DeepEMD: Differentiable Earth Mover's Distance for Few-Shot Learning

Deep learning has proved to be very effective in learning with a large amount of labelled data. Few-shot learning in contrast attempts to learn with only a few labelled data. In this work, we develop methods for few-shot image classification from a new perspective of optimal matching between image regions. We employ the Earth Mover's Distance (EMD) as a metric to compute a structural distance between dense image representations to determine image relevance. The EMD generates the optimal matching flows between structural elements that have the minimum matching cost, which is used to calculate the image distance for classification. To generate the important weights of elements in the EMD formulation, we design a cross-reference mechanism, which can effectively alleviate the adverse impact caused by the cluttered background and large intra-class appearance variations. To handle $k$-shot classification, we propose to learn a structured fully connected layer that can directly classify dense image representations with the proposed EMD. Based on the implicit function theorem, the EMD can be inserted as a layer into the network for end-to-end training. Our extensive experiments validate the effectiveness of our algorithm which outperforms state-of-the-art methods by a significant margin on four widely used few-shot classification benchmarks, namely, miniImageNet, tieredImageNet, Fewshot-CIFAR100 (FC100) and Caltech-UCSD Birds-200-2011 (CUB).Comment: Extended version of DeepEMD in CVPR2020 (oral