ABSTRACT
A new, more immersive, television experience is here. With higher resolution, wider color gamut and extended dynamic range, the new Ultra High Definition (UHD) TV standards define a container which allows content creators to offer the consumer a much more immersive visual experience.
Although very little content exploiting the full range of the container is yet available, some artifacts associated with the compression of high dynamic range (HDR) content have already been identified and reported in the literature.
Specifically, the chroma subsampling process has been shown to cause disturbing artifacts for image regions of certain color and luminance.
This paper quantifies the distortion and identifies regions of the extended color volume where artifacts associated with standard image processing techniques are more likely to occur. In doing so, it highlights that the problems will become greater as more content exploiting the full UHD container becomes available, requiring additional care and processing in content production and delivery. Finally, the paper references ways of overcoming these issues.
INTRODUCTION
The DVB UHD1 Phase 2 and ATSC 3.0 standards are becoming well established and allow significant benefits over High Definition television standards beyond simply more pixels.
The technological enhancements of High Dynamic Range (HDR), Wide Color Gamut (WCG) and High Frame Rate (HFR) all contribute to more life-like images and hence a more immersive viewing experience.
ITU-R Recommendation BT.2100 (Rec. 2100) defines parameters and formats for HDR television, using a minimum 𝑌’𝐶𝑏𝐶𝑟 component bit-depth of 10-bits, color primaries from ITU-R Recommendation BT.2020 (Rec. 2020), and two alternative Electro-Optical Transfer Functions (𝐸𝑂𝑇𝐹s): Perceptual Quantiser (PQ, standardised as SMPTE ST 2084) and Hybrid Log-Gamma (HLG, standardised as ARIB STD-B67).
This paper examines a system using the PQ transfer function, but many of the principles will also apply to HLG. The PQ transfer function is specifically designed to exploit the relative insensitivity of the human visual system to absolute differences in luminance when the luminance is high.
It achieves this by allocating more code words to lower values of the red, green and blue component signals, than to higher values, and as such is much more non-linear than traditional SDR transfer functions.
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