How to merge contours in opencv?

问题

Ok guys I have been working on this project for quite some time now.

I am building this bot that plays the chrome dinosaur game. So I tried other methods to detect the characters like matchTemplate and even made my own algorithm to locate the objects, but I like this one (findcontours) the most.

Here's what I have:

Can anyone help me find out how I should merge the two rectangles of the cacti?

img = screen_cap()
roi = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
ret, thresh = cv2.threshold(roi,127, 255, 0)
im2, contours, hierarchy = cv2.findContours(thresh, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
first = True
for cnt in contours:
    area = cv2.contourArea(cnt)
    if area > 200: #filtering contours
        x,y,w,h = cv2.boundingRect(cnt)
        if w/h < 4: # filtering even more
            cv2.rectangle(img,(x,y),(x+w,y+h),(255,0,0),2)

回答1:

This is an old question and seems like it has not been answered properly yet (apologies to fellow SOers who did it partially in comments though). To me it seems like the questioner has a two-part question:

Is there a opencv function that merges two rectangles?

Answer to this question is yes and no. Let me be clear; yes if you are using opencv C++ bindings. Simple & can be used to take a union and | for intersection of two rects. But Python bindings lack those functions.

How to then do it in Python?

def union(a,b):
    x = min(a[0], b[0])
    y = min(a[1], b[1])
    w = max(a[0]+a[2], b[0]+b[2]) - x
    h = max(a[1]+a[3], b[1]+b[3]) - y
    return (x, y, w, h)

def intersection(a,b):
    x = max(a[0], b[0])
    y = max(a[1], b[1])
    w = min(a[0]+a[2], b[0]+b[2]) - x
    h = min(a[1]+a[3], b[1]+b[3]) - y
    if w<0 or h<0: return () # or (0,0,0,0) ?
    return (x, y, w, h)
    # Please remember a and b are rects.

Source Code Credit: OpenCV union and intersaction on rects

回答2:

Sorry to be a little late to the party. However if I google "merge opencv contours" I find this; and I think there should be an answer.

You can merge any two contours by one of those recipes:

Get a list of points of each contour
append them
force them into cv2 contour format
get cv2.convexHull of that if you don't care too much about the details.

If you don't like the result of convexHull because the concave parts of the contours are important, then follow this recipe instead:

Get a list of points of each contour
append them
get a common center
sort all the points in clockwise order around the center
force them into cv2 contour format

If the two contours have a lot of concave shapes in them, this could yield a zigzag-pattern as the recipe goes through both contours disregarding their original structure. If that's the case, you need to follow a third recipe:

Get a list of points of each contour
get a common center
delete points of each contour which are inside the other contour
find the point which is closest to the common center in each contour.
go through the first contour as it is listed until you hit the closest point.
Then switch to the other list, starting from the closest point you go clockwise through the other contour until it is used up
switch back to the first contour and append the rest of their points.
force them into cv2 contour format

The next-more-complicated case is if you have multiple intersections between the contours and you want to preserve the holes between the two. Then it's better to make a black image and draw the contours in white via cv2.fillPoly(); then get the contours back out via cv2.findContours()

I sketched some steps for the first two recipes here

get a list of points of each contour:

import cv2
list_of_pts = [] 
for ctr in ctrs_to_merge
    list_of_pts += [pt[0] for pt in ctr]

order points clockwise

I use the function of this really great posting of MSeifert to order the points in clockwise order

center_pt = np.array(list_of_pts).mean(axis = 0)
list_of_pts = sorted(list_of_pts, key=clockwiseangle_and_distance)

force a list of points into cv2 format

import numpy as np
ctr = np.array(list_of_pts).reshape((-1,1,2)).astype(np.int32)

merge them with `cv2.convexHull` instead

If you use this then there is no need to order the points clockwise. However, the convexHull might lose some contour properties because it does not preserve concave corners of your contour.

# get a list of points
# force the list of points into cv2 format and then
ctr = cv2.convexHull(ctr) # done.

I think the function to merge two contours should be content of the opencv library. The recipe is quite straightforward and it is sad that many programmers using opencv will have to boilerplate code this.

来源：https://stackoverflow.com/questions/44501723/how-to-merge-contours-in-opencv

标签

python

OpenCV

artificial-intelligence