DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

In the sRGB standard, the
exponent is written (“advertised”)
as 1 ⁄2.4 (about 0.417).
However, the insertion of the
linear segment, and the offsetting
and scaling of the pure
power function segment of the
curve, cause an exponent of
about 0.45 to best describe the
overall curve. See sRGB transfer
function, on page 267.
Rendering intent in desktop computing
In the desktop computer environment, the ambient
condition is considerably brighter, and the surround is
brighter, than is typical of television viewing. An endto-end
exponent lower than the 1.25 of video is called
for; a value around 1.125 is generally suitable. However,
desktop computers are used in a variety of different
viewing conditions. It is not practical to originate every
image in several forms, optimized for several potential
viewing conditions! A specific encoding function needs
to be chosen. Achieving optimum reproduction in
diverse viewing conditions requires selecting a suitable
correction at display time. Technically, this is easy to
achieve: Modern computer display subsystems have
hardware lookup tables (LUTs) that can be loaded
dynamically with appropriate curves. However, it is
a challenge to train users to make a suitable choice.
In the development of the sRGB standard for desktop
computing, the inevitability of local, viewing-dependent
correction was not appreciated. That standard
promulgates an encoding standard with an effective
exponent of about 0.45, different from that of video.
We are now saddled with image data encoded with two
standards having comparable perceptual uniformity but
different rendering intents. Today, sRGB and video
(Rec. 709) coding are distinguished by the applications:
sRGB is used for still images, and Rec. 709 coding is
used for motion video images. But image data types are
converging, and this dichotomy in rendering intent is
bound to become a nuisance.
Video cameras, film cameras, motion picture cameras,
and digital still cameras all capture images from the real
world. When an image of an original scene or object is
captured, it is important to introduce rendering intent.
However, scanners used in desktop computing rarely
scan original objects; they usually scan reproductions
such as photographic prints or offset-printed images.
When a reproduction is scanned, rendering intent has
already been imposed by the first imaging process. It
may be sensible to adjust the original rendering intent,
but it is not sensible to introduce rendering intent that
would be suitable for scanning a real scene or object.
86 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES