data, your code will not work. In your situation

cv::Mat img = cv::loadImage("blah.jpg");
const cv::Mat& M = img;
for(int i = 0; i < rows; i++) 
{
    const Vec3b* Mi = M.ptr<Vec3b>(i); 
    for(int j = 0; j < cols; j++)
    {
        const Vec3b& Mij = Mi[j];
        std::cout<<"Row="<<i<<"Col="<<j<<"\t";
        std::cout<<"b="<<Mij[0]<<" g="<<Mij[1]<<" r="<<Mij[2]<<std::endl;
    }
}

is the fastest correct way. Otherwise you could use M.at<Vec3b>(i,j).