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You should using Canny detect edges, this is simple. https://docs.opencv.org/3.4.3/d6/d56/samples_2cpp_2edge_8cpp-example.html#a3

You can normal it with HoughLines after detected edges.

You should using Canny detect edges, this is simple. https://docs.opencv.org/3.4.3/d6/d56/samples_2cpp_2edge_8cpp-example.html#a3

You can normal it with HoughLines after detected edges.

You should using Canny detect edges, this is simple. https://docs.opencv.org/3.4.3/d6/d56/samples_2cpp_2edge_8cpp-example.html#a3

You can normal it with HoughLines after detected edges.

Or can using the detect line, look like bellow.

Result

You should using Canny detect edges, this is simple. https://docs.opencv.org/3.4.3/d6/d56/samples_2cpp_2edge_8cpp-example.html#a3

You can normal it with HoughLines after detected edges.

Or can using the detect line, look like bellow.

Result

Or you want to recognize digits. you can referent bellow. https://www.pyimagesearch.com/2017/02/13/recognizing-digits-with-opencv-and-python/