  The efficacy of building footprint segmentation from remotely sensed images
has been hindered by model transfer effectiveness. Many existing building
segmentation methods were developed upon the encoder-decoder architecture of
U-Net, in which the encoder is finetuned from the newly developed backbone
networks that are pre-trained on ImageNet. However, the heavy computational
burden of the existing decoder designs hampers the successful transfer of these
modern encoder networks to remote sensing tasks. Even the widely-adopted deep
supervision strategy fails to mitigate these challenges due to its invalid loss
in hybrid regions where foreground and background pixels are intermixed. In
this paper, we conduct a comprehensive evaluation of existing decoder network
designs for building footprint segmentation and propose an efficient framework
denoted as BFSeg to enhance learning efficiency and effectiveness.
Specifically, a densely-connected coarse-to-fine feature fusion decoder network
that facilitates easy and fast feature fusion across scales is proposed.
Moreover, considering the invalidity of hybrid regions in the down-sampled
ground truth during the deep supervision process, we present a lenient deep
supervision and distillation strategy that enables the network to learn proper
knowledge from deep supervision. Building upon these advancements, we have
developed a new family of building segmentation networks, which consistently
surpass prior works with outstanding performance and efficiency across a wide
range of newly developed encoder networks.
