The answer for this, like many other questions, is "it depends". Humans don't do it like this at all -- we try to recognize the subject first, then find it's edges. (I suspect.) It's difficult to define what the subject /is/ in many photographs. The best way to find a method is probably to load up your stuff into an image editor, try it's various edge-detecting methods, and decide which one does the best job. Then either find a method to simply use the graphics program in question (Hint: Gimp), or do some research and find out what method the graphics program used, and how to program it in perl (Hint: PDL).

The problem is really /much/ harder then you think, though. Look at The Kiss (Safe For Work, unless you have a very strict work), for example. It's even already monochrome. The hardest border, though, is between the girls' hair and the bed. Does that mean their hair is the subject of the photograph? No, in order to find the subject, you need to follow the borders of both girls -- even where there isn't much contrast, such as between the white shirts (esp on the girl on the right) and panties and the white bed.

Update: better link.

Warning: Unless otherwise stated, code is untested. Do not use without understanding. Code is posted in the hopes it is useful, but without warranty. All copyrights are relinquished into the public domain unless otherwise stated. I am not an angel. I am capable of error, and err on a fairly regular basis. If I made a mistake, please let me know (such as by replying to this node).

In reply to Re: the difference between two colors, and how to describe a color by theorbtwo
in thread the difference between two colors, and how to describe a color by pg

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