Multi-modal contrastive models such as CLIP achieve state-of-the-art performance in zero-shot classification by embedding input images and texts on a joint representational space. Recently, a modality gap has been reported in two-encoder contrastive models like CLIP, meaning that the image and text embeddings reside in disjoint areas of the latent space. Previous studies suggest that this gap exists due to 1) the cone effect, 2) mismatched pairs in the dataset, and 3) insufficient training. We show that, even when accounting for all these factors, and even when using the same modality, the contrastive loss actually creates a gap during training. As a result, We propose that the modality gap is inherent to the two-encoder contrastive loss and rename it the contrastive gap. We present evidence that attributes this contrastive gap to low uniformity in CLIP space, resulting in embeddings that occupy only a small portion of the latent space. To close the gap, we adapt the uniformity and alignment properties of unimodal contrastive loss to the multi-modal setting and show that simply adding these terms to the CLIP loss distributes the embeddings more uniformly in the representational space, closing the gap. In our experiments, we show that the modified representational space achieves better performance than default CLIP loss in downstream tasks such as zero-shot image classification and multi-modal arithmetic.

在多模态对比模型中存在模态差距（modality gap），且对比损失（contrastive loss）实际上导致了这种差距。为了解决这个问题，研究人员通过将单模态对比损失的均匀性和对齐性原则引入到多模态环境中，改进了对比损失函数（CLIP loss），使得嵌入更均匀地分布在表示空间中，从而消除了差距，进而在零样本图像分类和多模态算术等下游任务中实现了更好的性能。

非模态鸿沟：对比鸿沟的特征与解决