void CImgcxView::OnSobel()
{
    // TODO: Add your command handler code here
     if(!m_bFileOpen)
    {
        AfxMessageBox("Please open a grey image!");
        return;
    }

    if(::GetImageType() != 8)
    {
        AfxMessageBox("Not a grey image!");
        return;
    }  

    BYTE *Out_pImage = new BYTE[m_nXSize*m_nYSize];


    static int cx[9] //算子x(Roberts)
    = { -1, 0, 1,
        -2, 0, 2,
        -1, 0, 1};

    static int cy[9] //算子y(Roberts)
    = {  1,  2,  1,
         0,  0,  0,
        -1, -2, -1};

    int d[9];
    int i,j,dat;
    float xx, yy, zz;

    for (j = 1; j < m_nYSize - 1; j++)
    {
        for (i = 1; i < m_nXSize - 1; i++)
        {
            d[0] = *(m_pImage + (j-1)*m_nXSize + i-1);
            d[1] = *(m_pImage + (j-1)*m_nXSize + i);
            d[2] = *(m_pImage + (j-1)*m_nXSize + i+1);
            d[3] = *(m_pImage + j*m_nXSize + i-1);
            d[4] = *(m_pImage + j*m_nXSize + i);
            d[5] = *(m_pImage + j*m_nXSize + i+1);
            d[6] = *(m_pImage + (j+1)*m_nXSize + i-1);
            d[7] = *(m_pImage + (j+1)*m_nXSize + i);
            d[8] = *(m_pImage + (j+1)*m_nXSize + i+1);

            xx = (float)(cx[0]*d[0] + cx[1]*d[1] + cx[2]*d[2]
                        +cx[3]*d[3] + cx[4]*d[4] + cx[5]*d[5]
                        +cx[6]*d[6] + cx[7]*d[7] + cx[8]*d[8]);

            yy = (float)(cy[0]*d[0] + cy[1]*d[1] + cy[2]*d[2]
                        +cy[3]*d[3] + cy[4]*d[4] + cy[5]*d[5]
                        +cy[6]*d[6] + cy[7]*d[7] + cy[8]*d[8]);

            zz = (float)(1/4*sqrt(xx*xx + yy*yy));

            dat = (int)zz;

            if (dat > 255) dat = 255;

            *(Out_pImage + j*m_nXSize + i) = dat;
        }
    }

    ::Disp_image(Out_pImage);
    Invalidate();
}
这是边缘检测的代码，为什么图像处理后完全变黑？