  Virtual viewpoints synthesis is an essential process for many immersive
applications including Free-viewpoint TV (FTV). A widely used technique for
viewpoints synthesis is Depth-Image-Based-Rendering (DIBR) technique. However,
such techniques may introduce challenging non-uniform spatial-temporal
structure-related distortions. Most of the existing state-of-the-art quality
metrics fail to handle these distortions, especially the temporal structure
inconsistencies observed during the switch of different viewpoints. To tackle
this problem, an elastic metric and multi-scale trajectory based video quality
metric (EM-VQM) is proposed in this paper. Dense motion trajectory is first
used as a proxy for selecting temporal sensitive regions, where local geometric
distortions might significantly diminish the perceived quality. Afterwards, the
amount of temporal structure inconsistencies and unsmooth viewpoints
transitions are quantified by calculating 1) the amount of motion trajectory
deformations with elastic metric and, 2) the spatial-temporal structural
dissimilarity. According to the comprehensive experimental results on two FTV
video datasets, the proposed metric outperforms the state-of-the-art metrics
designed for free-viewpoint videos significantly and achieves a gain of 12.86%
and 16.75% in terms of median Pearson linear correlation coefficient values on
the two datasets compared to the best one, respectively.
