rmedWB: Free-Support Median by Weiszfeld Update with Barycentric...

In T4transport: Tools for Computational Optimal Transport

Free-Support Median by Weiszfeld Update with Barycentric Projection

Description

For a collection of empirical measures \lbrace \mu_k\rbrace_{k=1}^K, the free-support Wasserstein median, a minimizer to the following functional

\mathcal{F}(\nu) = \sum_{k=1}^K w_k \mathcal{W}_2 (\nu, \mu_k ),

is computed using the OT-adapted version of the Weiszfeld algorithm using the barycentric projection as a means to recover an optimal displacement map.

Usage

rmedWB(atoms, marginals = NULL, weights = NULL, num_support = 100, ...)

Arguments

atoms	a length-K list where each element is an (N_k \times P) matrix of atoms.
marginals	marginal distributions for empirical measures; if NULL (default), uniform weights are set for all measures. Otherwise, it should be a length-K list where each element is a length-N_i vector of nonnegative weights that sum to 1.
weights	weights for each individual measure; if NULL (default), each measure is considered equally. Otherwise, it should be a length-K vector.
num_support	the number of support points M for the barycenter (default: 100).
...	extra parameters including abstol stopping criterion for iterations (default: 1e-6). maxiter maximum number of iterations (default: 10).

Value

a list with three elements:

support

an (M \times P) matrix of the Wasserstein median's support points.

weight

a length-M vector of median's weights with all entries being 1/M.

history

a vector of cost values at each iteration.

Examples

## Not run: 
#-------------------------------------------------------------------
#     Free-Support Wasserstein Median of Multiple Gaussians
#
# * class 1 : samples from N((0,0),  Id)
# * class 2 : samples from N((20,0), Id)
#
#  We draw 8 empirical measures of size 50 from class 1, and 
#  2 from class 2. All measures have uniform weights.
#-------------------------------------------------------------------
## GENERATE DATA
#  8 empirical measures from class 1
input_measures = vector("list", length=10L)
for (i in 1:8){
  input_measures[[i]] = matrix(rnorm(50*2), ncol=2)
}
for (j in 9:10){
  base_draw = matrix(rnorm(50*2), ncol=2)
  base_draw[,1] = base_draw[,1] + 20
  input_measures[[j]] = base_draw
}

## COMPUTE
#  compute the Wasserstein median
run_median = rmedWB(input_measures, num_support = 50)
#  compute the Wasserstein barycenter
run_bary   = rbaryGD(input_measures, num_support = 50)

## VISUALIZE
opar <- par(no.readonly=TRUE)

#  draw the base points of two classes
base_1 = matrix(rnorm(80*2), ncol=2)
base_2 = matrix(rnorm(20*2), ncol=2)
base_2[,1] = base_2[,1] + 20
base_mat = rbind(base_1, base_2)
plot(base_mat, col="gray80", pch=19)

#  auxiliary information
title("estimated barycenter and median")
abline(v=0); abline(h=0)

#  draw the barycenter and the median
points(run_bary$support, col="red", pch=19)
points(run_median$support, col="blue", pch=19)
par(opar)

## End(Not run)

T4transport documentation built on Jan. 11, 2026, 9:07 a.m.