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Right now, it is a bug in OpenCV.

But if you are using only OpenCV-Python (not Java), there is a temporary solution. I have tested it only with master branch.

Go to opencv/modules/objdetect/include/opencv2/objdetect.hpp.

Its line 200 starts as follows :

void detectMultiScale( InputArray image,

Change it to following:

 CV_WRAP_AS(detectMultiScale2) void detectMultiScale( InputArray image,

So the final function definition will look like below (especially note 3 CV_OUT macros):

 CV_WRAP_AS(detectMultiScale2) void detectMultiScale( InputArray image,
                               CV_OUT std::vector<Rect>& objects,
                               CV_OUT std::vector<int>& rejectLevels,
                               CV_OUT std::vector<double>& levelWeights,
                               double scaleFactor = 1.1,
                               int minNeighbors = 3, int flags = 0,
                               Size minSize = Size(),
                               Size maxSize = Size(),
                               bool outputRejectLevels = false );

Now compile it. You will be able to call detectMultiScale2 instead of detectMultiScale in Python as follows:

rects,r,w = cascade.detectMultiScale2(img, scaleFactor=1.3, minNeighbors=4, minSize=(30, 30), flags = cv2.CASCADE_SCALE_IMAGE,outputRejectLevels = 1)