Visual-linguistic-stylistic Triple Reward for Cross-lingual Image Captioning

Generating image captions in different languages is worth exploring and essential for non-native speakers. Nevertheless, collecting paired annotation for every language is time-consuming and impractical, particularly for minor languages. To this end, the cross-lingual image captioning task is proposed, which leverages existing image-source caption annotation data and wild unrelated target corpus to generate satisfactory caption in the target language. Current methods perform a two-step translation process of image-to-pivot (source) and pivot-to-target. The distinct two-step process comes with certain caption issues, such as the weak semantic alignment between the image and the generated caption and the generated caption's non-target language style. To address these issues, we propose an end-to-end reinforce learning framework with Visual-linguistic-stylistic Triple Reward named TriR. In TriR, we jointly consider the visual, linguistic, and stylistic alignments to generate factual, fluent, and natural caption in the target language. To be specific, the image-source caption annotation provides factual semantic guidance, whereas the unrelated target corpus guides the language style of generated caption. To achieve this, we construct a visual reward module to measure the cross-modal semantic embedding of image and target caption, a linguistic reward module to measure the cross-linguistic embedding of source and target captions, and a stylistic reward module to imitate the presentation style of target corpus. The TriR can be implemented with either classical CNN-LSTM or prevalent Transformer architecture. Extensive experiments are conducted with four cross-lingual settings, i.e., Chinese-to-English, English-to-Chinese, English-to-German, and English-to-French. Experimental results demonstrate the remarkable superiority of our method, and sufficient ablation experiments validate the beneficial impact of every reward.