Sonam: I think the third condition is your problem. It's impossible for any "thick" structure, and will only be satisfied for regions that are already a line and can't be thinned anymore at all. Think about it, you are basically removing layer by layer of the outer perimeter until you get a line. If you have a cigar shape, it will get shortened - how could it not? It cannot go all the way out to the tips like it used to. If you need that, then that is a special, non-standard morphological operation that you need to do with custom code. For example, you could look at the last few pixels near the line endpoint and extrapolate (say with a line or quadratic) that a long way (long enough to poke outside your original shape), then AND (&) that image with your original binary image.
