That's interesting. But only 1/4 of the information required to tackle the problem.

Only one pair of images, and no reference as to you would rate them--are they 2% similar or 98% similar?

Isolating the pink in the images is relatively simple: cluster1 & cluster8.

One problem is that the graphs have been produced using anti-aliasing. A process that trades accuracy for aesthetic appearance. Which means that if you try to subtract one image from another you get a big blurry streak of color: cluster1 - cluster8

With some more processing it should be possible to retrieve a single, contiguous line from those fat, fuzzy, discontinuous streaks. Albiet that it might take several passes to do so.

But the show stopper as far as achieving your single similarity figure is the lack of any indication of how to rate the two posted images for similarity.

It is obvious that if the two lines, once reduced to continuous, single pixel wide lines, exactly overlaid each other, that would constitute 100% similarity. But how do you rate divergence?

Possibilities:

1. You might consider any two pixels that do not exactly align a percentage point of divergence.

   That is, in the following 10 pixel wide overlay:

   x        o
    x      o 
     x    o 
     oxoxox
    o      x
   o        x
   [download]

   Because X positions 4 through 7 overlap, that would be 40% similarity.

2. Or, you might consider not only the fact that two pixels diverge, but also by how much they diverge.

   Meaning that this overlay would rate as having a higher similarity than the one above:

   
    
   xxx    ooo 
     oxoxox
   oo      xx
   [download]

   Even though they overlap to the same extent.

   Meaning that:

   
   
   xxxxxxxxxx
   oooooooooo
   [download]

   Would have a much higher simlilarity rating than:

   xxxxxxxxxx
   
   
   
   oooooooooo
   [download]

   Despite there being no overlap at all.

So then the problem is, what do you consider to be 0% similarity? (Which is why I asked for two pairs of images and associated ratings in the first place.)