title: How to detect a grainy or faded image?
body: <p>I've seen questions about detecting blurry images, but what about faded/grainy images. I have a large dataset of scanned passport-style portrait photos, and a number of them are old, hence looking faded and grainy (i.e hard to recognize the person).</p>
  
  <p>Image quality metrics like BRISQUE and blur detection [<a href="https://www.pyimagesearch.com/2015/09/07/blur-detection-with-opencv/?submitted_comment=1#comment-523789" rel="nofollow noreferrer">link</a>] didn't work so well and were inconsistent. The criteria for classification would be whether the photo was good enough for an average person to tell who the person was from the image.
  So I tried face detection (HOG, etc), but it recognizes images where it's pretty much impossible to tell who the person is. 
  Ideally I'm looking for suggestions that is somewhat lightweight. <a href="https://i.stack.imgur.com/XsMpt.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/XsMpt.png" alt="enter image description here"></a></p>