Topology of the pixel connections (Sect. 2.5). Pixels of the red shape are 4-connected and thus the shape may represent a source. Pixels of the dark blue shape are not 4-connected: there are no paths connecting any pair of the blue pixels such that any neighboring pixels in the paths are connected to each other only by their sides. Note, however, that there are two 4-connected sub-areas in the blue shape, which could represent sources.
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