Sliced Wasserstein barycenter and gradient flow with PyTorch

In this exemple we use the pytorch backend to optimize the sliced Wasserstein loss between two empirical distributions [31].

In the first example one we perform a gradient flow on the support of a distribution that minimize the sliced Wassersein distance as poposed in [36].

In the second exemple we optimize with a gradient descent the sliced Wasserstein barycenter between two distributions as in [31].

[31] Bonneel, Nicolas, et al. “Sliced and radon wasserstein barycenters of measures.” Journal of Mathematical Imaging and Vision 51.1 (2015): 22-45

[36] Liutkus, A., Simsekli, U., Majewski, S., Durmus, A., & Stöter, F. R. (2019, May). Sliced-Wasserstein flows: Nonparametric generative modeling via optimal transport and diffusions. In International Conference on Machine Learning (pp. 4104-4113). PMLR.

# Author: Rémi Flamary <remi.flamary@polytechnique.edu>
#
# License: MIT License

# sphinx_gallery_thumbnail_number = 4

Loading the data

import numpy as np
import matplotlib.pylab as pl
import torch
import ot
import matplotlib.animation as animation

I1 = pl.imread('../../data/redcross.png').astype(np.float64)[::4, ::4, 2]
I2 = pl.imread('../../data/tooth.png').astype(np.float64)[::4, ::4, 2]

sz = I2.shape[0]
XX, YY = np.meshgrid(np.arange(sz), np.arange(sz))

x1 = np.stack((XX[I1 == 0], YY[I1 == 0]), 1) * 1.0
x2 = np.stack((XX[I2 == 0] + 60, -YY[I2 == 0] + 32), 1) * 1.0
x3 = np.stack((XX[I2 == 0], -YY[I2 == 0] + 32), 1) * 1.0

pl.figure(1, (8, 4))
pl.scatter(x1[:, 0], x1[:, 1], alpha=0.5)
pl.scatter(x2[:, 0], x2[:, 1], alpha=0.5)
plot sliced wass grad flow pytorch

Out:

<matplotlib.collections.PathCollection object at 0x7f5dbdc46c10>

Sliced Wasserstein gradient flow with Pytorch

device = "cuda" if torch.cuda.is_available() else "cpu"

# use pyTorch for our data
x1_torch = torch.tensor(x1).to(device=device).requires_grad_(True)
x2_torch = torch.tensor(x2).to(device=device)


lr = 1e3
nb_iter_max = 100

x_all = np.zeros((nb_iter_max, x1.shape[0], 2))

loss_iter = []

# generator for random permutations
gen = torch.Generator()
gen.manual_seed(42)

for i in range(nb_iter_max):

    loss = ot.sliced_wasserstein_distance(x1_torch, x2_torch, n_projections=20, seed=gen)

    loss_iter.append(loss.clone().detach().cpu().numpy())
    loss.backward()

    # performs a step of projected gradient descent
    with torch.no_grad():
        grad = x1_torch.grad
        x1_torch -= grad * lr / (1 + i / 5e1)  # step
        x1_torch.grad.zero_()
        x_all[i, :, :] = x1_torch.clone().detach().cpu().numpy()

xb = x1_torch.clone().detach().cpu().numpy()

pl.figure(2, (8, 4))
pl.scatter(x1[:, 0], x1[:, 1], alpha=0.5, label='$\mu^{(0)}$')
pl.scatter(x2[:, 0], x2[:, 1], alpha=0.5, label=r'$\nu$')
pl.scatter(xb[:, 0], xb[:, 1], alpha=0.5, label='$\mu^{(100)}$')
pl.title('Sliced Wasserstein gradient flow')
pl.legend()
ax = pl.axis()
Sliced Wasserstein gradient flow

Animate trajectories of the gradient flow along iteration

pl.figure(3, (8, 4))


def _update_plot(i):
    pl.clf()
    pl.scatter(x1[:, 0], x1[:, 1], alpha=0.5, label='$\mu^{(0)}$')
    pl.scatter(x2[:, 0], x2[:, 1], alpha=0.5, label=r'$\nu$')
    pl.scatter(x_all[i, :, 0], x_all[i, :, 1], alpha=0.5, label='$\mu^{(100)}$')
    pl.title('Sliced Wasserstein gradient flow Iter. {}'.format(i))
    pl.axis(ax)
    return 1


ani = animation.FuncAnimation(pl.gcf(), _update_plot, nb_iter_max, interval=100, repeat_delay=2000)