im_zerox - find the zero crossings of an image
int im_zerox(in, out, flag)
IMAGE *in, *out;
im_zerox() detects the positive and negative edges of zero crossings of
an image held by the image descriptor in, depending on the flag.
If flag is -1 the negative zero crossings are returned. If flag is 1
the positive zero crossings are returned.
The output image is byte with zero crossing set to 255 and all other
values set to zero. Input can have any number of channels, and be any
The function returns 0 on success and -1 on error.
N. Dessipris - 14/05/1991
14 May 1991