DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Many video engineers are
unfamiliar with color science. They
consider only the first of these two
purposes, and disregard, or remain
ignorant of, the great importance
of perceptually uniform coding.
Original
scene
Gamma in video
In a video system, gamma correction is applied at the
camera for the dual purposes of precompensating the
nonlinearity of the display’s CRT and coding into
perceptually uniform space. Figure 23.2 summarizes the
image reproduction situation for video. At the left,
gamma correction is imposed at the camera; at the
right, the display imposes the inverse function.
Coding into a perceptual domain was important in the
early days of television because of the need to minimize
the noise introduced by over-the-air analog transmission.
However, the same considerations of noise
visibility apply to analog videotape recording. These
considerations also apply to the quantization error that
is introduced upon digitization, when a signal representing
luminance is quantized to a limited number of
bits. Consequently, it is universal to convey video
signals in gamma-corrected form.
processing, and
transmission
system
Scanner/
camera Recording,
Display
Reproduced
image
Figure 23.2 Image reproduction in video.
Luminance from the scene is reproduced at the
display, with a suitable scale factor because we do not seek
to reproduce the absolute luminance level of the scene. However,
the ability of vision to detect that two luminance levels differ is not
uniform from black to white, but is approximately a constant ratio – about 1% –
of the luminance. In video, luminance from the scene is transformed by a function similar
to a square root into a nonlinear, perceptually uniform signal that is transmitted. The
camera is designed to mimic the human visual system, in order to “see” lightness in the
scene the same way that a human observer would. Noise introduced by the transmission
system then has minimum perceptual impact. The nonlinear signal is transformed back to
luminance at the display. In a CRT, a 2.5-power function is intrinsic.
Observer
CHAPTER 23 GAMMA 261