How to find contour of black regions in white background ?

asked 2020-03-02 04:21:49 -0600

I have image like this, I want to find contours of black regions. How can I do it ?
I use following code

                 im = cv2.imread(img)
                    plt.imshow(im)
                    print(type(im))
                    gray = cv2.cvtColor(im, cv2.COLOR_BGR2GRAY)
                    blurred = cv2.GaussianBlur(gray, (5, 5), 0)
                    thresh = cv2.threshold(im, 60, 255, cv2.THRESH_BINARY_INV)
                    cnts = cv2.findContours(thresh, cv2.RETR_EXTERNAL,cv2.CHAIN_APPROX_SIMPLE)

How ever it gives an error like "TypeError: Expected Ptr<cv::umat> for argument 'image' ". Can you help me solve it ? Thnaks image description

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answered 2020-03-02 04:58:24 -0600

berak
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updated 2020-03-02 11:36:38 -0600

please take a look at the docs, cv2.threshold returns a ret, image tuple, you have to use it like:

[edit] correct invocation, see comments below:

gray = cv2.cvtColor(im, cv2.COLOR_BGR2GRAY)
blurred = cv2.GaussianBlur(gray, (5, 5), 0)
value, thresh = cv2.threshold(blurred, 60, 255, cv2.THRESH_BINARY_INV)
# opencv 4.x:
cnts, hierarchy = cv2.findContours(thresh, cv2.RETR_EXTERNAL,cv2.CHAIN_APPROX_SIMPLE)
# opencv 3.x:
# im2, cnts, hierarchy = cv2.findContours(thresh, cv2.RETR_EXTERNAL,cv2.CHAIN_APPROX_SIMPLE)

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Thanks, It solved for that line but for the next line -" error: OpenCV(4.2.0) C:\projects\opencv-python\opencv\modules\imgproc\src\contours.cpp:197: error: (-210:Unsupported format or combination of formats) [Start]FindContours supports only CV_8UC1 images when mode != CV_RETR_FLOODFILL otherwise supports CV_32SC1 images only in function 'cvStartFindContours_Impl' ". I think I should change the mode of image, if it is right could you tell me to what ? thanks

kanankhan ( 2020-03-02 05:51:39 -0600 )edit

ah, easy: cv.threshold(gray,...) not cv.threshold(im,...)

then:

 >>>  help(cv.findContours)

for the next problem (return values from that), i can already see it, hehehehe ;)

berak ( 2020-03-02 06:01:38 -0600 )edit

Thanks, it solved my problem, I also laugh because it is first time that I use OPENCV xD

kanankhan ( 2020-03-02 06:08:07 -0600 )edit

@berak. It is not gray. It should be cv.threshold(blurred,...) not cv.threshold(gray,...). The reasoning is why. You needed to get rid of noises.before threshold.

supra56 ( 2020-03-02 07:52:01 -0600 )edit

^^ sure, right. at least NOT the color img ...

berak ( 2020-03-02 08:20:17 -0600 )edit

Thanks, I have one more problem, I want to get coordinates of the centers of the regions those I found. what can I use ? I am sorry today I started to use opencv thats why I dont have much experience in this field. Thanks in advance

kanankhan ( 2020-03-02 10:57:22 -0600 )edit

centers of the regions

either the center of the bounding box (geometric center)
or from moments() (center of mass)
even from summing up all points of the contour, and divide by N (another way to calculate center of mass)

we have nice tutorials here

berak ( 2020-03-02 11:14:25 -0600 )edit

Thanks I used moments like this:

              for c in range(0,len(cnts)):
                    cnt = cnts[c]
                    M = cv2.moments(cnt)

                    cx = int(M['m10']/M['m00'])
                    cy = int(M['m01']/M['m00'])

And it worked now I need to crop this region from image how can I do it using Opencv or I should use PIL library or it is just nmpy slicing ? which one will be easy ? Thanks

kanankhan ( 2020-03-03 02:01:24 -0600 )edit

I used OpenCV, but no third party.

supra56 ( 2020-03-03 08:05:57 -0600 )edit

Thanks, I used numpy slicing as suggested in OpenCV, my program , runs for the first image and it works perfectle as I expected but when it moves the second image it gives an error like: " error: OpenCV(4.2.0) C:\projects\opencv-python\opencv\modules\imgcodecs\src\loadsave.cpp:715: error: (-215:Assertion failed) !_img.empty() in function 'cv::imwrite' " could please help me how should I deal with it ? thanks

kanankhan ( 2020-03-03 14:44:41 -0600 )edit

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How to find contour of black regions in white background ?

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[edit] correct invocation, see comments below:

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[edit] correct invocation, see comments below:

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How to find contour of black regions in white background ?