天天向上

The Objects(BLOB) are sorted by Circularity from small to big

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The Objects(BLOB) are sorted by Convexity from small to big

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[2] In simple words, convexity defect is a cavity in an object (blob, contour) segmented out from an image. That means an area that do not belong to the object but located inside of its outer boundary -convex hull (read documentation of cv::convexHull function for details).

The image below shows it better than million of words. Areas between fingers (all marked with arrows) in this schematics of a hand contour are the convexity defects.

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The Objects(BLOB) are sorted by Inertia from small to big

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[3] Based on the descriptions of other blob analyzers, 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.

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 this page on the properties of binary images sums it up fairly well (blob detection is done by converting the input image to a series of binary images):

The ratio gives us some idea of how rounded the object is. This ratio will be 0 for a line and 1 for a circle.

Reference:

[1] Blob Detection Using OpenCV ( Python, C++ )

[2] What is the deffinition of convexity defect in image processing?

[3] opencv SimpleBlobDetector filterByInertia meaning?