Unconstrained Open Vocabulary Image Classification: Zero-Shot Transfer from Text to Image via CLIP Inversion

Philipp Allgeuer, Kyra Ahrens, Stefan Wermter; Proceedings of the Winter Conference on Applications of Computer Vision (WACV), 2025, pp. 8206-8217

We introduce NOVIC an innovative real-time uNconstrained Open Vocabulary Image Classifier that uses an autoregressive transformer to generatively output classification labels as language. Leveraging the extensive knowledge of CLIP models NOVIC harnesses the embedding space to enable zero-shot transfer from pure text to images. Traditional CLIP models despite their ability for open vocabulary classification require an exhaustive prompt of potential class labels restricting their application to images of known content or context. To address this we propose an "object decoder" model that is trained on a large-scale 92M-target dataset of templated object noun sets and LLM-generated captions to always output the object noun in question. This effectively inverts the CLIP text encoder and allows textual object labels from essentially the entire English language to be generated directly from image-derived embedding vectors without requiring any a priori knowledge of the potential content of an image and without any label biases. The trained decoders are tested on a mix of manually and web-curated datasets as well as standard image classification benchmarks and achieve fine-grained prompt-free prediction scores of up to 87.5% a strong result considering the model must work for any conceivable image and without any contextual clues.