python opencv TypeError: Layout of the output array incompatible with cv::Mat

I'm using the selective search here: http://koen.me/research/selectivesearch/ This gives possible regions of interest where an object might be. I want to do some processing and retain only some of the regions, and then remove duplicate bounding boxes to have a final neat collection of bounding boxes. To discard unwanted/duplicated bounding boxes regions, I'm using the `grouprectangles` function of opencv for pruning. Once I get the interesting regions from Matlab from the "selective search algorithm" in the link above, I save the results in a `.mat` file and then retrieve them in a python program, like this: ``` import scipy.io as sio inboxes = sio.loadmat('C:\\PATH_TO_MATFILE.mat') candidates = np.array(inboxes['boxes']) # candidates is 4 x N array with each row describing a bounding box like this: # [rowBegin colBegin rowEnd colEnd] # Now I will process the candidates and retain only those regions that are interesting found = [] # This is the list in which I will retain what's interesting for win in candidates: # doing some processing here, and if some condition is met, then retain it: found.append(win) # Now I want to store only the interesting regions, stored in 'found', # and prune unnecessary bounding boxes boxes = cv2.groupRectangles(found, 1, 2) # But I get an error here ``` The error is: ``` boxes = cv2.groupRectangles(found, 1, 2) TypeError: Layout of the output array rectList is incompatible with cv::Mat (step[ndims-1] != elemsize or step[1] != elemsize*nchannels) ``` What's wrong? I did something very similar in another piece of code which gave no errors. This was the error-free code: ``` inboxes = sio.loadmat('C:\\PATH_TO_MY_FILE\\boxes.mat') boxes = np.array(inboxes['boxes']) pruned_boxes = cv2.groupRectangles(boxes.tolist(), 100, 300) ``` The only difference I can see is that `boxes` was a numpy array which I then converted to a list. But in my problematic code, `found` is already a list.