python - compatibility issue with contourArea in openCV 3

Question

I am trying to do a simple area calculation of contours I get from findContours. My openCv version is 3.1.0

My code is:

cc = cv2.findContours(im_bw.copy(), cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
cv2.contourArea(cc[0])

error: 'C:\builds\master_PackSlaveAddon-win32-vc12-static\opencv\modules\imgproc\src\shapedescr.cp...: error: (-215) npoints >= 0 && (depth == CV_32F || depth == CV_32S) in function cv::contourArea
'

Cant seem to solve it, I have a feeling its just typecasting altough I expect the findContours result to match the type of contourArea

Thanks :)

EDIT: turns out I need to take the 2nd argument of findContours

 im2, cc, hierarchy = cv2.findContours(im_bw.copy(), cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)

Answer 1

In Opencv 3 API version the cv2.findContours() returns 3 objects

• image
• contours
• hierarchy

So you need to rewrite your statement as:

image, contours, hierarchy = cv2.findContours(im_bw.copy(), cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)