Double-Stream Position Learning Transformer Network for Image Captioning

Image captioning has made significant achievement through developing feature extractor and model architecture. Recently, the image region features extracted by object detector prevail in most existing models. However, region features are criticized for the lacking of background and full contextual information. This problem can be remedied by providing some complementary visual information from patch features. In this paper, we propose a Double-Stream Position Learning Transformer Network (DSPLTN) which exploits the advantages of region features and patch features. Specifically, the region-stream encoder utilizes a Transformer encoder with Relative Position Learning (RPL) module to enhance the representations of region features through modeling the relationships between regions and positions respectively. As for the patch-stream encoder, we introduce convolutional neural network into the vanilla Transformer encoder and propose a novel Convolutional Position Learning (CPL) module to encode the position relationships between patches. CPL improves the ability of relationship modeling by combining the position and visual content of patches. Incorporating CPL into the Transformer encoder can synthesize the benefits of convolution in local relation modeling and self-attention in global feature fusion, thereby compensating for the information loss caused by the flattening operation of 2D feature maps to 1D patches. Furthermore, an Adaptive Fusion Attention (AFA) mechanism is proposed to balance the contribution of enhanced region and patch features. Extensive experiments on MSCOCO demonstrate the effectiveness of the double-stream encoder and CPL, and show the superior performance of DSPLTN.