Partially-Supervised Image Captioning

Image captioning models are becoming increasingly successful at describing the content of images in restricted domains. However, if these models are to function in the wild - for example, as aids for the visually impaired - a much larger number and variety of visual concepts must be understood. In this work, we teach image captioning models new visual concepts with partial supervision, such as available from object detection and image label datasets. As these datasets contain text fragments rather than complete captions, we formulate this problem as learning from incomplete data. To flexibly characterize our uncertainty about the unobserved complete sequence, we represent each incomplete training sequence with its own finite state automaton encoding acceptable complete sequences. We then propose a novel algorithm for training sequence models, such as recurrent neural networks, on incomplete sequences specified in this manner. In the context of image captioning, our method lifts the restriction that previously required image captioning models to be trained on paired image-sentence corpora only, or otherwise required specialized model architectures to take advantage of alternative data modalities. Applying our approach to an existing neural captioning model, we achieve state of the art results on the novel object captioning task using the COCO dataset. We further show that we can train a captioning model to describe new visual concepts from the Open Images dataset while maintaining competitive COCO evaluation scores.