Beyond Pixel Norm-Balls: Parametric Adversaries using an Analytically Differentiable Renderer
Hsueh-Ti Derek Liu
arXiv e-Print archive - 2018 via Local arXiv
cs.LG, cs.CV, cs.GR, stat.ML
First published: 2018/08/08 (4 years ago) Abstract: Many machine learning image classifiers are vulnerable to adversarial
attacks, inputs with perturbations designed to intentionally trigger
misclassification. Current adversarial methods directly alter pixel colors and
evaluate against pixel norm-balls: pixel perturbations smaller than a specified
magnitude, according to a measurement norm. This evaluation, however, has
limited practical utility since perturbations in the pixel space do not
correspond to underlying real-world phenomena of image formation that lead to
them and has no security motivation attached. Pixels in natural images are
measurements of light that has interacted with the geometry of a physical
scene. As such, we propose the direct perturbation of physical parameters that
underly image formation: lighting and geometry. As such, we propose a novel
evaluation measure, parametric norm-balls, by directly perturbing physical
parameters that underly image formation. One enabling contribution we present
is a physically-based differentiable renderer that allows us to propagate pixel
gradients to the parametric space of lighting and geometry. Our approach
enables physically-based adversarial attacks, and our differentiable renderer
leverages models from the interactive rendering literature to balance the
performance and accuracy trade-offs necessary for a memory-efficient and
scalable adversarial data augmentation workflow.
Liu et al. propose adversarial attacks on physical parameters of images, which can be manipulated efficiently through differentiable renderer. In particular, they propose adversarial lighting and adversarial geometry; in both cases, an image is assumed to be a function of lighting and geometry, generated by a differentiable renderer. By directly manipulating these latent variables, more realistic looking adversarial examples can be generated for synthetic images as shown in Figure 1.
Figure 1: Comparison of the proposed attack with known attacks applied to large perturbations, $L_\infty \approx 0.82$.
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