Is it possible to find the edge of a “spotty” region in emgucv?

Question

I have an image that looks like this:

and I want to find the edges of the dark part so lik

Answer 1

Yes, it's possible, and here is how you could do it:

• Change the contrast of the image to make the lighter part disappear:

• Then, convert it to HSV to perform a threshold operation on the Saturation channel:

• And execute erode & dilate operations to get rid of the noises:

At this point you'll have the result you were looking for. For testing purposes, at the end I execute the bounding box technique to show how to detect the beggining and the end of the area of interest:

I didn't have the time to tweak the parameters and make a perfect detection, but I'm sure you can figure it out. This answer provides a roadmap for achieving that!

This is the C++ code I came up with, I trust you are capable of converting it to C#:

#include <cv.h>
#include <highgui.h>

int main(int argc, char* argv[])
{
    cv::Mat image = cv::imread(argv[1]);
    cv::Mat new_image = cv::Mat::zeros(image.size(), image.type());

    /* Change contrast: new_image(i,j) = alpha*image(i,j) + beta */

    double alpha = 1.8;     // [1.0-3.0]
    int beta = 100;         // [0-100]
    for (int y = 0; y < image.rows; y++)
    { 
        for (int x = 0; x < image.cols; x++)
        { 
        for (int c = 0; c < 3; c++)
        {
            new_image.at<cv::Vec3b>(y,x)[c] = 
            cv::saturate_cast<uchar>(alpha * (image.at<cv::Vec3b>(y,x)[c]) + beta);
        }
        }
    }
    cv::imshow("contrast", new_image);

    /* Convert RGB Mat into HSV color space */

    cv::Mat hsv;
    cv::cvtColor(new_image, hsv, CV_BGR2HSV);
    std::vector<cv::Mat> v;
    cv::split(hsv,v);

    // Perform threshold on the S channel of hSv    
    int thres = 15;
    cv::threshold(v[1], v[1], thres, 255, cv::THRESH_BINARY_INV);
    cv::imshow("saturation", v[1]);

    /* Erode & Dilate */

    int erosion_size = 6;   
    cv::Mat element = cv::getStructuringElement(cv::MORPH_CROSS,
                          cv::Size(2 * erosion_size + 1, 2 * erosion_size + 1), 
                          cv::Point(erosion_size, erosion_size) );
    cv::erode(v[1], v[1], element);
    cv::dilate(v[1], v[1], element);    
    cv::imshow("binary", v[1]);

    /* Bounding box */

    // Invert colors
    cv::bitwise_not(v[1], v[1]);

    // Store the set of points in the image before assembling the bounding box
    std::vector<cv::Point> points;
    cv::Mat_<uchar>::iterator it = v[1].begin<uchar>();
    cv::Mat_<uchar>::iterator end = v[1].end<uchar>();
    for (; it != end; ++it)
    {
        if (*it) points.push_back(it.pos());
    }    

    // Compute minimal bounding box
    cv::RotatedRect box = cv::minAreaRect(cv::Mat(points));

    // Draw bounding box in the original image (debug purposes)
    cv::Point2f vertices[4];
    box.points(vertices);
    for (int i = 0; i < 4; ++i)
    {
        cv::line(image, vertices[i], vertices[(i + 1) % 4], cv::Scalar(0, 255, 0), 2, CV_AA);
    }

    cv::imshow("box", image);    
    cvWaitKey(0);

    return 0;
}