Get darker lines of an image using opencv
<p>How can we remove lighter lines of an image and get only darker lines?</p>

<p>There is a sudoku game in the following image (taken from <a href="http://docs.opencv.org/3.1.0/d7/d4d/tutorial_py_thresholding.html" rel="nofollow noreferrer">here</a>). Adaptive Gaussian Thresholding gives really good result but it includes gray (lighter) lines too.</p>

<p><em>Note: I tried canny but it removes numbers too.</em></p>

<p><a href="https://i.stack.imgur.com/ttehZ.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ttehZ.jpg" alt="ee"></a></p>