 <p>I've been wondering this for a while also; the OpenCV documentation isn't very helpful when it comes to blob detection.</p> <p>Based on the descriptions of <a href="http://www1.adept.com/main/KE/Data/ACE/AdeptSight_User/BlobAnalyzer_Results.html#Intrinsic_Inertia_Results">other blob analyzers</a>, the inertia of a blob is "the inertial resistance of the blob to rotation about its principal axes". It depends on how the mass of the blob (I guess in this case the area) is distributed throughout the blob's shape.</p> <p>There's a lot of mathy stuff involved -- most of which I don't remember how to do -- but the result at the bottom of <a href="http://homepages.inf.ed.ac.uk/rbf/CVonline/LOCAL_COPIES/OWENS/LECT2/node3.html">this page on the properties of binary images</a> sums it up fairly well (blob detection is done by converting the input image to a series of binary images):</p> <blockquote> <p>The ratio <img src="https://chart.googleapis.com/chart?cht=tx&amp;chl=I_%7Bmin%7D%2FI_%7Bmax%7D&amp;chf=bg,s,65432100" alt="I_min/I_max"> gives us some idea of how rounded the object is. This ratio will be 0 for a line and 1 for a circle.</p> </blockquote> <p>So basically, by specifying <code>minInertiaRatio</code> and <code>maxInertiaRatio</code> you can filter the blobs based on how elongated they are. An inertia ratio of 0 will yield elongated blobs (closer to lines) and an inertia ratio of 1 will yield blobs where the area is more concentrated toward the center (closer to circles).</p>