The Minimum Blur filter shrinks highlight regions in the image, and broadens darker areas. It works by comparing each pixel to its neighbour and replaces lighter pixels with darker pixels. This is most useful for modifying masks.
Settings
The following settings can be adjusted on the context toolbar:
Radius—controls intensity of the filter.
Circular—if this option is off (default), darker regions expand to form square areas. When selected, expanded darker regions are circular.