SGM-MDE: Semi-global optimization for classification-based monocular depth estimation

Depth estimation plays a crucial role in robotic applications that require environment perception. With the introduction of convolutional neural networks, monocular depth estimation (MDE) methods have become viable alternatives to LiDAR and stereo reconstruction-based solutions. Such methods require less equipment, fewer resources and do not need additional sensor alignment requirements. However, due to the ill-posed formulation of MDE, such algorithms can only rely on learning mechanisms, which makes them less reliable and less robust.In this work we propose a novel method to cope with the lack of geometric constraints inherent to monocular depth computation. Towards this goal, we initially mathematically transform the feature vectors from the last layer inside a MDE CNN such that a 3D stereo-like cost volume is generated. We then adapt the semi-global stereo optimization to the aforementioned volume, global consistency of the map being ensured. Furthermore, we enhance the results by adding a sub-pixel stereo post-processing be means of interpolation functions, a larger range of depth values being obtained. Our method can be applied to any classification-based MDE, experiments showing an increase in accuracy with an additional time cost of only 8 ms on a regular GPU, making the technique usable for real-time applications.