Distance Between Two Point Patterns
Given two point patterns, find the distance between them based on optimal point matching.
pppdist(X, Y, type = "spa", cutoff = 1, q = 1, matching = TRUE, ccode = TRUE, precision = NULL, approximation = 10, show.rprimal = FALSE, timelag = 0)
- Two point patterns (objects of class
- A character string giving the type of distance to be computed.
"mat", indicating whether the algorithm should find the optimal matching based on "subpattern assignment", "
- The value $> 0$ at which interpoint distances are cut off.
- The order of the average that is applied to the interpoint distances.
Inf, in which case the maximum of the interpoint distances is taken.
- Logical. Whether to return the optimal matching or only the associated distance.
- Logical. If
FALSE, Rcode is used which allows for higher precision, but is much slower.
- Index controlling accuracy of algorithm. The
q-th powers of interpoint distances will be rounded to the nearest multiple of
10^(-precision). There is a sensible default which depends on
q = Inf, compute distance based on the optimal matching for the corresponding distance of order
approximation. Can be
Inf, but this makes computations extremely slow.
- Logical. Whether to display a plot showing the iterative solution of the restricted primal problem.
- Time lag, in seconds, between successive displays of the iterative solution of the restricted primal problem.
Computes the distance between point patterns
on finding the matching between them
which minimizes the average of the distances between matched points
q=1), the maximum distance between matched points
q=Inf), and in general the
q-th order average
1/qth power of the sum of
qth powers) of the distances between matched points.
Distances between matched points are Euclidean distances cut off at
the value of
type controls the behaviour of the algorithm if
the cardinalities of the point patterns are different. For the type
(subpattern assignment) the subpattern of the point pattern
with the larger cardinality $n$ that is closest to the point pattern
with the smaller cardinality $m$ is determined; then the
average is taken over $n$ values: the $m$ distances of matched points
and $n-m$ "penalty distances" of value
the unmatched points. For the type
"ace" (assignment only if
cardinalities equal) the matching is empty and the distance returned is equal
cutoff if the cardinalities differ. For the
"mat" (mass transfer) each point pattern is assumed
to have total mass $m$ (= the smaller cardinality) distributed evenly
among its points; the algorithm finds then the "mass transfer plan" that
q-th order weighted average of the distances, where
the weights are given by the transferred mass divided by $m$. The result is a fractional matching (each match of two points has a weight in $(0,1]$)
with the minimized quantity as the associated distance.
The computations for all three types rely heavily on a specialized
primal-dual algorithm (described in Luenberger (2003), Section 5.9)
for Hitchcock's problem of optimal transport of a product from a number
of suppliers to a number of (e.g. vending) locations. The C implementation
used by default can handle patterns with a few hundreds of points, but
should not be used with thousands of points. By setting
show.rprimal = TRUE,
some insight in the working of the algorithm can be gained.
For moderate and large values of
can be numerical issues based on the fact that the
q-th powers of
distances are taken and some positive values enter the optimization algorithm
as zeroes because they are too small in comparison with the larger values.
In this case the number of zeroes introduced is given in a warning message,
and it is possible then that the matching obtained is not optimal and the associated
distance is only a strict upper bound of the true distance.
As a general guideline (which can be very wrong in special situations) a small
number of zeroes (up to about 50 percent of the smaller point pattern
usually still results in the right matching, and the number can even be quite a bit
higher and usually still provides a highly accurate upper bound for the distance.
problems can be reduced by enforcing (much slower) Rcode via the
ccode = FALSE.
q = Inf there is no fast algorithm available, which is why approximation is
normally used: for finding the optimal matching,
set to the value of
resulting distance is still given as the maximum rather than the
q-th order average in the corresponding distance computation.
approximation = Inf, approximation is suppressed and a very inefficient
exhaustive search for the best matching is performed.
The value of
precision should normally not be supplied by the user. If
ccode = TRUE, this value is preset to the highest exponent of 10 that
the C code still can handle (usually $9$). If
ccode = FALSE, the value is
preset according to
q (usually $15$ if
q is small),
which can sometimes be changed to obtain less severe warning messages.
- Normally an object of class
pppmatchingthat contains detailed information about the parameters used and the resulting distance. See
pppmatching.objectfor details. If
matching = FALSE, only the numerical value of the distance is returned.
Hitchcock F.L. (1941), The distribution of a product from several sources to numerous localities, J. Math. Physics 20, 224--230.
Luenberger D.G. (2003). Linear and nonlinear programming. Second edition. Kluwer.
# equal cardinalities X <- runifpoint(100) Y <- runifpoint(100) m <- pppdist(X, Y) m plot(m) # differing cardinalities X <- runifpoint(14) Y <- runifpoint(10) m1 <- pppdist(X, Y, type="spa") m2 <- pppdist(X, Y, type="ace") m3 <- pppdist(X, Y, type="mat") summary(m1) summary(m2) summary(m3) m1$matrix m2$matrix m3$matrix # q = Inf X <- runifpoint(10) Y <- runifpoint(10) mx1 <- pppdist(X, Y, q=Inf)$matrix mx2 <- pppdist(X, Y, q=Inf, ccode=FALSE, approximation=50)$matrix mx3 <- pppdist(X, Y, q=Inf, approximation=Inf)$matrix ((mx1 == mx2) && (mx2 == mx3)) # TRUE if approximations are good