OpenCV Error "Assertion Failed"

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VisualC++

OpenCV

Hi,
I have write a class to manage images and to verify value of pixel on a Mat image, in a test with if and else.

This is a part of the code that I've write, the specific procedure that creates problems:

bool cubo::verifica_punti(Mat& I, double TC[4][4], double fc[2],double KC[5], double cc[2],double alpha){
    for (int i=0;i<8;i++){
        //normalizzazione
        double PN[2] = { V[i].x/V[i].z, V[i].y/V[i].z};

        //calcolo delle componenti
        double r2 = pow(PN[0],2.0) + pow(PN[1],2.0);
        //distorsione tangenziale
        double dx[2]={((2*KC[2]*PN[0]*PN[1])+(KC[3]*(r2+2*pow(PN[0],2.0)))),(KC[2]*(r2+2*pow(PN[1],2.0))+2*KC[3]*PN[0]*PN[1])};
        //componenti sul piano
        double xd[2]={((1+KC[0]*r2+KC[1]*pow(r2,2.0)+KC[4]*pow(r2,3.0))*PN[0]+dx[0]),((1+KC[0]*r2+KC[1]*pow(r2,2.0)+KC[4]*pow(r2,3.0))*PN[1]+dx[1])};

        //calcolo delle coordinate in pixel
        int xp = ceil(fc[0]*(xd[0]+alpha*xd[1])+cc[0]);
        int yp = ceil(fc[1]*xd[1]+cc[1]);

        //cout<<endl<<"XP = "<<xp<<" YP = "<<yp<<endl;
        //valore temporaneo per testare
        //uchar pix = 1;
        if ((yp>720||yp<0))
            {
            return false;
            }
        else if ((xp>1280||xp<0)){
            return false;
            }
        else
        {
            Scalar pix = I.at<uchar>(yp,xp);

            if (pix.val[0]==0)
            {
            return false;
            }
        else
            return true;
        }
    }

};

This is a method of my class CUBO.

When program run, I receive this error and program crash:

http://imageshack.com/i/10r512j[^]

can someone help me??