Show, Tell, and Polish: Ruminant Decoding for Image Captioning

The encoder-decoder framework has been the base of popular image captioning models, which typically predicts the target sentence based on the encoded source image one word at a time in sequence. However, such a single-pass decoding framework encounters two problems. First, mistakes in the predicted words cannot be corrected and may propagate to the entire sentence. Second, because the single-pass decoder cannot access the following un-generated words, it can only perform local planning to choose every single word according to the preceding words, while lacks the global planning ability as for maintaining the semantic consistency and fluency of the whole sentence. In order to address the above two problems, in this work, we design a ruminant captioning framework which contains an image encoder, a base decoder, and a ruminant decoder. Specifically, the outputs of the former/base decoder are utilized as the global information to guide the words prediction of the latter/ruminant decoder, in an attempt to mimic human polishing process. We enable jointly training of the whole framework and overcome the non-differential problem of discrete words by designing a novel reinforcement learning based optimization algorithm. Experiments on two datasets (MS COCO and Flickr30 k) demonstrate that our ruminant decoding method can bring significant improvements over traditional single-pass decoding based models and achieves state-of-the-art performance.