int im_dif_std(im, xp, yp, xs, ys, dx, dy, mean, std)
IMAGE *im; int xp, yp, xs, ys; int dx, dy; double *mean, *std;
calculates the mean and the standard deviation of the difference image
created by the displacement vector (dx,dy) on the box (xp,yp;xs,ys) defined
on the image pointed by im. More specifically the difference image is
of an image given the dispacement vector (dx, dy) is defined as follows:
For each point of the original image, the start of the vector (dx,dy) is
set on that point. The difference image at this point is defined as the
difference of the values pointed by the vector (end value - start value).
The function returns the mean and the standard deviation of the
difference images measured on the area (xp,yp;xs,ys) of im.
Input im should be one band unsigned char image and it should
have been set by a call to im_mmapin(3) or im_setbuf(3).