Google Research

Improved Image Captioning via Policy Gradient optimization of SPIDEr


Current image captioning methods are usually trained via (penalized) maximum likelihood estimation. However, the log-likelihood score of a caption does not correlate well with human assessments of quality. Standard syntactic evaluation metrics, such BLEU, METEOR and ROUGE, are also not well correlated. The SPICE and CIDEr metrics are better correlated, but have traditionally been hard to optimize for. In this paper, we show how to use a policy gradient (PG) algorithm to directly optimize a combination of SPICE and CIDEr (a combination we call SPIDEr): the SPICE score ensures our captions are semantically faithful to the image, and the CIDEr score ensures our captions are syntactically fluent. The PG algorithm we propose improves on the prior MIXER approach, by using Monte Carlo rollouts instead of mixing ML training with PG. We show empirically that our algorithm leads to improved results compared to MIXER. Finally, we shoow that using our PG algorithm to optimize the novel SPIDEr metric results in image captions that are strongly preferred by human raters compared to captions generated by the same model but trained using different objective functions.

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