  The FlowNet demonstrated that optical flow estimation can be cast as a
learning problem. However, the state of the art with regard to the quality of
the flow has still been defined by traditional methods. Particularly on small
displacements and real-world data, FlowNet cannot compete with variational
methods. In this paper, we advance the concept of end-to-end learning of
optical flow and make it work really well. The large improvements in quality
and speed are caused by three major contributions: first, we focus on the
training data and show that the schedule of presenting data during training is
very important. Second, we develop a stacked architecture that includes warping
of the second image with intermediate optical flow. Third, we elaborate on
small displacements by introducing a sub-network specializing on small motions.
FlowNet 2.0 is only marginally slower than the original FlowNet but decreases
the estimation error by more than 50%. It performs on par with state-of-the-art
methods, while running at interactive frame rates. Moreover, we present faster
variants that allow optical flow computation at up to 140fps with accuracy
matching the original FlowNet.
