Adherent raindrop detection - Level Sets

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I'm attempting to detect adherent raindrops on a camera's field of view. Intensity change over time might be a good feature for detecting adherent rain drops, since intensity will be "more constant" on water droplets area. See image bellow.

On the right there is the intensity change image, and the darker circular-ish areas correspond to adherent raindrop areas.

How would I proceed to locate and segment these areas? Any thoughts? I've read something about Level Sets, which are ways to segment image regions according to their intensity, but I'm really clueless as to how to implement such technique. Any thoughts?

Other ways to detect adherent raindrops are also welcome.