  Omnidirectional (or 360-degree) images and videos are emergent signals in
many areas such as robotics and virtual/augmented reality. In particular, for
virtual reality, they allow an immersive experience in which the user is
provided with a 360-degree field of view and can navigate throughout a scene,
e.g., through the use of Head Mounted Displays. Since it represents the full
360-degree field of view from one point of the scene, omnidirectional content
is naturally represented as spherical visual signals. Current approaches for
capturing, processing, delivering, and displaying 360-degree content, however,
present many open technical challenges and introduce several types of
distortions in these visual signals. Some of the distortions are specific to
the nature of 360-degree images, and often different from those encountered in
the classical image communication framework. This paper provides a first
comprehensive review of the most common visual distortions that alter
360-degree signals undergoing state of the art processing in common
applications. While their impact on viewers' visual perception and on the
immersive experience at large is still unknown ---thus, it stays an open
research topic--- this review serves the purpose of identifying the main causes
of visual distortions in the end-to-end 360-degree content distribution
pipeline. It is essential as a basis for benchmarking different processing
techniques, allowing the effective design of new algorithms and applications.
It is also necessary to the deployment of proper psychovisual studies to
characterise the human perception of these new images in interactive and
immersive applications.
