The hausdorff distance is not zero even when 2 same images are passed

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I am passing 2 same images in hausdorff and shapeContextDistanceExtractor to calculate distance. We are expecting the answer to be zero since the images are same but we are getting a non-zero value. Can somebody please explain why.

            // Hausdorff Distant Extractor

            Ptr<HausdorffDistanceExtractor> model = createHausdorffDistanceExtractor();

            dist = model->computeDistance(c1, c2);

I am passing 2 images and extracting the points using samplecounters. Here c1 and c2 are the vector of countour points which are passed to the computeDistance function. The 2 images that I pass are same yet the distance I am getting in dist is non-zero. Please help.