Non connecting morphological filter

Question

After some simple preprocessing I am receiving boolean mask of segmented images.

I want to \"enhance\" borders of the mask and make them more smooth. For that

Answer 1

Without applying a morphological filter first, you can try to detect the external contours of the image. Now you can draw these external contours as filled contours and then apply your morphological filter. This works because now you don't have any holes to fill. This is fairly simple.

Another approach:

• find external contours
• take the x, y of coordinates of the contour points. you can consider these as 1-D signals and apply a smoothing filter to these signals

In the code below, I've applied the second approach to a sample image.

Input image

External contours without any smoothing

After applying a Gaussian filter to x and y 1-D signals

C++ code

Mat im = imread("4.png", 0);

Mat cont = im.clone();
Mat original = Mat::zeros(im.rows, im.cols, CV_8UC3);
Mat smoothed = Mat::zeros(im.rows, im.cols, CV_8UC3);

// contour smoothing parameters for gaussian filter
int filterRadius = 5;
int filterSize = 2 * filterRadius + 1;
double sigma = 10;      

vector<vector<Point> > contours;
vector<Vec4i> hierarchy;
// find external contours and store all contour points
findContours(cont, contours, hierarchy, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_NONE, Point(0, 0));
for(size_t j = 0; j < contours.size(); j++)
{
    // draw the initial contour shape
    drawContours(original, contours, j, Scalar(0, 255, 0), 1);
    // extract x and y coordinates of points. we'll consider these as 1-D signals
    // add circular padding to 1-D signals
    size_t len = contours[j].size() + 2 * filterRadius;
    size_t idx = (contours[j].size() - filterRadius);
    vector<float> x, y;
    for (size_t i = 0; i < len; i++)
    {
        x.push_back(contours[j][(idx + i) % contours[j].size()].x);
        y.push_back(contours[j][(idx + i) % contours[j].size()].y);
    }
    // filter 1-D signals
    vector<float> xFilt, yFilt;
    GaussianBlur(x, xFilt, Size(filterSize, filterSize), sigma, sigma);
    GaussianBlur(y, yFilt, Size(filterSize, filterSize), sigma, sigma);
    // build smoothed contour
    vector<vector<Point> > smoothContours;
    vector<Point> smooth;
    for (size_t i = filterRadius; i < contours[j].size() + filterRadius; i++)
    {
        smooth.push_back(Point(xFilt[i], yFilt[i]));
    }
    smoothContours.push_back(smooth);

    drawContours(smoothed, smoothContours, 0, Scalar(255, 0, 0), 1);

    cout << "debug contour " << j << " : " << contours[j].size() << ", " << smooth.size() << endl;
}

Answer 2

Not 100% sure what you are trying to achieve, but this may be an avenue to explore... the tool potrace takes images and converts them to vectorised images which involves smoothing. It prefers PGM format input files so I use ImageMagick to prepare them. Anyway, here is an example of the command and the result so see what you think:

convert disks.png pgm:- | potrace - -s -o out.svg

I have converted the resulting SVG file to a PNG so I can upload it to SO.