Difference-aware Adaptive Prompt Learning and Dense Alignment for Weakly Supervised Building Change Detection
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摘要: 遥感影像建筑物变化检测可为城市规划、土地资源管理和违章建筑排查等提供重要信息,但像素级标注成本高昂,限制了其在大规模场景中的应用。针对现有图像级弱监督建筑物变化检测方法存在语义先验利用不足、变化区域缺乏空间约束以及定位结果不完整等问题,设计一种差异感知提示学习与密集语义对齐的弱监督建筑物变化检测方法DAPL−CD(Difference-aware Adaptive Prompt Learning and Dense Alignment for Change Detection)。为了在降低像素级标注依赖的同时提升变化区域定位能力,该方法利用对比语言-图像预训练模型(Contrastive Language-Image Pre-training, CLIP)的跨模态表征能力,将文本语义与双时相影像差异特征映射至统一匹配空间,为变化区域定位提供语义参照。设计差异感知可学习文本提示自适应双时相影像中的复杂差异语义,缓解固定文本模板难以适配遥感场景的问题。针对CLIP全局图文对齐缺乏局部定位约束的问题,引入像素−文本密集对齐机制,约束局部视觉特征与前景、背景文本特征之间的匹配关系,增强变化区域与未变化区域的语义判别能力。实验结果表明,在仅依赖图像级标签的情况下,所提方法在WHU−CD和LEVIR−CD两个数据集上均取得了较优结果,能获得更完整、准确的变化区域定位效果,为大规模城市监测与违建排查等应用提供了一种低标注成本的技术思路。Abstract:
Objective Building change detection from bi-temporal high-resolution remote sensing images is an important task for urban planning, land resource management, illegal construction monitoring, and disaster damage assessment. Existing fully supervised change detection methods usually achieve high detection accuracy by relying on pixel-level annotations. However, obtaining pixel-level labels for large-scale remote sensing images is labor-intensive and time-consuming, which limits their practical application in large-scale monitoring scenarios. Image-level weakly supervised change detection reduces annotation costs by using only image-level labels that indicate whether an image pair contains changes. Nevertheless, the lack of spatial supervision makes it difficult to accurately locate changed regions. Current weakly supervised methods generally rely on class activation maps (CAMs) to generate pseudo labels, but CAMs tend to highlight only the most discriminative regions and may ignore complete change areas or introduce background noise. Vision-language models provide a new way to introduce semantic priors into weakly supervised learning. However, directly applying Contrastive Language-Image Pre-training (CLIP) to change detection is still challenging. Fixed text prompts are difficult to adapt to the difference semantics of bi-temporal images, and the original CLIP objective mainly focuses on global image-text alignment rather than pixel-level localization. To address these problems, this paper proposes a Difference-aware Adaptive Prompt Learning and Dense Alignment method for weakly supervised building Change Detection, named DAPL-CD. Methods The framework introduces CLIP-based cross-modal semantic knowledge into image-level weakly supervised building change detection. For a pair of bi-temporal remote sensing images, a shared CLIP visual encoder is first used to extract visual representations from the two temporal images. The local visual features are fused along the channel dimension to obtain bi-temporal difference features, which contain semantic information related to changed and unchanged regions. Based on the difference characteristics of building change detection, a difference-aware adaptive prompt learning strategy is designed. Instead of manually designed fixed text templates, learnable context vectors are inserted into text prompts while preserving category-related semantic words. The generated foreground and background text embeddings are used as semantic prototypes to provide adaptive semantic guidance for change localization. Furthermore, a pixel-text dense alignment mechanism is introduced to extend CLIP’s global image-text alignment capability to local feature matching. The initial CAM generated by the classification branch is used to obtain preliminary foreground and background regions. Then, visual-text positive and negative sample pairs are constructed between local difference features and text embeddings. An InfoNCE-based dense alignment loss is employed to pull matched visual and textual features closer and push mismatched features apart. Finally, the classification branch and segmentation branch are jointly optimized using classification loss, global alignment loss, dense alignment loss, and segmentation loss. To avoid unreliable pseudo labels during early training, the segmentation loss is introduced after the CAM quality becomes relatively stable. Results and Discussions Experiments are conducted on WHU-CD and LEVIR-CD, two public benchmark datasets for building change detection. During training, only image-level labels are used, while pixel-level annotations are used only for evaluation. Overall Accuracy (OA), F1-score, and Intersection over Union (IoU) are adopted as the main evaluation metrics. Because changed buildings usually account for a small proportion of remote sensing images, OA can be strongly affected by the dominant unchanged background pixels. Therefore, F1-score and IoU are emphasized to evaluate the detection quality of changed regions. Quantitative comparison results show that DAPL-CD achieves 94.7% OA, 82.8% F1-score, and 70.6% IoU on the WHU-CD dataset, and obtains 92.3% OA, 68.0% F1-score, and 51.5% IoU on the LEVIR-CD dataset, outperforming the compared weakly supervised change detection methods in terms of F1-score and IoU ( Table 1 ). Visual comparison results further demonstrate that the proposed method produces more complete responses for large-scale building changes and more continuous predictions for small and scattered changed buildings (Fig.3 ,Fig.4 ). Ablation experiments verify the effectiveness of the proposed adaptive prompt learning and pixel-text dense alignment mechanisms. The baseline model, which uses fixed text prompts without foreground or background alignment, achieves an F1-score of 63.2% and an IoU of 46.2%. Introducing both foreground and background alignment improves the two metrics to 66.3% and 49.6%, respectively, demonstrating that dense semantic matching between local visual features and text prototypes enhances the discrimination of changed regions. After adaptive prompt learning is incorporated, the F1-score and IoU increase to 68.0% and 51.5%, indicating that learnable prompts alleviate the semantic mismatch between fixed text descriptions and bi-temporal difference features. Under the learnable-prompt setting, foreground alignment alone achieves an F1-score of 64.4% and an IoU of 47.5%, whereas background alignment alone obtains 65.3% and 48.5%, respectively. Combining the two alignment branches achieves the best F1-score and IoU, demonstrating that foreground and background semantic constraints provide complementary guidance for change localization (Table 2 ). The CAM results show that dense alignment produces stronger and more complete responses over actual changed regions while suppressing irrelevant background activations (Fig. 5 ).Conclusions A weakly supervised building change detection framework based on difference-aware adaptive prompt learning and pixel-text dense semantic alignment is proposed. By introducing CLIP-based vision-language semantic priors, the proposed method effectively transforms text-level semantic knowledge into local change localization capability. The adaptive prompt learning strategy improves the representation of change-related semantic descriptions, while the dense alignment mechanism establishes direct correspondence between local difference features and foreground/background semantic prototypes. Experimental results on WHU-CD and LEVIR-CD demonstrate that DAPL-CD achieves competitive performance under image-level supervision and improves the completeness and accuracy of changed building localization. The proposed framework provides an effective solution for reducing annotation requirements in large-scale remote sensing change detection. Future research will focus on improving pseudo-label reliability, reducing dependence on large-scale pre-trained models, and extending the method to multi-temporal or multi-spectral remote sensing data. -
表 1 不同数据集的定量结果对比
表 2 重要模块的消融实验结果
前景对齐 背景对齐 可学习文本提示 上下文长度$ M $ OA F1 IoU × × × − 89.6 63.2 46.2 × × √ 16 89.5 63.7 46.7 √ × √ 16 89.6 64.4 47.5 × √ √ 16 90.1 65.3 48.5 √ √ × − 91.2 66.3 49.6 √ √ √ 8 92.7 67.6 51.0 √ √ √ 16 92.3 68.0 51.5 √ √ √ 32 92.2 67.1 50.5 -
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