ThesRGBcolourspaceisrelativelysmall,sothere’s plenty of room for improvement with HDRD65
REC.2020
DCI-P3
sRGB
ADOBE RGB
On the surface, the definition of HDR
is a quite simple one: it’s blacker
blacks, brighter whites and more
colour in between. However, defining
the standards behind these basic
ideas requires us to look in depth
at three fundamental definitions of
computer displays: colour depth,
colour space and contrast.
Colour depth is how accurately we
can define colours. For each of the three
primary colours (red, green and blue)
that are used to recreate all the colours
we see on modern computers, each
colour channel can be set to one of 256
levels (8-bit). When combined, these can
define up to 16,777,216 colours. With most
HDR standards, the accuracy with which
colours can be described is increased
from 8-bit per channel to 10-bit, resulting
in a total of 1,073,741,824 colours.
As for colour space, this defines not
the accuracy of colours but the overall
range of them. So, a larger colour
space allows for richer-looking colours
while a small space results in a narrow,
duller-looking range. Most such colour
spaces are defined in reference to
the CIE 1931 xy colour space, which
approximately describes the full range
of colours visible to the human eye.
The default for computing is sRGB; as
you can see from the colour space chart
above right, this only covers a relatively
small area of the CIE 1931 xy space -
35.9% to be exact. Another popular colour
space used a lot in the photography and
print industries is Adobe RGB, which
covers 52.1%, with it greatly extending
the range of green colour representation
in particular. With HDR, a range of colour
spaces are used by different standards,
but the most common are DCI-P3 and
Rec. 2020. DCI-P3 covers 45.5% of CIE
1931 xy while Rec. 2020 covers 63.3%,
What is
HDR?
making it comfortably the most demanding
of the most common colour spaces.
It’s this greater span of colour that
necessitates moving to a more granular
colour depth. A depth of 8-bit is enough
to cover all the visible transitions
between colours for sRGB but for larger
colour spaces, more numbers are
needed to define all those colours.
The final piece of the puzzle is contrast,
which is the difference between the
brightest and darkest colours a display
can show. Most LCD displays can producea contrast ratio of 1000:1, so the
brightest pixel on screen can be
1000x brighter than the darkest
(although some VA-type LCD displays
improve on this, with up to 5,000:1
contrast ratios). Meanwhile, most
HDR standards look to extend this
to 10,000:1 and beyond. However,
crucially, not all standards require
any improvement on this 1000:1
level, which is largely why the arrival
of HDR has proved so confusing,
and sometimes underwhelming.