  To properly evaluate the performance of 360VR-specific encoding and
transmission schemes, and particularly of the solutions based on viewport
adaptation, it is necessary to consider not only the bandwidth saved, but also
the quality of the portion of the scene actually seen by users over time. With
this motivation, we propose a robust, yet flexible methodology for accurately
assessing the quality within the viewport along the visualization session. This
procedure is based on a complete analysis of the geometric relations involved.
Moreover, the designed methodology allows for both offline and online usage
thanks to the use of different approximations. In this way, our methodology can
be used regardless of the approach to properly evaluate the implemented
strategy, obtaining a fairer comparison between them.
