DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

What are loosely called JPEG files
use the JPEG File Interchange
Format (JFIF), cited in the margin
of page 459. Version 1.02 of that
specification states that linear-light
coding (gamma 1.0) is used. That
is seldom the case in practice;
instead, encoding power laws of
0.45 (sRGB) or 0.58 (i.e., 1.45 ⁄2.5)
are usually used. See page 273.
shown at the left; a photometer reading luminance
displayed by a JPEG system is shown at the right. In
PHIGS and CGM, the displayed luminance is halfway up
the physical scale, a relative luminance of 0.5. In the
JPEG case, displayed luminance is halfway up the
perceptual scale, only about 0.18 in relative luminance.
The PHIGS and CGM standards are obsolete; however,
the problem persists that many graphics image files do
not carry any transfer function information. If you
exchange RGB image data without regard for transfer
functions, huge differences will result when image data
is displayed.
The digital image-processing literature rarely discriminates
between linear and nonlinear coding. Also, when
intensity is mentioned, be suspicious: Image data may
be represented in linear-light form, proportional to
intensity. However, a pixel component value is usually
associated with a small area of a sensor or a display, so
its units should include a per square meter (·m –2) term.
Pixel component values are ordinarily properly represented
as radiance, luminance, relative luminance, or
tristimulus value.
280 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES