Simple Blob Detector: How is a blob measured?

crackwitz · July 16, 2025, 2:37pm

looks like the size of the KeyPoint (second argument, if the first is a Point) is set to be the “diameter”, which is twice the “radius”, and “radius” is calculated thusly:

github.com/opencv/opencv

modules/features2d/src/blobdetector.cpp

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          KeyPoint kpt(sumPoint, (float)(centers[i][centers[i].size() / 2].radius) * 2.0f);

github.com/opencv/opencv

modules/features2d/src/blobdetector.cpp

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          center.location = Point2d(moms.m10 / moms.m00, moms.m01 / moms.m00);

github.com/opencv/opencv

modules/features2d/src/blobdetector.cpp

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          //compute blob radius
          {
              std::vector<double> dists;
              for (size_t pointIdx = 0; pointIdx < contours[contourIdx].size(); pointIdx++)
              {
                  Point2d pt = contours[contourIdx][pointIdx];
                  dists.push_back(norm(center.location - pt));
              }
              std::sort(dists.begin(), dists.end());
              center.radius = (dists[(dists.size() - 1) / 2] + dists[dists.size() / 2]) / 2.;
          }
          
          centers.push_back(center);

that looks like it’s taking the barycenter (from moments calculation), then taking all the distances to all the contour points, and taking the median of that.

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Simple Blob Detector: How is a blob measured?

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