# Python: Merge overlapping bounding boxes, replace by surrounding bounding box

I'm working on comparing bounding boxes and combining boxes that overlap too much. I got it working with this code I found, basically a non-max-suppression:

```
def non_max_suppression_fast(boxes, overlapThresh):
# if there are no boxes, return an empty list
if len(boxes) == 0:
return []
# if the bounding boxes integers, convert them to floats --
# this is important since we'll be doing a bunch of divisions
if boxes.dtype.kind == "i":
boxes = boxes.astype("float")
#
# initialize the list of picked indexes
pick = []
# grab the coordinates of the bounding boxes
x1 = boxes[:,0]
y1 = boxes[:,1]
x2 = boxes[:,2]
y2 = boxes[:,3]
# compute the area of the bounding boxes and sort the bounding
# boxes by the bottom-right y-coordinate of the bounding box
area = (x2 - x1 + 1) * (y2 - y1 + 1)
idxs = np.argsort(y2)
# keep looping while some indexes still remain in the indexes
# list
while len(idxs) > 0:
# grab the last index in the indexes list and add the
# index value to the list of picked indexes
last = len(idxs) - 1
i = idxs[last]
pick.append(i)
# find the largest (x, y) coordinates for the start of
# the bounding box and the smallest (x, y) coordinates
# for the end of the bounding box
xx1 = np.maximum(x1[i], x1[idxs[:last]])
yy1 = np.maximum(y1[i], y1[idxs[:last]])
xx2 = np.minimum(x2[i], x2[idxs[:last]])
yy2 = np.minimum(y2[i], y2[idxs[:last]])
# compute the width and height of the bounding box
w = np.maximum(0, xx2 - xx1 + 1)
h = np.maximum(0, yy2 - yy1 + 1)
# compute the ratio of overlap
overlap = (w * h) / area[idxs[:last]]
# delete all indexes from the index list that have
idxs = np.delete(idxs, np.concatenate(([last],
np.where(overlap > overlapThresh)[0])))
# return only the bounding boxes that were picked using the
# integer data type
return boxes[pick].astype("int")
```

But this code does not replace two overlapping boxes by a new box that is as big as the two overlapping boxes combined. It only deletes one of the two boxes and keeps the other one. How can I fix this? I can't figure out how to change the "delete" line appropriately. My idea was to make a new list and append new boxes to it, but I can't figure out how to change the code posted above. Another idea is to keep the delete-line and replace the remaining box parameters by the parameters of the new bounding box. I get the new parameters by this code:

```
xx1 = np.minimum(x1[i], x1[idxs[:last]])
yy1 = np.minimum(y1[i], y1[idxs[:last]])
xx2 = np.maximum(x2[i], x2[idxs[:last]])
yy2 = np.maximum(y2[i], y2[idxs[:last]])
```

Thanks in advance!

can you try to look up "intersection over union" ?

also, you could try to use existing functionality

Since I don't know how many rectangles will be above each other, or if boxes will overlap at all, I don't see how groupRectangles() would work in my case. Also, the rectangles in my example most probably won't have a similar size. How would you use it?

The intersecting over union method does exactly what I posted above, I think. It also saves the intersecting area, not the box that contains both overlapping boxes.