zero_crossing — Extract zero crossings from an image.
zero_crossing returns the zero crossings of the input
image as a region. A pixel is accepted as a zero crossing if its
gray value (in
Image) is zero, or if at least one of its
neighbors of the 4-neighborhood has a different sign.
This operator is intended to be used after edge operators returning
the second derivative of the image (e.g.,
which were possibly followed by a smoothing operator. In this case,
the zero crossings are (candidates for) edges.
→object (int1 / int2 / int4 / real)
zero_crossing usually returns the value 2 (H_MSG_TRUE). If
necessary, an exception is raised.