Shadows interfering with image processing

Question

I am currently working on processing a tic tac toe board to have a robot play a human user. As the lighting changed throughout the day, the image became more difficult to process. Is there a way to avoid this and only get the pen lines?

Answer 1

Here's one way:

img=Import["https://i.stack.imgur.com/LBVmD.jpg"]
Dilation[EdgeDetect[img], 2]