Suppose we have to come up with some kind of function that defines how different two probability distributions are. One such function is the Kullback-Leibler divergence. It is an asymmetric function: it gives a different value for probability distribution $A$ given probability distribution $B$ versus the other way around. It is henceforth not a true distance (which is symmetric), but a so-called divergence. A divergence also does not satisfy the “triangle inequality”: $$D(x + y) \leq D(x) + D(y)$$ is not necessarily true for all $x$ and $y$. It does satisfy however two other important conditions. A divergence is always zero or larger and the divergence is only zero if and only if $$x = y$$.

## Earth Mover’s distance

There is another function that defines how different two probability distributions are. It is called the Earth Mover’s Distance. This name has been suggested by Jorge Stol when he was working with CAD programs that have a function to compute the optimal earth displacement from roadcuts to roadfills. The concept is much older though (from Gaspard Monge in 1781, more than two centuries ago).

## 1st Wasserstein distance

Mathematically this function is equivalent to the 1st Wasserstein distance. The 1st Wasserstein distance between two probability measures $\mu$ and $\nu$ is defined as:

$W(\mu, \nu) = \inf_{\lambda \in \Gamma(\mu,\nu)} \int_{M \times M} d(x,y) d\lambda(x,y)$

where $\Gamma(\mu,\nu)$ is the collection of all measures $M \times M$ with marginals $\mu$ and $\nu$. This set $\Gamma(\mu,\nu)$ is called the set of all couplings of $\mu$ and $\nu$. We can write it also as:

$W(\mu, \nu) = \inf_{\lambda \in \Gamma(\mu,\nu)} \int_\mu \int_\nu d(x,y) \lambda(x,y) dx dy$

where we assume $\lambda$ to be well-behaved.

Informally, given all kind of ways to send over “stuff” from position $x$ to position $y$ we find the one that minimizes (mathematically fancy the infimum) the distance $d(x,y)$ so that everything is moved. Note, that $\mu$ and $\nu$ are probability measures. They sum to one. There is no mass lost or gained in the process.

We can use the Wasserstein distance to calculate the difference between point clouds. The above distance becomes now discrete:

$W(\mu, \nu) = \min_{\lambda \in \Gamma(\mu,\nu)} \sum_{x \in \mu} \sum_{y \in \nu} d(x,y) \lambda(x,y)$

## Assignment problem

This can also be formulated as an assignment problem or a matching problem on a (complete) bipartite graph. The nodes $x \in \mu$ on the left, the nodes $y \in \nu$ on the right. The edges between the nodes in $\mu$ and the nodes in $\nu$ have weights corresponding to their distance $d(x,y)$. Now, define a mapping $\lambda$ that minimizes $$W(\mu,\nu)$$.

We can write the constraints on that indicate that each vertex is adjacent to exactly one edge:

$\sum_{x \in \mu} \lambda(x,y) = 1 \quad \text{for} \quad y \in \nu$ $\sum_{y \in \nu} \lambda(x,y) = 1 \quad \text{for} \quad x \in \mu$ $0 \leq \lambda(x,y) \leq 1 \quad \text{for} \quad x \in \mu, y \in \nu$ $\lambda(x,y) \in \mathbb{Z} \quad \text{for} \quad x \in \mu, y \in \nu$

The last constraint can actually be removed. When we solve it without this constraint, we will end up with an optimal solution that satisfies this condition nevertheless.

## Gromov-Wasserstein

The Gromov-Wasserstein distance can compare point clouds of different dimension. Suppose we have a point cloud A in 2D and a point cloud B in 3D, it can assign a distance to it. How is this possible? The distance between two points in any dimension is just a scalar. Hence, if we do not work with the point coordinates themselves, but only with the pairwise distances between points, we can minimize a function between the pairwise distances in A and the pairwise distances in B.

# Challenge

Now, the kind of distance we would like to formulate is one that can be used for multiple objects.