Sample animation....(14Mb - it may take a while to load)

Six seed points were selected by hand, and the fast marching algorithm is run. It's like a flood fill, but is more clever in selecting which direction to extend the border - it tastes the neighbourhood of the entire border and picks the path of least resistance, going faster in "flat" areas than areas where the intensity changes rapidly.

Note that by construction this is finding 6 objects, and flooding around local intracellular densities