  Most of the existing semantic segmentation approaches with image-level class
labels as supervision, highly rely on the initial class activation map (CAM)
generated from the standard classification network. In this paper, a novel
"Progressive Patch Learning" approach is proposed to improve the local details
extraction of the classification, producing the CAM better covering the whole
object rather than only the most discriminative regions as in CAMs obtained in
conventional classification models. "Patch Learning" destructs the feature maps
into patches and independently processes each local patch in parallel before
the final aggregation. Such a mechanism enforces the network to find weak
information from the scattered discriminative local parts, achieving enhanced
local details sensitivity. "Progressive Patch Learning" further extends the
feature destruction and patch learning to multi-level granularities in a
progressive manner. Cooperating with a multi-stage optimization strategy, such
a "Progressive Patch Learning" mechanism implicitly provides the model with the
feature extraction ability across different locality-granularities. As an
alternative to the implicit multi-granularity progressive fusion approach, we
additionally propose an explicit method to simultaneously fuse features from
different granularities in a single model, further enhancing the CAM quality on
the full object coverage. Our proposed method achieves outstanding performance
on the PASCAL VOC 2012 dataset e.g., with 69.6$% mIoU on the test set), which
surpasses most existing weakly supervised semantic segmentation methods. Code
will be made publicly available here https://github.com/TyroneLi/PPL_WSSS.
