Explanation for the cv2.HoughCircles function parameters

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But I call the function with only 4 parameters like this : cv2.HoughCircles(gray, cv2.cv.HOUGH_GRADIENT, 1.2, 100) I understand the first two but the 1.2 and 100 I don't understand. To what do they correspond in the c++ function ? And can I specify the radius in the Python function ?

please look again at the docs.

and sure, you can specify minRadius=... and maxRadius=... (you just have to use key=value args in python, if you omit some in the middle)

So what you are saying is that the Python and the c++ version of this function have the exact same parameters(in the same order also, of course except for the 2nd parameter in the c++ version)?

yes (minus the output image), as you said.