Mirror, mirror on the wall, tell me, is the error small?

Do object part localization methods produce bilaterally symmetric results on mirror images? Surprisingly not, even though state of the art methods augment the training set with mirrored images. In this paper we take a closer look into this issue. We first introduce the concept of mirrorability as the ability of a model to produce symmetric results in mirrored images and introduce a corresponding measure, namely the \textit{mirror error} that is defined as the difference between the detection result on an image and the mirror of the detection result on its mirror image. We evaluate the mirrorability of several state of the art algorithms in two of the most intensively studied problems, namely human pose estimation and face alignment. Our experiments lead to several interesting findings: 1) Surprisingly, most of state of the art methods struggle to preserve the mirror symmetry, despite the fact that they do have very similar overall performance on the original and mirror images; 2) the low mirrorability is not caused by training or testing sample bias - all algorithms are trained on both the original images and their mirrored versions; 3) the mirror error is strongly correlated to the localization/alignment error (with correlation coefficients around 0.7). Since the mirror error is calculated without knowledge of the ground truth, we show two interesting applications - in the first it is used to guide the selection of difficult samples and in the second to give feedback in a popular Cascaded Pose Regression method for face alignment.Comment: 8 pages, 9 figure

A phase conjugate mirror inspired approach for building cloaking structures with left-handed materials

In this paper, we propose and examine a new cloaking method, which was inspired by the close correspondence between a phase conjugate mirror and the interface between a pair of matched right-handed material (RHM) and left-handed material (LHM) media. Using this method, we show that a symmetric conducting shell embedded in the interface junction of an isotropic RHM layer and an isotropic negative index or LHM layer can serve as a limited cloaking structure. The proposed structure presents an anomalously small scattering cross-section to an incident propagating electromagnetic (EM) field. The interior of the shell can be used to shield small objects from interrogation. We report the results of 2D finite-element-method (FEM) simulations that were performed to verify the principle, and discuss the limitations of the proposed structure