compatibility issue with contourArea in openCV 3

问题:

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\n'

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)

回答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)

回答2:

This problem is caused by the different return value of cv2.findContours in different OpenCV versions.

In OpenCV 4.0.0, this error may looks like cv2.error: OpenCV(4.0.0) C:\projects\opencv-python\opencv\modules\imgproc\src\convhull.cpp:137: error: (-215:Assertion failed) total >= 0 && (depth == CV_32F || depth == CV_32S) in function 'cv::convexHull'

You can find a detailed explanation and solutions here: How to use `cv2.findContours` in different OpenCV versions?

回答3:

Depending on the OpenCV version, cv2.findContours() has varying return signatures.

In OpenCV 3.4.X, cv2.findContours() returns 3 items

image, contours, hierarchy = cv.findContours(image, mode, method[, contours[, hierarchy[, offset]]])

In OpenCV 2.X and 4.1.X, cv2.findContours() returns 2 items

contours, hierarchy = cv.findContours(image, mode, method[, contours[, hierarchy[, offset]]])

You can easily obtain the contours regardless of the version like this:

cnts = cv2.findContours(image, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
cnts = cnts[0] if len(cnts) == 2 else cnts[1]

回答4:

Thanks for @ZdaR; By the way, you can do the following in OpenCV 4.1:

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