  Free-Viewpoint Video (FVV) systems allow the viewers to freely change the
viewpoints of the scene. In such systems, view synthesis and compression are
the two main sources of artifacts influencing the perceived quality. To assess
this influence, quality evaluation studies are often carried out using
conventional displays and generating predefined navigation trajectories
mimicking the possible movement of the viewers when exploring the content.
Nevertheless, as different trajectories may lead to different conclusions in
terms of visual quality when benchmarking the performance of the systems,
methods to identify critical trajectories are needed. This paper aims at
exploring the impact of exploration trajectories (defined as Hypothetical
Rendering Trajectories: HRT) on perceived quality of FVV subjectively and
objectively, providing two main contributions. Firstly, a subjective assessment
test including different HRTs was carried out and analyzed. The results
demonstrate and quantify the influence of HRT in the perceived quality.
Secondly, we propose a new objective video quality assessment measure to
objectively predict the impact of HRT. This measure, based on Sketch-Token
representation, models how the categories of the contours change spatially and
temporally from a higher semantic level. Performance in comparison with
existing quality metrics for FVV, highlight promising results for automatic
detection of most critical HRTs for the benchmark of immersive systems.
