Learning without Seeing nor Knowing: Towards Open Zero-Shot Learning
This addresses the problem of making zero-shot learning more applicable to real-world scenarios where not all categories are known in advance, representing an incremental extension of GZSL.
The paper tackles the limitation of Generalized Zero-Shot Learning (GZSL) requiring known class embeddings for unseen categories by proposing Open Zero-Shot Learning (OZSL), which extends GZSL to open-world settings by rejecting instances from unknown categories without visual data or class embeddings, and achieves this through unknown feature generation.
In Generalized Zero-Shot Learning (GZSL), unseen categories (for which no visual data are available at training time) can be predicted by leveraging their class embeddings (e.g., a list of attributes describing them) together with a complementary pool of seen classes (paired with both visual data and class embeddings). Despite GZSL is arguably challenging, we posit that knowing in advance the class embeddings, especially for unseen categories, is an actual limit of the applicability of GZSL towards real-world scenarios. To relax this assumption, we propose Open Zero-Shot Learning (OZSL) to extend GZSL towards the open-world settings. We formalize OZSL as the problem of recognizing seen and unseen classes (as in GZSL) while also rejecting instances from unknown categories, for which neither visual data nor class embeddings are provided. We formalize the OZSL problem introducing evaluation protocols, error metrics and benchmark datasets. We also suggest to tackle the OZSL problem by proposing the idea of performing unknown feature generation (instead of only unseen features generation as done in GZSL). We achieve this by optimizing a generative process to sample unknown class embeddings as complementary to the seen and the unseen. We intend these results to be the ground to foster future research, extending the standard closed-world zero-shot learning (GZSL) with the novel open-world counterpart (OZSL).