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isContourConvex always returns "False"?

asked 2018-07-06 06:54:07 -0500

Maren gravatar image

updated 2018-07-06 07:06:25 -0500

I have tried to use cv2.isContourConvex on several different shapes, but I get "False" all the time. I think I might have misunderstood how cv2.isContourConvex is working. Can anyone help?

Here is how I have tried to use it on the image below (which should be convex, right?):

img = cv2.imread('convex.png')

ret, thresh = cv2.threshold(img_gray,0,255,cv2.THRESH_BINARY+cv2.THRESH_OTSU)
image,contours, hierarchy = cv2.findContours(thresh,cv2.RETR_TREE,cv2.CHAIN_APPROX_NONE)
cnt = contours[0]

print cv2.isContourConvex(cnt)

image description

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answered 2018-07-06 07:13:56 -0500

LBerger gravatar image

updated 2018-07-06 07:25:03 -0500

which should be convex, right? NO

Convex : If you choose two points inside shape then All points of line segment defined by end points belong to shape.

your shape : image description

this segment proove that shape is not convex

image description

Discrete geometry : should convex right ? I'm agree but reality it is not convex

this one is convex :

Point p1(10,10),p4(10,100), p3(100,100), p2(100,10) ;
vector<Point> ctr;


if (isContourConvex(ctr))
    cout << "Contour is convex\n";
    cout << "Contour is not convex\n";
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Oh I see, thank you! :) Will all images give the result "False" then because of pixelation? And is it any other method I could use to check if shapes in images is convex or not, without getting the problem with pixels?

Maren gravatar imageMaren ( 2018-07-06 07:37:06 -0500 )edit

I gave you an example of convex shape. Now you should use convexHull. You can compare surface of convex hull and contours. tutorial here

In convexhull results you gan get distance between contour and convex hull.

LBerger gravatar imageLBerger ( 2018-07-06 07:44:11 -0500 )edit
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Last updated: Jul 06 '18