Algorithm Design for Gamut Mapping From UHDTV to HDTV

With the advent of Recommendation ITU-R BT.2020 (Rec. 2020) color space for ultra-high definition television (UHDTV), the development of a gamut mapping algorithm for wide to standard gamuts is greatly needed. In wide-gamut UHDTV production, particularly under the assumption of simultaneous live UHDTV and high-definition television (HDTV) broadcast scenarios, real-time gamut mapping from UHDTV to HDTV is vital. Designing such an algorithm requires a careful consideration of the shape and size of significantly different gamuts. In this paper, we set some requirements for gamut mapping from UHDTV to HDTV, and propose a hue-preserved gamut mapping algorithm. The mapping paths are designed in the CIE 1976 ${L^{\ast}}{a^{\ast}}{b^{\ast}}$ (CIELAB) color space through modifications of the hue prediction based on our subjective experimental results on the constant perceived hue. To prevent an excessive chroma reduction, the lightness of the out-of-gamut colors is changed. The design process and mathematical expressions of our proposed gamut mapping algorithm are provided. In addition, we demonstrate the improvements in image quality.

I. Introduction

Ultra-High definition television (UHDTV) is a new television system that provides a better viewing experience than a high-definition television (HDTV) system. In August 2012, the ITU-R issued Recommendation BT.2020 (Rec. 2020) [1] specifying the video parameter values for UHDTV production and international program exchange. The important features of Rec. 2020 include a wider color gamut than that of HDTV specified in Recommendation ITU-R BT.709 (Rec. 709) [2], in addition to high pixel counts of 4K ( pixels) and 8K ( pixels), a higher frame frequency of 120 Hz, and a higher bit depth of 12 bits.