This filter is found in
It works on color borders. It spreads pixels that have a Value between selected thresholds, in selected directions.
The examples will be about the following image (zomm x8):
More White: Pixels will be propagated from upper Value pixels towards lower Value pixels. So bright areas will enlarge.
More Black: Pixels will be propagated from lower Value pixels towards upper Value pixels. So dark areas will enlarge.
Middle Value to Peaks: On a border between the selected thresholds, it blends the two colors.
Foregound to Peaks: The propagated areas will be filled with the foreground color of the toolbox.
Only Foreground: Only areas with the Foreground color will propagate.
Only Background: Only areas with the Background color will propagate.
Lower Threshold and Upper Threshold: These commands allow you to set the Value range that will be concerned by Propagate.
Propagating Rate: That's the propagating amount. The higher it will be the more colored the propagation will be.
Propagating Direction: You can select one or more directions.