Improving Image Captioning Systems with Postprocessing Strategies

Image captioning (IC) systems are generally based on encoder–decoder architecture where convolutional neural networks (CNNs) are employed to represent an image with discriminative features and recurrent neural networks (RNNs) sequentially generate a sentence description. Even though a lot of effort has been devoted lately to designing reliable IC systems, the task is far from being solved. The generated descriptions can be affected by different errors related to the attributes and the objects present in the scene. Moreover, once an error occurs, it can be propagated in the recurrent layers of the decoder generating non-accurate descriptions. To solve this problem, we propose two postprocessing strategies applied to the generated descriptions to rectify the errors and improve their quality. The proposed postprocessing strategies are based on hidden Markov models (HMMs) and Viterbi algorithm. The proposed postprocessing strategies can be applied to any encoder–decoder IC system. They are applied at test time once the IC system is trained. In particular, we propose to rectify a sentence once it is fully generated (post-generation strategy) or at each time instant of the generation process (in-generation strategy). Experiments conducted on four different IC datasets confirm the promising capabilities of the proposed postprocessing strategies to rectify the output of a simple encoder–decoder by generating more coherent descriptions. The achieved results are competitive and sometimes better than complex IC systems.