title: Detecting scratch on image with much noise
body: <p>I am having problem to detect scratch on these images. Actually, it is very easy to see by human eyes. However, when applying some algorithms, there are a lot of noise and I couldn't extract the scratch only.</p>
  
  <p>Here are these images:
  <a href="https://i.stack.imgur.com/Kn9wT.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/Kn9wT.jpg" alt="enter image description here"></a></p>
  
  <p><a href="https://i.stack.imgur.com/LriFW.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/LriFW.jpg" alt="enter image description here"></a></p>
  
  <p>At present, I tried some kinds of filter (smoothing, average, median, Gaussian filter or Sobel edge detector) to erase noise and detect scratch but they don't help much.
  Could you suggest some idea?
  Some tools or algorithms that I should consider?</p>