1. Introduction
360° cameras are becoming very popular, thanks to emerging Virtual Reality (VR) technologies and applications. More than a dozen new 360° cameras were released in 2016 [1], and the market is expected to grow by more than 100% per year in the next few years [2]. Watching panoramic photos and videos is becoming a common experience on content distribution sites like YouTube and Facebook, and many content creators are adopting the new medium. For example, BBC News distributes news in 360° videos online [3]. Whereas traditional cameras are restricted to a field of view (FOV) even narrower than human perception, a 360° camera captures the entire visual world from its optical center.