Don't have any handy, but look in books about  machine vision.

I know that commercial GIS applications have tools for doing this for finding
stream centerlines.  You may try searching for algorithms to do stream
centerline determination.  I don't know of free/open source code to do that.

I don't know of any pure C# libs. We had similar problems and solved
them just by wrapping IPPi (Intel Performance Primitives for image
processing) and OpenCV which have plenty of low- and high level image
processing functionality (i.e. morphological operations, distance
calculation) which you could use to implement thinning/skeletonizing
very easily and with high performance!

hth,
-- Henon
------
http://www.eqqon.com/index.php/User:Henon

I wrote a similar algorithm for identifying edges of coastline,
and finding areas wich were 100km from any large water body,
this was for radio propogation and interference purposes.

it just found any large body of water wich would be like your white space,
then then took a walk from a point in the water till it hit land
then id a walk along the coast with a simple algorithm of trying
all the 9 adjacent pixels in a clockwise order from the last
position till it found a land pixel again, and so on.

was quite interesting work, was pushing the limits of a pc at the time
I needed 32mb of ram, my boss gasped saying his hard drive was only 20mb lol

Colin =^.^=