IEEE Consumer Electronics Magazine - October 2015 - 81

challenges have been addressed by this group such as how to
set up a subjective evaluation for HDR content, which objective metrics correlate well with subjective results, displays
capabilities, etc. In terms of HDR coding techniques, three
categories have been defined:
▼ compression efficiency improvement over HEVC Main 10
profile
▼ backward-compatible solutions
▼ optimization of the performance of the existing Main 10
Profile and/or Scalable Main 10 Profile (non-normative
changes only, pre-/postprocessing, encoder only modifications, etc.).
This group issued a call for evidence (CfE) in February
2015 to determine if there is enough evidence that current
standards are not efficient enough to address the requirements
for HDR use-case scenarios [33]. Several proposals were
submitted addressing the above-mentioned three categories.
Preliminary subjective evaluations of these proposals provided evidence that significant compression improvements
over the HEVC standard can be achieved [34]. Following
the conclusion of the CfE in the 112th MPEG meeting, the
effort on HDR/WCG video coding has been put on a fast
track with the objective to complete the standardization of
HDR compression technologies quickly. Note that standardization of the compression and distribution of HDR content
is mandatory for the widespread adoption of this technology
by the end consumer.

DISPLAYS
Similar to acquisition technology, current display technologies are not capable of representing the full color gamut and
luminance range existing in the real world. Most of the existing displays rely on 8 b for each of the three color components and can, therefore, represent 65,025 colors over 255
levels of gray. In addition to the limited code values available
to encode different colors or gray levels, displays have two
other limitations: their peak luminance and their contrast. The
peak luminance indicates the maximum amount of brightness
that can be perceived by an observer while staring at the display. It is achieved when displaying the maximum code value
represented by the three color components of (R = 255,

HDR
Content

Codec

Codec-1

Color and Tone
Mapping
Distribution

G = 255, and B = 255), also known as the white point of a
display. Similarly, the black point of a display is represented
by the three color components of (R = 0, G = 0, and B = 0).
On an LCD-based display, the black point is never truly 0
cd/m 2 because black is simulated by the crystals of the panel
being completely shut, along with a polarized layer behind the
crystals, to prevent light from the backlight to go through. Yet,
precisely because the backlight is still on and the structure of
the crystal array is not perfect, some amount of light will leak
through the crystal and be seen. The contrast of a display is the
ratio between the white and black point of a display. Figure 2
illustrates the range of commonly achieved luminance displayable in comparison to what the human eye can see.
To avoid using techniques that reduce the quality of the
displayed HDR images, Seetzen et al. proposed in [35] a
design to create HDR displays by combining two light modulation devices: a modulated backlight and an LCD panel. The
effective dynamic range of the image created is going to be a
product of the dynamic range of both modulators (see [36]
for more details). With this approach, the backlight emits
white light that is later filtered by the LCD panel using color
filters to achieve any color within the gamut of the display.
Two types of backlight can be used for creating an HDR display. The first approach employs a digital projector producing a modulated backlight that later falls onto the back of an
LCD panel. In a second design, a hexagonal matrix of individually controlled white light-emitting diodes (LEDs) is
used to produce the backlight for the LCD panel. This technique has been used in the first commercially available HDR
display that can achieve 4,000 cd/m 2 peak luminance [37].
In addition to increasing the dynamic range, HDR displays
should be able to cover most of the full visible gamut. However, using white LED along with the LCD panel prevents
achieving highly saturated colors. This is due to the imperfection of the current LCD panels, where at a given pure color
(for example, red), light leakage from crystals associated with
other colors (for example, green and blue) can reduce the saturation. Moreover, the saturation of the primaries (i.e., gamut
primaries) depends on the white LED and its color filters. To
address these issues, one approach is to replace each white
LED with three or more organic LEDs (OLEDs) that emit

HDR
Display

HD/UHD
TV (LDR)

Inverse Color
and Tone Mapping
HD or
UHD
Content
(LDR)

Codec

Codec-1

HDR
Display
HD/UHD
TV (LDR)

Distribution
(a)

(b)

FIGURE 16. (a) The distribution of HDR content and derivation of LDR content at the display stage and (b) the distribution of LDR content and derivation of HDR content at the display stage.

october 2015

IEEE Consumer Electronics Magazine

Table of Contents for the Digital Edition of IEEE Consumer Electronics Magazine - October 2015