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Title: ACE: Artificial Checkerboard Enhancer to Induce and Evade Adversarial Attacks

Paper content:
The checkerboard phenomenon is one of the well-known visual artifacts in the computer vision field.
The origins and solutions of checkerboard artifacts in the pixel space have been studied for a long time, but their effects on the gradient space have rarely been investigated.
In this paper, we revisit the checkerboard artifacts in the gradient space which turn out to be the weak point of a network architecture.
We explore image-agnostic property of gradient checkerboard artifacts and propose a simple yet effective defense method by utilizing the artifacts.
We introduce our defense module, dubbed Artificial Checkerboard Enhancer (ACE), which induces adversarial attacks on designated pixels.
This enables the model to deflect attacks by shifting only a single pixel in the image with a remarkable defense rate.
We provide extensive experiments to support the effectiveness of our work for various attack scenarios using state-of-the-art attack methods.
Furthermore, we show that ACE is even applicable to large-scale datasets including ImageNet dataset and can be easily transferred to various pretrained networks.
The checkerboard phenomenon is one of the well-known artifacts that arise in various applications such as image super resolution, generation and segmentation BID29 BID27 .
In general, the checkerboard artifacts indicate an uneven pattern on the output of deep neural networks (DNNs) that occurs during the feed-forward step.
BID21 have investigated in-depth the origin of the phenomenon that the artifacts come from the uneven overlap of the deconvolution operations (i.e., transposed convolution, ) on pixels.
Its solutions have been suggested in various studies BID35 BID0 .Interestingly
, a possible relationship between the checkerboard artifacts and the robustness of neural networks has been noted by BID21 but not has it been seriously investigated. Moreover, while
the previous works BID16 BID20 BID21 BID8 have concentrated on the artifacts in the pixel space, studies have been rare on the artifacts in the gradient space that occur during the backward pass of the convolution operation.To show that the gradient checkerboard artifacts phenomenon is crucial for investigating the network robustness and is indeed a weak point of a neural network, we focus on analyzing its effects on the gradient space in terms of adversarial attack and defense. By explicitly visualizing
the gradients, we demonstrate that the phenomenon is inherent in many contemporary network architectures such as ResNet BID12 , which use strided convolutions with uneven overlap. It turns out that the gradient
checkerboard artifacts substantially influence the shape of the loss surface, and the effect is image-agnostic.Based on the analysis, we propose an Artificial Checkerboard Enhancer module, dubbed ACE. This module further boosts or
creates the checkerboard artifacts in the target network and manipulates the gradients to be caged in the designated area. Because ACE guides the attacks
to the intended environment, the defender can easily dodge the attack by shifting a single pixel (Figure 1 ) with a negligible accuracy loss. Moreover, we demonstrate that
our module is scalable to large-scale datasets such as ImageNet BID6 ) and also transferable to other models in a plug and play fashion without additional fine-tuning of pretrained networks. Therefore, our module is highly
practical in general scenarios in that we can easily plug any pretrained ACE module into the target architecture.Base Network (e.g. ResNet, VGG) ACE Module
In this paper, we have investigated the gradient checkerboard artifacts (GCA) which turned out to be a potential threat to the network robustness.
Based on our observations, we proposed a novel Artificial Checkerboard Enhancer (ACE) which attracts the attack to the pre-specified domain.
ACE is a module that can be plugged in front of any pretrained model with a negligible performance drop.
Exploiting its favorable characteristics, we provide a simple yet effective defense method that is even scalable to a large-scale dataset such as ImageNet.
Our extensive experiments show that the proposed method can deflect various attack scenarios with remarkable defense rates compared with several existing adversarial defense methods.

assistant: text: We propose a novel aritificial checkerboard enhancer (ACE) module which guides attacks to a pre-specified pixel space and successfully defends it with a simple padding operation.