Detecting a stain
Hello.
I'm trying to detect a stain in an image. The original image has a marble background and the stain its almost the same color, sometimes the stain is darker and sometimes brighter than the marble background.
background. See some examples:
http://upload.wikimedia.org/wikipedia/commons/f/f3/SierraXenolith.JPG
http://us.123rf.com/400wm/400/400/losalex/losalex0711/losalex071100030/2057779-marble-structure-a-stain-a-strip-stone.jpg
http://image2.yell.ru/responses/5/7/1/r_670743_pr8rse37rffkijk9skb7.jpg
https://lh3.googleusercontent.com/-48VlxENbnkY/SjXKwlNTtyI/AAAAAAAABGs/fFIwLb21bu8/s640/Texture%20-%20Granite.JPG
http://us.123rf.com/400wm/400/400/lobanov/lobanov1108/lobanov110800013/10391208-a-texture-of-marble-with-rust-stains.jpg
I would like to know if there is a function that can detect such stain. The difficulty is distinguish the stain from the marble texture background. Can somebody help me?
Thanks.
Rafhael