a definition module configured to define for each local region one or more local black points, wherein {HEAD ~ each local black point} corresponds to a single darkest pixel or {TAIL ~ local cluster} of the darkest pixels in the given local region 
corresponds to
a definition module configured to define for each local region one or more local black points , wherein { HEAD ~ each local black point } corresponds to a single darkest pixel or { TAIL ~ local cluster } of the darkest pixels in the given local region
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH