Blur Is an Ensemble: Spatial Smoothings to Improve Accuracy, Uncertainty, and Robustness

Namuk Park, Songkuk Kim

Keywords: bayesian neural network, uncertainty, uncertainty estimation, uncertainty quantification

Abstract: Bayesian neural networks (BNNs) have shown success in the areas of uncertainty estimation and robustness. However, a crucial challenge prohibits their use in practice. Bayesian NNs require a large number of predictions to produce reliable results, leading to a significant increase in computational cost. To alleviate this issue, we propose spatial smoothing, a method that ensembles neighboring feature map points of CNNs. By simply adding a few blur layers to the models, we empirically show that spatial smoothing improves accuracy, uncertainty estimation, and robustness of BNNs across a whole range of ensemble sizes. In particular, BNNs incorporating spatial smoothing achieve high predictive performance merely with a handful of ensembles. Moreover, this method also can be applied to canonical deterministic neural networks to improve the performances. A number of evidences suggest that the improvements can be attributed to the stabilized feature maps and the flattening of the loss landscape. In addition, we provide a fundamental explanation for prior works—namely, global average pooling, pre-activation, and ReLU6—by addressing them as special cases of spatial smoothing. These not only enhance accuracy, but also improve uncertainty estimation and robustness by making the loss landscape smoother in the same manner as spatial smoothing.

One-sentence Summary: To improve accuracy, uncertainty, and robustness, we propose spatial smoothing, a method that aggregates spatially nearby feature maps of CNNs. This spatial ensemble of feature maps—even GAP—helps in optimization by flattening the loss landscape.