opencv+python: Assertion failure when findcontours

Question

I am working on an image processing project.

I am using opencv 2.4.11 + python 2.7

The code is simple as

def preprocessing(src):
    src = cv2.resize(src, (0,0), fx=6, fy=6)
    kernel = np.ones((5,5), np.uint8)

    dilate = cv2.dilate(src, kernel, iterations = 1)
    erode = cv2.erode(dilate, kernel, iterations = 1)

    cv2.imshow("dilate", dilate)
    cv2.imshow("erode", erode)

    th, thresh = cv2.threshold(erode, 200, 255, cv2.THRESH_BINARY)
    cv2.imshow("thresh", thresh)

    contours, hierarchy = cv2.findContours(thresh,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)
    cv2.drawContours(src,contours,-1,(0,255,0),3)

    cv2.imshow("src", src)
    cv2.waitKey()


print "==== start image processing ====="
im2 = cv2.imread("training1.png",cv2.CV_LOAD_IMAGE_GRAYSCALE)
preprocessing(im2)

But the findcontours returns assertion failure

The output is

OpenCV Error: Assertion failed (step[dims-1] == (size_t)CV_ELEM_SIZE(flags)) in create, file /tmp/opencv20160107-29960-t5glvv/opencv-2.4.12/modules/core/src/matrix.cpp, line 236
Traceback (most recent call last):
  File "process.py", line 55, in <module>
preprocessing(im2)
  File "process.py", line 44, in preprocessing
contours, hierarchy =  cv2.findContours(thresh,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)
cv2.error: /tmp/opencv20160107-29960-t5glvv/opencv-2.4.12/modules/core/src/matrix.cpp:236: error: (-215) step[dims-1] == (size_t)CV_ELEM_SIZE(flags) in function create

Can anyone help? Thank you very much

(a) The error message does not match the code shown. (b) The code, as shown, works fine for me. — John1024
– John1024, Commented May 7, 2016 at 6:21
@John1024 sorry, i just pick part of the code here. And what version of python and opencv are you using? I found the same question here stackoverflow.com/questions/32922944/… — Mix
– Mix, Commented May 7, 2016 at 6:26
@John1024 But how can it output the correct result from previous functions. And if I just remove the png from the directory. The cv2.resize() function will throw the exception. — Mix
– Mix, Commented May 7, 2016 at 7:13

tfv · Accepted Answer · 2016-05-07 06:41:46Z

1

The code works fine. The reason is that the image file is missing.

answered May 7, 2016 at 6:41

tfv

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1 Comment

Mix Over a year ago

But how did I get results from previous imshow() function? Is that a bug for opencv?

Mix · Accepted Answer · 2016-05-10 02:03:58Z

1

Solution

Unlink opencv from brew.
Download Anaconda package: http://continuum.io/downloads#all and install.
Install openCV: "conda install opencv" which is version 2.4.8.

If there is anaconda conflict with bottleneck, remove the reference and install it again by conda install command.

I still don't know why opencv 2.4.12 throws such exception, I guess it is because the numpy is conflict against opencv.

answered May 10, 2016 at 2:03

Mix

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Comments

Chandan Chaurasia · Accepted Answer · 2020-03-31 11:23:17Z

0

You need to convert the image into gray scale and then try contour .It worked for me

answered Mar 31, 2020 at 11:23

Chandan Chaurasia

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