  Videos obtained by rolling-shutter (RS) cameras result in spatially-distorted
frames. These distortions become significant under fast camera/scene motions.
Undoing effects of RS is sometimes addressed as a spatial problem, where
objects need to be rectified/displaced in order to generate their correct
global shutter (GS) frame. However, the cause of the RS effect is inherently
temporal, not spatial. In this paper we propose a space-time solution to the RS
problem. We observe that despite the severe differences between their xy
frames, a RS video and its corresponding GS video tend to share the exact same
xt slices -- up to a known sub-frame temporal shift. Moreover, they share the
same distribution of small 2D xt-patches, despite the strong temporal aliasing
within each video. This allows to constrain the GS output video using
video-specific constraints imposed by the RS input video. Our algorithm is
composed of 3 main components: (i) Dense temporal upsampling between
consecutive RS frames using an off-the-shelf method, (which was trained on
regular video sequences), from which we extract GS "proposals". (ii) Learning
to correctly merge an ensemble of such GS "proposals" using a dedicated
MergeNet. (iii) A video-specific zero-shot optimization which imposes the
similarity of xt-patches between the GS output video and the RS input video.
Our method obtains state-of-the-art results on benchmark datasets, both
numerically and visually, despite being trained on a small synthetic RS/GS
dataset. Moreover, it generalizes well to new complex RS videos with motion
types outside the distribution of the training set (e.g., complex non-rigid
motions) -- videos which competing methods trained on much more data cannot
handle well. We attribute these generalization capabilities to the combination
of external and internal constraints.
