  The Digital Video Broadcasting (DVB) has proposed to introduce the Ultra-High
Definition services in three phases: UHD-1 phase 1, UHD-1 phase 2 and UHD-2.
The UHD-1 phase 2 specification includes several new features such as High
Dynamic Range (HDR) and High Frame-Rate (HFR). It has been shown in several
studies that HFR (+100 fps) enhances the perceptual quality and that this
quality enhancement is content-dependent. On the other hand, HFR brings several
challenges to the transmission chain including codec complexity increase and
bit-rate overhead, which may delay or even prevent its deployment in the
broadcast echo-system. In this paper, we propose a Variable Frame Rate (VFR)
solution to determine the minimum (critical) frame-rate that preserves the
perceived video quality of HFR video. The frame-rate determination is modeled
as a 3-class classification problem which consists in dynamically and locally
selecting one frame-rate among three: 30, 60 and 120 frames per second. Two
random forests classifiers are trained with a ground truth carefully built by
experts for this purpose. The subjective results conducted on ten HFR video
contents, not included in the training set, clearly show the efficiency of the
proposed solution enabling to locally determine the lowest possible frame-rate
while preserving the quality of the HFR content. Moreover, our VFR solution
enables significant bit-rate savings and complexity reductions at both encoder
and decoder sides.
