IEEE Consumer Electronics Magazine - May 2018 - 41

human visual contrast sensitivities over a specific range of
luminance values. The PQ specification achieves a very wide
range of brightness levels for a given bit depth using a nonlinear TF that is finely tuned to match the HVS.

Scene
Light

Artistic
Adjustments

OETF

aDaptive ReSHapeR
The purpose of the adaptive reshaper, intricately described in
[2] and [8], is to modify the post-EOTF signal. The reshaper is
also able to create SDR from HDR signal. This capability will
not be investigated during our experiments. PQ functions are
designed for a reference display with dynamic range from 0 to
10,000 cd/m2. However, due to current display limitations, the
video dynamic range might be smaller. The reshaper is
designed to solve the problem of the range mapping of the
content. In addition, HDR and WCG lead to larger color volumes. Therefore, the reshaper addresses color and intensity
compression efficiency.
The reshaper is modeled for luma as a piecewise polynomial of the second order, with up to eight pieces. For chroma
components, a piecewise linear model with up to 32 pieces is
used. The reshaper improves the coding efficiency of a signal
as the aforementioned parametric reshaping models
implement the following two features: 1) adaptive codeword
redistribution based on pixel brightness and 2) signal requantization among luma and chroma components, which ultimately
changes the bit rate allocation for the three components.
The overall process of the reshaper starts with the preprocessing of the signal and its content and intent analysis. The
signal is then adjusted, and the transform curve is derived.
The curve is then modeled in the luma and chroma polynomials that are transmitted along with the signal. The signal is
applied to an encoder and compressed by normal means.
Upon decoding, the polynomials are used to create a look-up
table (LUT), and the decoded pixels are modulated by the
LUT. The result of the postprocess LUT is a representation of
the original input signal. A comprehensive description of the
reshaper process is detailed in [21].

SuBjecTIVe ASSeSSMeNTS
In this section, visual quality subjective assessments comparing the performance of two different aspects are presented. First, the perceived quality of HDR video sequences
represented in two color spaces, Y'CbCr and ICtCp, is evaluated. Then, the performance of a reshaper in each investigated color space is addressed. This investigation is, to the
best of our knowledge, the first attempt to subjectively
assess the performance of these algorithms. The goal is to

Display
Light

Artistic OOTF
(a)

HlG
The HLG was designed based on the model shown in Figure 3(c),
where the OOTF is considered to be on the display side. This
system is then defined by its OETF. A comprehensive
description and analysis of this system is presented in [3].
HLG specification offers a degree of compatibility with legacy displays more closely matching the previous established
television transfer curves.

EOTF

Scene
Light

OOTF

Inverse
EOTF

EOTF

Display
Light

OETF
Encoding
Camera

Scene
Light

Decoding
Display

Signal
(b)
Inverse
OETF

OETF

OOTF

EOTF
Encoding
Camera

Display
Light

Decoding
Signal
(c)

Display

FIgure 3. The operating diagrams of ooTF functions [14]. (a) a
reference ooTF, (b) ooTF in the camera (e.g., pQ), and (c) ooTF
in the display (e.g., HlG).

determine whether the use of ICtCp color space and the
integration of a reshaper can be justified in future HDR compression standards.
As previously mentioned, the ICtCp color space is
designed to overcome constraints of the Y'CbCr space, such
as luminance and chrominance channels correlation, bitdepth restriction, and hue nonlinearity, among others. Moreover, the design of the ICtCp color space reflects the
properties of the HVS. The objective quality benchmarks of
ICtCp performance in terms of compression efficiency were
presented in [20]. Nevertheless, to reliably compare the visual quality of the proposed algorithms, subjective assessments
need to be performed. This allows for completely and comprehensively appraising the benefits of using the ICtCp color
space for HDR and WCG.
Another way to increase compression efficiency in HDR and
WCG imaging is to use a reshaper. In general, a reshaper is a
mapping function used to adjust the pixel intensity values in a
way that optimizes compression. The objective quality benchmarks of the reshaper performance in terms of compression
efficiency were presented in [21]. The metrics used during the
objective quality benchmarks of the reshaper performance and
ICtCp compression efficiency were DE100, L100, X, Y, Z peak
signal-to-noise ratio, and optical signal-to-noise ratio metrics.
Our aim is to verify the efficiency of the reshaper in terms of bit
rate saving while preserving the perceived visual quality.

teSt MateRial
Five 16-bit full HD clips of roughtly 10 s were selected for
our experiments (see Figure 4). Three Moving Picture Experts
Group (MPEG) contents, namely Market3, BalloonFestival,
may 2018

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