  Deep neural networks have lead to a breakthrough in depth estimation from
single images. Recent work often focuses on the accuracy of the depth map,
where an evaluation on a publicly available test set such as the KITTI vision
benchmark is often the main result of the article. While such an evaluation
shows how well neural networks can estimate depth, it does not show how they do
this. To the best of our knowledge, no work currently exists that analyzes what
these networks have learned.
  In this work we take the MonoDepth network by Godard et al. and investigate
what visual cues it exploits for depth estimation. We find that the network
ignores the apparent size of known obstacles in favor of their vertical
position in the image. Using the vertical position requires the camera pose to
be known; however we find that MonoDepth only partially corrects for changes in
camera pitch and roll and that these influence the estimated depth towards
obstacles. We further show that MonoDepth's use of the vertical image position
allows it to estimate the distance towards arbitrary obstacles, even those not
appearing in the training set, but that it requires a strong edge at the ground
contact point of the object to do so. In future work we will investigate
whether these observations also apply to other neural networks for monocular
depth estimation.
