larrykite

Mat image;
Mat result;
cv::Mat_<cv::Vec3b>::const_iterator it = image.begin<cv::Vec3b>();
cv::Mat_<cv::Vec3b>::const_iterator itend = image.end<cv::Vec3b>();
cv::Mat_<uchar>::iterator itout=result.begin<uchar>();

for ( ; it!=itend; ++it, ++itout){
    if (getDistance(*it) < minDist){
        *itout = 255;

larrykite

Mat  img  =  imread("image.jpg");
Mat  grey;
cvtColor(img,  grey,  CV_BGR2GREY);
Mat  sobelx;
Sobel(grey,  sobelx,  CV_32F,  1,  0);
double  minVal,  maxVal;
minMaxLoc(sobelx,  &minVal,  &maxVal);  //find  minimum  and  maximum  intensities
Mat  draw;
sobelx.convertTo(draw,  CV_8U,  255.0/(maxVal  -  minVal),  -minVal);