A detector for detecting the foreground or background vector near a
discontinuity in a vector field has a covering-uncovering detector (3)for
determining whether covering or uncovering situation exists. The detector
further has calculating means (1,2,4) for calculating at the position
x.sub.1 of the discontinuity:
a first position x.sub.a in the previous (covering) or next (uncovering)
vector field by shifting x.sub.1 over the first vector at one side of the
discontinuity,
a second position x.sub.b in the previous (covering) or next (uncovering)
vector field by shifting x.sub.1 over the second vector at the other side
of the discontinuity
and a third intermediate position between x.sub.a and x.sub.b.
Furthermore, the detector has means (5) for fetching the background vector
with v.sub.av at the third position in the previous (covering) or next
(uncovering) vector field.
