  We revisit and advance visual prompting (VP), an input prompting technique
for vision tasks. VP can reprogram a fixed, pre-trained source model to
accomplish downstream tasks in the target domain by simply incorporating
universal prompts (in terms of input perturbation patterns) into downstream
data points. Yet, it remains elusive why VP stays effective even given a
ruleless label mapping (LM) between the source classes and the target classes.
Inspired by the above, we ask: How is LM interrelated with VP? And how to
exploit such a relationship to improve its accuracy on target tasks? We peer
into the influence of LM on VP and provide an affirmative answer that a better
'quality' of LM (assessed by mapping precision and explanation) can
consistently improve the effectiveness of VP. This is in contrast to the prior
art where the factor of LM was missing. To optimize LM, we propose a new VP
framework, termed ILM-VP (iterative label mapping-based visual prompting),
which automatically re-maps the source labels to the target labels and
progressively improves the target task accuracy of VP. Further, when using a
contrastive language-image pretrained (CLIP) model, we propose to integrate an
LM process to assist the text prompt selection of CLIP and to improve the
target task accuracy. Extensive experiments demonstrate that our proposal
significantly outperforms state-of-the-art VP methods. As highlighted below, we
show that when reprogramming an ImageNet-pretrained ResNet-18 to 13 target
tasks, our method outperforms baselines by a substantial margin, e.g., 7.9% and
6.7% accuracy improvements in transfer learning to the target Flowers102 and
CIFAR100 datasets. Besides, our proposal on CLIP-based VP provides 13.7% and
7.1% accuracy improvements on Flowers102 and DTD respectively. Our code is
available at https://github.com/OPTML-Group/ILM-VP.
