Feature Purification Matters: Suppressing Outlier Propagation for Training-Free Open-Vocabulary Semantic Segmentation

Training-free open-vocabulary semantic segmentation has advanced with vision-language models like CLIP, which exhibit strong zero-shot abilities. However, CLIP's attention mechanism often wrongly emphasises specific image tokens, namely outliers, which results in irrelevant over-activation. Existing approaches struggle with these outliers that arise in intermediate layers and propagate through the model, ultimately degrading spatial perception. In this paper, we propose a Self-adaptive Feature Purifier framework (SFP) to suppress propagated outliers and enhance semantic representations for open-vocabulary semantic segmentation. Specifically, based on an in-depth analysis of attention responses between image and class tokens, we design a self-adaptive outlier mitigator to detect and mitigate outliers at each layer for propagated feature purification. In addition, we introduce a semantic-aware attention enhancer to augment attention intensity in semantically relevant regions, which strengthens the purified feature to focus on objects. Further, we introduce a hierarchical attention integrator to aggregate multi-layer attention maps to refine spatially coherent feature representations for final segmentation. Our proposed SFP enables robust outlier suppression and object-centric feature representation, leading to a more precise segmentation. Extensive experiments show that our method achieves state-of-the-art performance and surpasses existing methods by an average of 4.6% mIoU on eight segmentation benchmarks. The code will be released.