Non connecting morphological filter - opencv

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 I am using OPEN morphology filter with a rather big circle kernel , it works very well until the distance between segmented objects is enough. But In alot of samples objects stick together. Is there exists some more or less simple method to smooth such kind of images without changing its morphology ?

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.