  Human can infer the 3D geometry of a scene from a sketch instead of a
realistic image, which indicates that the spatial structure plays a fundamental
role in understanding the depth of scenes. We are the first to explore the
learning of a depth-specific structural representation, which captures the
essential feature for depth estimation and ignores irrelevant style
information. Our S2R-DepthNet (Synthetic to Real DepthNet) can be well
generalized to unseen real-world data directly even though it is only trained
on synthetic data. S2R-DepthNet consists of: a) a Structure Extraction (STE)
module which extracts a domaininvariant structural representation from an image
by disentangling the image into domain-invariant structure and domain-specific
style components, b) a Depth-specific Attention (DSA) module, which learns
task-specific knowledge to suppress depth-irrelevant structures for better
depth estimation and generalization, and c) a depth prediction module (DP) to
predict depth from the depth-specific representation. Without access of any
real-world images, our method even outperforms the state-of-the-art
unsupervised domain adaptation methods which use real-world images of the
target domain for training. In addition, when using a small amount of labeled
real-world data, we achieve the state-ofthe-art performance under the
semi-supervised setting.
