Training binary neural networks with real-to-binary convolutions

This paper shows how to train binary networks to within a few percent points ( 3-5 %) of the full precision counterpart with a negligible increase in the computational cost. In particular, we first show how to build a strong baseline, which already achieves state-of-the-art accuracy, by combining recently proposed advances, and carefully tuning the optimization procedure. Secondly, we show that by attempting to minimize the discrepancy between the output of the binary and the corresponding real-valued convolution additional significant accuracy gains can be obtained. We materialize this idea in two complementary ways: (1) with a loss function, during training, by matching the spatial attention maps computed at the output of the binary and real-valued convolutions, and (2) in data-driven manner, by using the real-valued activations being available during inference prior to the binarization process for re-scaling the activations right after the binary convolution. Finally, we show that, when putting all of our improvements together, the resulting model reduces the gap to its real-valued counterpart to less than 3% and 5% top-1 error on CIFAR-100 and ImageNet, respectively, when using a ResNet-18 architecture.