# The World is Continuously Varying

Imagine we want to train a self-driving car in New York so that we can take it
all the way to Seattle without tediously driving it for over 48 hours. We hope
our car can handle all kinds of environments on the trip and send us safely to
the destination. We know that road conditions and views can be very different.
It is intuitive to simply collect road data of this trip, let the car learn
from every possible condition, and hope it becomes the perfect self-driving car
for our New York to Seattle trip. It needs to understand the traffic and
skyscrapers in big cities like New York and Chicago, more unpredictable weather
in Seattle, mountains and forests in Montana, and all kinds of country views,
farmlands, animals, etc. However, how much data is enough? How many cities
should we collect data from? How many weather conditions should we consider? We
never know, and these questions never stop.

Figure 1: Domains boundaries are rarely clear. Therefore, it is hard to set up
definite domain descriptions for all possible domains.

As for the weather, the number of different conditions can be infinite, while
the words that can be used to describe weather conditions (such as sunny and
rainy) are always limited. Some conditions have subtle differences that even
humans can not tell. For example, overcast v.s. cloudy, the transition state of
rainy to non-rainy, light snow v.s. light rain, etc. We can always try to
collect data for every possible weather condition that we can imagine and use
conditions. But our imagination is limited when it comes to weather, and the
main reason is that there are no clear boundaries between weather conditions.
In that case, none of the traditional domain adaptation approaches, where clear
boundaries are assumed, can help us in real-world weather adaption. Therefore,
we start rethinking machine learning and domain adaptation systems, and try to
introduce a continuous learning protocol under domain adaptation scenario.

# Open Compound Domain Adaptation (OCDA)

The goal of domain adaptation is to adapt the model learned on the training
data to the test data of a different distribution. Such a distributional gap is
often formulated as a shift between discrete concepts of well defined data
domains, e.g., images collected in sunny weather versus those in rainy weather.
Though domain generalization and latent domain adaptation have attempted to
tackle complex target domains, most existing works usually assume that there is
a known clear distinction between domains. Such a known and clear distinction
between domains is hard to define in practice, e.g., test images could be
collected in mixed, continually varying, and sometimes never seen weather
conditions. With numerous factors jointly contributing to data variance, it
becomes implausible to separate data into discrete domains.

We propose to study Open Compound Domain Adaptation (OCDA), a continuous
and more realistic setting for domain adaptation (Figure 2). The task is to
learn a model from labeled source domain data and adapt it to unlabeled
compound target domain data which could differ from the source domain on
various factors. Our target domain can be regarded as a combination of multiple
traditionally homogeneous domains where each is distinctive on one or two major
factors, and yet none of the domain labels are given. For example, the five
well-known datasets on digits recognition (SVHN, MNIST, MNIST-M, USPS, and
SynNum) mainly differ from each other by the backgrounds and text fonts. It is
not necessarily the best practice, and not feasible under some scenarios, to
consider them as distinct domains. Instead, our compound target domain pools
them together. Furthermore, at the inference stage, OCDA tests the model not
only in the compound target domain but also in open domains that have
previously unseen during training.

Figure 2: Open Compound Domain Adaptation problem. Unlike existing
domain adaptation which assumes clear distinctions between discrete domains,
our compound target domain is a combination of multiple traditionally
homogeneous domains without any domain labels. We also allow novel domains to
show up at the inference time.

While OCDA has not been defined in the literature, there are two closely
related tasks which are often studied in isolation: single-target domain
differences. The newly proposed Open Compound Domain Adaptation (OCDA) serves
as a more comprehensive and more realistic touchstone for evaluating domain

Figure 3: The differences between single-target domain adaptation, multi-target

# The Importance of Curriculum & Memory

In our OCDA setting, the target domain no longer has a predominantly uni-modal
distribution, posing challenges to existing domain adaptation methods. We
propose a novel approach based on two technical insights into OCDA: 1) a
curriculum domain adaptation strategy to bootstrap generalization across
domain distinction in a data-driven self-organizing fashion and 2) a memory
module
to increase the model’s agility towards novel domains, as illustrated
in Figure 4.

Firstly, unlike existing curriculum adaptation methods that rely on some
holistic measure of instance difficulty, we schedule the learning of unlabeled
instances in the compound target domain according to their individual gaps to
the labeled source domain, so that we solve an incrementally harder domain
adaptation problem till we cover the entire target domain.

Our second technical insight is to prepare our model for open domains during
inference with a memory module that effectively augments the representations of
an input for classification. Intuitively, if the input is close enough to the
source domain, the feature extracted from itself can most likely already result
in accurate classification. Otherwise, the input-activated memory features can
step in and play a more important role. Consequently, this memory-augmented
network is more agile at handling open domains than its vanilla counterpart.

Figure 4: Overview of our approach. 1) We separate characteristics specific to
domains from those discriminative between classes. The teased out domain
feature is used to construct a curriculum for domain-robust learning. 2) We
enhance our network with a memory module that facilitates knowledge transfer
from the source domain to target domain instances, so that the network can
dynamically balance the input information and the memory-transferred knowledge
for more agility towards previously unseen domains.

# Robustness to Compound and Open Domains

We control the complexity of the compound and open target domain by varying the
number of traditional target domains / datasets in it. Here we gradually
increase constituting domains from a single target domain to two, and
eventually three. As demonstrated in Figure 5, we observe that our approach is
more resilient to the various numbers of compound and open domains. 1) The
learned curriculum enables gradual knowledge transfer that is capable of coping
with complex structures in the compound target domain. 2) Furthermore, the
domain indicator module in our framework helps dynamically calibrate the
embedding, thus enhancing the robustness to open domains.

Figure 5: The performance change w.r.t. the number of compound and open
domains.

# Visualization of Learning Dynamics

Here is a visualization of disentangling domain characteristics. We separate
characteristics specific to domains from those discriminative between classes.
It is achieved by a class-confusion algorithm in an unsupervised manner. Figure
6 (a) and (b) visualize the examples embedded by the class encoder and domain
encoder, respectively. The class encoder places instances in the same class in
a cluster, while the domain encoder places instances according to their common
appearances, regardless of their classes.

Figure 6: t-SNE Visualization of our (a) class-discriminative features, and (b)
domain features. Our framework disentangles the mixed-domain data into
class-discriminative factors and domain-focused factors.

Acknowledgements: We thank all co-authors of the paper “Open Compound Domain
Adaptation” for their contributions and discussions in preparing this blog. The
views and opinions expressed in this blog are solely of the authors of this
paper.

This blog post is based on the following paper which will be presented at IEEE
Conference on Computer Vision and Pattern Recognition (CVPR 2020) as an oral
presentation
: