  Deep learning based methods have significantly boosted the study of automatic
building extraction from remote sensing images. However, delineating vectorized
and regular building contours like a human does remains very challenging, due
to the difficulty of the methodology, the diversity of building structures, and
the imperfect imaging conditions. In this paper, we propose the first
end-to-end learnable building contour extraction framework, named BuildMapper,
which can directly and efficiently delineate building polygons just as a human
does. BuildMapper consists of two main components: 1) a contour initialization
module that generates initial building contours; and 2) a contour evolution
module that performs both contour vertex deformation and reduction, which
removes the need for complex empirical post-processing used in existing
methods. In both components, we provide new ideas, including a learnable
contour initialization method to replace the empirical methods, dynamic
predicted and ground truth vertex pairing for the static vertex correspondence
problem, and a lightweight encoder for vertex information extraction and
aggregation, which benefit a general contour-based method; and a well-designed
vertex classification head for building corner vertices detection, which casts
light on direct structured building contour extraction. We also built a
suitable large-scale building dataset, the WHU-Mix (vector) building dataset,
to benefit the study of contour-based building extraction methods. The
extensive experiments conducted on the WHU-Mix (vector) dataset, the WHU
dataset, and the CrowdAI dataset verified that BuildMapper can achieve a
state-of-the-art performance, with a higher mask average precision (AP) and
boundary AP than both segmentation-based and contour-based methods.
