Visually anayzing network diagrams.

edit

I would suggest to do better preprocessing because of non uniform background and to avoid blur because thin arcs can disappear. When you have a good mask for your drawing you can use massive morphology operation to isolate nodes and arcs Finally you have to design a logic to build the chart connections.

edit: small changes in descriptions

Below is my DiagramAnalyzer to convert your image into a Graphviz/dot chart. Here is the result

graph G{ 
    N0 N1 N2 N3 N4 
    N1 -- N0
    N2 -- N0
    N3 -- N2
    N3 -- N1
    N4 -- N3
}

Enumerating: Layout by Graphviz/dot

int DiagramAnalyzer()
{
    Mat src, bw;
    src = imread("../img/diagram-x5.jpg");

==PREPROCESSING==

Option1: Remove non uniform background/illumination. You can achieve this using the wonderful quadratic background removal by @LBerger. Than get a mask of the drawing using a simple threshold

#if 1
    Mat srcGray, dstGray;
    cvtColor(src, srcGray, CV_BGR2GRAY);
    BackgroundRemoveQuadratic(srcGray, dstGray);
    threshold(dstGray, bw, 100, 255, THRESH_OTSU);

Quadratic result threshold

OR Option2: Get the HSV planes and threshold high values in S (>=95 percentile) to get a mask of the drawing. You can achieve this using clipHistPercent=5 with BrightnessAndContrastAuto than get a binary mask looking at very high values in saturation

#else
    Mat src_hsv, saturation;
    vector<cv::Mat> hsv_planes;
    cvtColor(src, src_hsv, CV_BGR2HSV);
    cv::split(src_hsv, hsv_planes);
    saturation = hsv_planes[1];
    BrightnessAndContrastAuto(saturation, saturation, 5);
    bw = saturation >= 254;
#endif

Saturation result (look at values of drawing) threshold

fill circular structures using small circle

int nodeSize = 15; //avg node diameter in px
Morph(bw, bw, MORPH_CLOSE, MORPH_ELLIPSE, cvRound(nodeSize / 5));
imshow("Preprocessing", bw);

Preprocessing (result of quadratic)

GET MASK OF NODES: remove boundary structures smaller than nodeSize . Should remove the arcs

cv::Mat nodesMask;
Morph(bw, nodesMask, MORPH_OPEN, MORPH_ELLIPSE, cvRound(nodeSize / 5));
imshow("Nodes-Mask", nodesMask);

Node Mask

GET MASK OF ARCS: dilate nodes a bit (ensure node vs arcs separation) than subtract them from the global mask

cv::Mat arcs, nodesLarger;
Morph(nodesMask, nodesLarger, MORPH_DILATE, MORPH_ELLIPSE, 1);
arcs = bw - nodesLarger;
imshow("Arcs-Mask", arcs);

Arcs Mask

GET INTERSECTIONS: ensure node vs arcs intersection doubling size of nodes than get a mask of shared pixels

cv::Mat intersections;
Morph(nodesLarger, nodesLarger, MORPH_DILATE, MORPH_ELLIPSE, nodeSize);
intersections = nodesLarger & arcs;

GET CONTOURS OF ELEMENTS

vector<vector<Point> > nodesContours, arcsContours, intesectsContours;
findContours(nodesLarger, nodesContours, CV_RETR_EXTERNAL, CHAIN_APPROX_SIMPLE);
findContours(arcs, arcsContours, CV_RETR_EXTERNAL, CHAIN_APPROX_SIMPLE);
findContours(intersections, intesectsContours, CV_RETR_EXTERNAL, CHAIN_APPROX_SIMPLE);
vector<Point> nodesCenter(nodesContours.size(), Point(-1, -1));
vector<double> nodesArea(nodesContours.size(), -1);
vector<double> arcsLenght(arcsContours.size(), -1);

DRAW ENUMERATED CONTOURS and get center of nodes

Moments m;
Point center;
Scalar cl = Scalar(0, 0, 255);
double val,minvalue = nodeSize/3;
for (size_t i = 0; i < nodesContours.size(); i++)
{
    val = contourArea(nodesContours[i]);
    if (val < minvalue) continue; //skip if it's small
    nodesArea[i] = val;
    m = moments(nodesContours[i], true);
    //if (m.m00 == 0) continue;
    center = Point(m.m10 / m.m00, m.m01 / m.m00);
    nodesCenter[i] = center;
    drawMarker(src, center, cl, MARKER_TILTED_CROSS, 5);
    putText(src, "N" + to_string(i), center, FONT_HERSHEY_PLAIN, 1, cl);
}
cl = Scalar(0, 255, 0);
minvalue = nodeSize/5 ...