im_sign - find the unit vector in the direction of value
int im_sign( in, out )
IMAGE *in, *out;
im_sign(3) finds the unit vector in the direction of the pixel value.
For non-complex images, it returns a signed char image with values -1,
0, and 1 for negative, zero and positive pixels. For complex images it
returns a complex normalised to length 1.
The function returns 0 on success and -1 on error.
im_abs(3), im_cmulnorm(3), im_c2amph(3)