title: what is the simplest/most efficient way of detecting black lines in image?
body: <p>I have a collection of signal images with 
  signal as a black line, grid of red lines in the background, and black letters and numbers that are of no importance to me.</p>
  
  <p>I need to extract the black curvy signal line as a time series, and completely ignore the letters and numbers.
  Something like the graph below, but printed and scanned, therefore not so crisp. </p>
  
  <p>Ideally, the output should be a CSV of the black line.</p>
  
  <p>...26.931, 24.740, 25.806, 24.364, 24.477, 23.901, 23.175, 23.227...</p>
  
  <p>Is there an off the shelf method? what is the simplest/easiest way?<a href="https://i.stack.imgur.com/GRA7N.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/GRA7N.png" alt="enter image description here"></a></p>