you just did not see the next argument to drawContours, there is 'thickness' . if you set it to -1, your contour will get filled even.
opencv line size of drew contour
Question
I would like to draw contours thicker on an image on iOS, but I could not see any option in function cv::drawContours(<#InputOutputArray image#>, <#InputArrayOfArrays contours#>, <#int contourIdx#>, <#const Scalar &color#>)
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Solution
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