CLIPScore: A Reference-free Evaluation Metric for Image Captioning

Image captioning has conventionally relied on reference-based automatic evaluations, where machine captions are compared against captions written by humans. This is in contrast to the reference-free manner in which humans assess caption quality. In this paper, we report the surprising empirical finding that CLIP (Radford et al., 2021), a cross-modal model pretrained on 400M image+caption pairs from the web, can be used for robust automatic evaluation of image captioning without the need for references. Experiments spanning several corpora demonstrate that our new reference-free metric, CLIPScore, achieves the highest correlation with human judgements, outperforming existing reference-based metrics like CIDEr and SPICE. Information gain experiments demonstrate that CLIPScore, with its tight focus on image-text compatibility, is complementary to existing reference-based metrics that emphasize text-text similarities. Thus, we also present a reference-augmented version, RefCLIPScore, which achieves even higher correlation. Beyond literal description tasks, several case studies reveal domains where CLIPScore performs well (clip-art images, alt-text rating), but also where it is relatively weaker in comparison to reference-based metrics, e.g., news captions that require richer contextual knowledge.

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Task Dataset Model Metric Name Metric Value Global Rank Result Benchmark
Human Judgment Correlation Flickr8k-CF RefCLIP-S Kendall's Tau-b 36.4 # 2
Human Judgment Correlation Flickr8k-CF CLIP-S Kendall's Tau-b 34.4 # 3
Human Judgment Correlation Flickr8k-Expert RefCLIP-S Kendall's Tau-c 53.0 # 2
Human Judgment Correlation Flickr8k-Expert CLIP-S Kendall's Tau-c 51.2 # 3
Hallucination Pair-wise Detection (4-ref) FOIL RefCLIP-S Mean Accuracy 92.6 # 1
Hallucination Pair-wise Detection (1-ref) FOIL CLIP-S Mean Accuracy 91 # 1
Hallucination Pair-wise Detection (4-ref) FOIL CLIP-S Mean Accuracy 87.2 # 3
Human Judgment Classification Pascal-50S CLIP-S Mean Accuracy 80.7 # 3
Human Judgment Classification Pascal-50S RefCLIP-S Mean Accuracy 83.1 # 2


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