Paper:
Recent studies show that machine learning models are vulnerable to adversarial examples. In 2D image domain, these examples are obtained by adding imperceptible noises to natural images. This paper studies adversarial generation of point clouds by learning to deform those approximating object surfaces of certain categories. As 2D manifolds embedded in the 3D Euclidean space, object surfaces enjoy the general properties of smoothness and fairness. We thus argue that in order to achieve imperceptible surface shape deformations, adversarial point clouds should have the same properties with similar degrees of smoothness/fairness to the benign ones, while being close to the benign ones as well when measured under certain distance metrics of point clouds. To this end, we propose a novel loss function to account for imperceptible, geometry-aware deformations of point clouds, and use the proposed loss in an adversarial objective to attack representative models of point set classifiers. Experiments show that our proposed method achieves stronger attacks than existing methods, without introduction of noticeable outliers and surface irregularities. In this work, we also investigate an opposite direction that learns to deform point clouds of object surfaces in the same geometry-aware, but cooperative manner. Cooperatively generated point clouds are more favored by machine learning models in terms of improved classification confidence or accuracy. We present experiments verifying that our proposed objective succeeds in learning cooperative shape deformations.

Review:
This paper looks at the task of (adversarially or cooperatively) perturbing a point cloud in context of a classification task. It follows the prevailing paradigm of changing the original input in the direction of a high positive/negative gradient while staying ‘close’ to the original input, and the key contribution here is to define a different notion of ‘closeness’.
While previous work (Xiang et. al., at least for ‘point addition attack’) used a combination of chamfer and Hausdorff distance as the notion of closeness, this paper additionally includes change in curvature (which is an intuitive term to include) when computing the adversarial/cooperative updates to the point cloud. The obtained results do visually look less perturbed compared to the previous approach, and the obtained adversarial shapes are more robust against two defenses studied.
Concerns/Questions:
1) If a point cloud P’ is only a (small) perturbation of a point cloud P, then the Chamfer distance / Hausdorff distance is essentially the  L_2 / L_infinity norm of their difference. While the use of curvature terms is different, I feel the claims of importance of using ‘distance metric of point clouds’ is not very well justified (as I’d expect essentially same results if these two terms were replaced by L2 and L_infinity norms instead). I think the use of these terms was more necessary in the work of Xiang et. al., as they allowed point addition, so the ‘norm of difference of point clouds’ is not well defined.
2) This paper uses a different (more aggressive) adversarial term (in Eqn. 8) compared to Xiang et. al., so it is not surprising that the results in Table 1 indicate more robustness to defenses.
3) In addition to the above comments about specifics, I feel this work’s contribution over prior work is not significant. While Xiang et. al. did use L2 norm for their perturbation case, they did investigate the Chamfer/Hausdorff distances for another scenario, and therefore the main contribution here is an additional loss term.
4) This is perhaps a hard concern to address, but simply showing some qualitative results to highlight that the changes are ‘imperceptible’ is not sufficient. Ideally, one should report a curve on ‘change perceptibility’ vs ‘attack success rate’ (though this would require some notion of perceptibility that was not used in optimization). Alternately, one could compare methods via A/B testing on mechanical turk, asking ‘Which are these two shapes are closer to the original one?’, and ablate for a certain level of confidence on the wrong class, which approach led to less changes. The current results simply show some examples, but provide no empirical way of judging which approach actually leads to more imperceptible changes.
Overall, though the results are perceptually encouraging, I have slight concerns the empirical results reported. However, the primary issue is that the contribution regarding the additional term, while intuitive, is not a significant one in its own right.
While the rating here only allows me to give a ‘3’ as a weak reject, I am perhaps a bit more towards borderline (though leaning towards reject) than that indicates.