  Supervised crowd counting relies heavily on costly manual labeling, which is
difficult and expensive, especially in dense scenes. To alleviate the problem,
we propose a novel unsupervised framework for crowd counting, named CrowdCLIP.
The core idea is built on two observations: 1) the recent contrastive
pre-trained vision-language model (CLIP) has presented impressive performance
on various downstream tasks; 2) there is a natural mapping between crowd
patches and count text. To the best of our knowledge, CrowdCLIP is the first to
investigate the vision language knowledge to solve the counting problem.
Specifically, in the training stage, we exploit the multi-modal ranking loss by
constructing ranking text prompts to match the size-sorted crowd patches to
guide the image encoder learning. In the testing stage, to deal with the
diversity of image patches, we propose a simple yet effective progressive
filtering strategy to first select the highly potential crowd patches and then
map them into the language space with various counting intervals. Extensive
experiments on five challenging datasets demonstrate that the proposed
CrowdCLIP achieves superior performance compared to previous unsupervised
state-of-the-art counting methods. Notably, CrowdCLIP even surpasses some
popular fully-supervised methods under the cross-dataset setting. The source
code will be available at https://github.com/dk-liang/CrowdCLIP.
