title: Adaptive threshold of blurry image
body: <p>I have a fairly blurry 432x432 image of a Sudoku puzzle that doesn't adaptively threshold well (take the mean over a block size of 5x5 pixels, then subtract 2):</p>
  
  <p><img src="https://i.stack.imgur.com/NjcGn.png" alt="enter image description here"></p>
  
  <p>As you can see, the digits are slightly distorted, there are a lot of breakages in them, and a few 5s have fused into 6s and 6s into 8s.  Also, there's a ton of noise.  To fix the noise, I have to make the image even blurrier using a Gaussian blur.  However, even a fairly large Gaussian kernel and adaptive threshold blockSize (21x21, subtract 2) fails to remove all the breakages and fuses the digits together even more:</p>
  
  <p><img src="https://i.stack.imgur.com/NHZvS.png" alt="enter image description here"></p>
  
  <p>I've also tried dilating the image after thresholding, which has a similar effect to increasing the blockSize; and <a href="https://stackoverflow.com/questions/4993082/how-to-sharpen-an-image-in-opencv">sharpening the image</a>, which doesn't do much one way or the other.  What else should I try?</p>