Activation to Saliency: Forming High-Quality Labels for Unsupervised Salient Object Detection

Unsupervised Salient Object Detection (USOD) is of paramount significance for both industrial applications and downstream tasks. Existing deep-learning (DL) based USOD methods utilize some low-quality saliency predictions extracted by several traditional SOD methods as saliency cues, which mainly capture some conspicuous regions in images. Furthermore, they refine these saliency cues with the assistant of semantic information, which is obtained from some models trained by supervised learning in other related vision tasks. In this work, we propose a two-stage Activation-to-Saliency (A2S) framework that effectively generates high-quality saliency cues and uses these cues to train a robust saliency detector. More importantly, no human annotations are involved in our framework during the whole training process. In the first stage, we transform a pretrained network (MoCo v2) to aggregate multi-level features to a single activation map, where an Adaptive Decision Boundary (ADB) is proposed to assist the training of the transformed network. To facilitate the generation of high-quality pseudo labels, we propose a loss function to enlarges the feature distances between pixels and their means. In the second stage, an Online Label Rectifying (OLR) strategy updates the pseudo labels during the training process to reduce the negative impact of distractors. In addition, we construct a lightweight saliency detector using two Residual Attention Modules (RAMs), which refine the high-level features using the complementary information in low-level features, such as edges and colors. Extensive experiments on several SOD benchmarks prove that our framework reports significant performance compared with existing USOD methods. Moreover, training our framework on 3000 images consumes about 1 hour, which is over 30x faster than previous state-of-the-art methods.