IEEE Consumer Electronics Magazine - October 2015 - 73

image courtesy of freeimages.com/shadman ahmed

The half-float pixel representation is used
in the new Academy Color Encoding System
standard, which is currently undergoing industrywide production trials by major Hollywood studios.

In a nutshell, HDR technology aims at capturing, distributing, and displaying a range of luminance
and color values that better correspond to what the human eye can perceive. Here, the term luminance
stands for the photometric quantity of light arriving at the human eye measured in candela per square
meter (cd/m 2) or nits. The color refers to all the weighted combinations of spectral wavelengths,
expressed in nanometers (nm), emitted by the sun that are visible by the human eye (see Figure 1).
The human eye can perceive a dynamic range of over 14 orders of magnitude (i.e., the difference in
powers of ten between highest and lowest luminance value) in the real world through adaptation.
However, at a single adaptation time, the human eye can only resolve up to five orders of magnitude,
as illustrated in Figure 2. Dynamic range denotes the ratio between the highest (Ymax) and lowest
(Ymin) luminance value. As reported in Table 1, there are different interpretations for dynamic range,
depending on the application. For instance, in photography, dynamic range is measured in terms of
f-stops, which correspond to the number of times that the light intensity can be doubled.
HDR image and video technology can capture all the light and color information of a scene, contrary
to the regular low-dynamic-range (LDR) acquisition that can capture and reproduce only a limited dynamic range and color information present in a real-world scene (which loses details through under-/overexposed pixels). These limitations are imposed by capturing and displaying technologies as well as the
representation of pixels throughout the distribution pipeline. While many manufacturers already claim the
introduction of HDR-enabled devices, there is still uncertainty around what constitutes HDR, with standardization bodies working hard trying to define its specifications. This uncertainty comes from the fact
that for a long time, and in the absence of HDR display technology, different techniques have been proposed to display HDR content on traditional LDR displays. These techniques, known as tone mapping
operators (TMOs), adapt HDR content to the limitations of LDR displays [1]. Tone mapping results in a
loss of information as well as quality in the final content. However, as Figure 3 illustrates, the amount of
detail information reproduced in an HDR tone-mapped image is far more than what is captured by an
LDR device, attesting that HDR has the potential to revolutionize the digital media market.
Through tone mapping, the quality of content distributed to the end user can be increased; however, a commercial HDR distribution pipeline is not yet viable, since from acquisition to display, only
prototype devices exist. Although the HDR displays presented at commercial shows are only a
glimpse into what future displays can achieve, the quality of the displayed content is quite stunning
compared to LDR displays. Furthermore, HDR technology is about not only increasing the content
quality but also elevating creativity at the mastering stage. This arises from the fact that HDR corresponds to the physical values of the light intensity falling on a sensor or emitted by a display, allowing
content creation/mastering to not be tied to the limitations of the display but rather match with original
physical values. Consequently, the end consumer can potentially enjoy the content as originally imagined by its creator and not as its interpretation by a display. Increased creative freedom is accomplished because HDR technology can accurately represent a higher dynamic range and color gamut
than LDR technology. In fact, LDR cameras can usually capture more than what displays can achieve
(see Figure 2); hence, the content creator has to squeeze information in a constrained LDR space. A
tradeoff is usually mandatory to preserve contrast where it is most needed while losing information in
highlights and shadows.
From the HDR distribution and transmission point of view, while existing video compression
standards are capable of compressing HDR video content, their performance is optimized for LDR
video content. Thus, further investigations are required to ensure the efficiency of these standards
for HDR video content transmission. Furthermore, cohabitation in the pipeline between HDR and
LDR video content is mandatory because the transition between those two representations will not
be instantaneous.
october 2015

IEEE ConsumEr ElECtronICs magazInE

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