
ECCV 2022 Workshop on Adversarial Robustness in the Real World, Tel Aviv, Israel.
Attacking Motion Estimation with Adversarial Snow
Jenny Schmalfuss Lukas Mehl Andr´
es Bruhn
Institute for Visualization and Interactive Systems, University of Stuttgart, Germany
firstname.lastname@vis.uni-stuttgart.de
Abstract
Current adversarial attacks for motion estimation (opti-
cal flow) optimize small per-pixel perturbations, which are
unlikely to appear in the real world. In contrast, we ex-
ploit a real-world weather phenomenon for a novel attack
with adversarially optimized snow. At the core of our at-
tack is a differentiable renderer that consistently integrates
photorealistic snowflakes with realistic motion into the 3D
scene. Through optimization we obtain adversarial snow
that significantly impacts the optical flow while being indis-
tinguishable from ordinary snow. Surprisingly, the impact
of our novel attack is largest on methods that previously
showed a high robustness to small Lpperturbations.
1. Introduction
Adversarial attacks, which are a severe threat to neural
networks, have recently been introduced in the context of
optical flow. There, the goal is to compute the pixel-wise
2D motion fbetween two consecutive frames of an image
sequence at times tand t+1. Current attacks [11,14] modify
these two frames in the 2D space and consequently ignore
the actual 3D geometry of the scene and the objects moving
within. Moreover, when modifying pixels, they do not im-
pose visual constraints, yielding attacked images that lack
naturalism. Therefore, the conclusions drawn from robust-
ness analyses with these attacks might not necessarily re-
flect the robustness of optical flow methods in the real world
– where perturbations are more likely to appear in the form
of weather phenomena.
This work aims to answer the question whether a natu-
rally occurring weather effect like snow can be manipulated
to serve as an adversarial sample for motion estimation. To
this end, we propose an adversarial attack that augments
images with falling snowflakes featuring a high degree of
realism: We create snowflakes with a view-consistent 3D
motion over time, insert them into the 3D scene in a depth-
aware manner, and ensure photo-realism through visual ef-
fects (see Fig. 1). This enables us to generate adversari-
ally manipulated snow that significantly deteriorates optical
Original
Frame tFrame t+1 Optical flow
Random snow
Adversarial snow
Figure 1. Snow attack with 3000 snowflakes that are first placed
randomly in the 3D scene (Random snow) and then optimized (Ad-
versarial snow) to perturb optical flow estimation with GMA [6].
flow predictions, while still satisfying the spatio-temporal
and visual constraints of naturalistic snow. We consider
snow as representative weather effect where single particles
move independent of the remaining scene content, but note
that the proposed attack procedure could also be used to
model rain or sleet.
Related work. Current optical flow methods based on neu-
ral networks [5,6,10] were recently shown to be suscep-
tible to adversarially modified input images, which alter
the resulting attacked flow ˇ
fto resemble a specified tar-
get flow fT. The few existing adversarial attacks on optical
flow methods generate either perturbations with small Lp
norms [14,15] or adversarial patches [11], while adversarial
weather attacks are completely unexplored. In contrast, ad-
versarial perturbations that imitate snow effects have been
investigated in the context of classification [7,8] or hu-
man pose estimation [16]. However, for these applications,
weather attacks [7,8] or snow augmentations [4,9] only
have to be applied to single images rather than sequences.
For optical flow estimation, a realistic motion of the weather
effect over multiple frames and camera perspectives is
required, imposing certain geometric constraints in time,
which prevents the direct application of existing single-
image adversarial weather generation schemes. Also, learn-
ing models for variations in images from data rather than
modeling them explicitly has been explored for adversar-
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arXiv:2210.11242v1 [cs.CV] 20 Oct 2022