How to detect and draw contours using OpenCV in Python?

Question

I wrote the following code to detect and draw contours:

img = cv2.imread('test2.tif');

if not img is None:
    imgray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY);
    ret,thresh = cv2.threshold(imgray,127,255,0);
    contours,hierarchy = cv2.findContours(thresh,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE);

    #draw a three pixel wide outline 
    cv2.drawContours(img,contours,-1,(0,255,0),3);

And here is the error I received:

Traceback (most recent call last): File "C:/Users/R.K.singh/Desktop/Image processing/intro-to-contours.py", line 10, in contours,hierarchy = cv2.findContours(thresh,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE); ValueError: too many values to unpack

What is wrong? I am using Python 2.7 and OpenCV 3.1.0

Jeru Luke · Accepted Answer · 2017-01-05 14:47:22Z

2

In order to emphasize Selchuk's point, the syntax involving OpenCV 3.x has changed a little bit. It has a different return value when it comes to cv2.findContours. It returns the following image, contours, hierarchy.

Previous versions of OpenCV, however, return only contours, hierarchy. They do not return the image.

answered Jan 5, 2017 at 14:47

Jeru Luke

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Community · Accepted Answer · 2017-01-05 15:03:20Z

2

Change the following line. You are using OpenCV 3.1.0 but you have coded using OpenCV 2.7.x.

(cnts, _) = cv2.findContours(thresh.copy(), cv2.RETR_TREE,
      cv2.CHAIN_APPROX_SIMPLE)

Also this link will help you.

edited Jan 5, 2017 at 15:03

CommunityBot

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answered Jan 5, 2017 at 13:54

user6542516

1 Comment

singhuist Over a year ago

what's edged? It isn't defined in my code. @selçuk-İlhan-aydi

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