Given two point patterns in three-dimensional space,
this function computes the Euclidean distance from each point
in the first pattern to each point in the second pattern,
and returns a matrix containing these distances. This is a method for the generic function crossdist
for three-dimensional point patterns (objects of class "pp3").
This function expects two
point patterns X and Y, and returns the matrix
whose [i,j] entry is the distance from X[i] to
Y[j].
Alternatively if periodic=TRUE, then provided the windows
containing X and Y are identical and are rectangular,
then the distances will be computed in the `periodic'
sense (also known as `torus' distance): opposite edges of the
rectangle are regarded as equivalent.
This is meaningless if the window is not a rectangle.