soundandvision.com (^) [ 29
rience? In simplest terms, it means
there is an increase in the number
of brightness levels the display is
capable of reproducing. This results
in a more “true to life” image: no more
muted headlights on dark highways,
or dim stars against the black of
space. But it doesn’t stop there.
While HDR photography is well
established, HDR video is relatively
new. Much of it results from work
done by Dolby Laboratories, which
first launched HDR in theaters with
Dolby Cinema and later brought it to
Ultra HD Blu-ray and streaming via
Dolby Vision. The most common HDR
format is HDR10, a bare-bones
version derived from the work the
company did with Dolby Vision that
was provided to the industry
royalty-free. Additional HDR formats
include Hybrid Log Gamma (HLG),
Technicolor’s Advanced HDR, and
HDR10+, but all of these are either in
early development or have limited
available content. I’ll touch on them
more in a bit.
LIGHT, BRIGHT: SDR VS. HDR
What does HDR bring to the table?
HDR’s key benefits include an ability
to render much brighter highlights,
deliver vastly increased color volume
(both saturation and brightness), and
boost video fidelity through higher
bit depths. The standard dynamic
range (SDR) provided by the HDTV
format—the one we are used to
watching—is mastered with a peak
video brightness level of 100 nits.
HDR, in contrast, can reach up to
10,000 nits! An early bit of misinfor-
mation surrounding the format was
that HDR’s added brightness would
create eyestrain, but that isn’t how it
works. Much of the brightness range
in the video image remains in the
same range as it does with SDR, but
HDR provides additional headroom
for brightness “peaks” (streetlights,
luminance, or light output, HDR
provides that delivers a wider color
range.
Another HDR component is higher
video bit depth. HD/SDR video
content was limited to 8-bit resolution,
with video from black to white (100
nits) divided up into 256 levels. But
the HDR10 format uses 10-bit
resolution, which increases the range
to 1024 levels. The main benefit here
is the elimination of banding artifacts
(typically seen in the blue gradients of
skylines, or in dark scenes with subtle
tonal variations). The use of 10-bit
encoding also allows for more precise
rendering of near-black tones, which
create a particular challenge during
the video mastering process.
BEYOND GAMMA
While the benefits of HDR are
substantial, it doesn’t come without
caveats. First off, HDR doesn’t
employ the traditional gamma
system used by HDTV and the
earlier NTSC video format, which
takes into account the display’s
peak brightness and lowest black
level capability and then adjusts
the in-between values to create a
desired balance. HDR instead uses
an absolute system called Perceptual
Quantizer (PQ) where each video
level is mapped one-to-one with the
display. PQ makes the bold assump-
tion that the display will provide
enough range at both the high and
or the sparkle of sun reflected off
chrome, for example). These peaks,
also called specular highlights, are
what give images a natural look that
more closely matches what we see
in the world around us every day.
While new digital cameras used for
cinematography are responsible
for much of HDR’s impact, older
film-based content also contains
sufficiently high dynamic range to
allow for mastering in the format.
A WIDER WORLD OF COLOR
Color is the next key component of
HDR. The Ultra HDTV format uses
a color gamut called BT.2020 that’s
notably wider than the Rec. 709 one
used for standard HDTV. However,
the added brightness provided by
HDR extends the volume of color
as well. While color in the A/V
world is traditionally represented in
two-dimensional space with data
points for saturation and hue, it also
extends out in the vertical direction
with luminance. It’s the increased
Opposite page,
bottom: HDR
formats like
Dolby Vision
bring the
dynamic
range of the
video delivery
infrastructure
closer to that of
the human eye.
Top
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ty.T
V Because of the
nature of PQ,
there’s an ideal
fixed relationship
between video
signal values and
the light output of
the display. The
reality of what a
display can do
might look like
the red trace in
the graph at right.
Above: HDR vs.
SDR color volume.
The increased
luminance that
HDR provides
delivers a wider
color range.
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