The filter subtracts the original image from the closed one (“Black Hat” transformation). In fact, it shows areas which are darker than others and is used for their isolation.
Filter Black Hat transformation is suited to marking the details in halftone images, for example, by restoring the images, etc. You can use Top Hat and Black Hat transformations together for increasing the contrast.
Size (size) parameter determines the area of filter influence around each point.
Iteration (iteration) parameter determines the amount of iterations.