CLIP doesn't do captioning, it just generates embeddings. And it's contrastive, so it would work poorly for this kind of task: anything 'relational' falls apart

CLIP doesn't do captioning, it just generates embeddings. And it's contrastive, so it would work poorly for this kind of task: anything 'relational' falls apart immediately. (See for example the DALL-E 2 results for these kinds of captions/tasks.) It's almost certainly a VQ-VAE-style encoding of the image itself into a sequence of tokens, as was done by DALL-E 1, CM3, Gato and a whole bunch of more recent models. It's the very obvious thing to do, and their context window is more than large enough now.