DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

The Macintosh computer by
default implements a 1 ⁄1.45-power
function at the output LUT. John
Knoll’s Gamma Control Panel can
load the output LUT. When set to
a gamma value g, the Control
Panel loads an output LUT with
a power function whose exponent
is 2.61 ⁄g . Strangely, gamma on
Macintosh computers has come to
be quoted as the exponent applied
prior to the framebuffer (whereas
in other computers it is the exponent
of the table loaded into the
output LUT). So, the Mac’s default
gamma is said to be 1.8, not 1.45.
A Macintosh can be set to handle
video (or PC) R’G’B’ data by
loading a ramp into its output
LUT. Using Knoll’s control panel,
this is accomplished by setting
gamma to 2.61.
JFIF files originated on Macintosh
ordinarily represent R, G, and B
display tristimulus values raised to
the 1 ⁄1.72 power.
Computer graphics systems generally store tristimulus
values in the framebuffer, and use hardware LUTs, in the
path to the display, to gamma-correct on the fly. This is
illustrated in the second row. Typically, a 1 ⁄2.2-power
function is loaded into the output LUT; in this case,
rendering intent of 1.14 is achieved.
Macintosh computers use the approach shown in the
bottom row. The output LUT is, by default, loaded with
a 1 ⁄1.45-power function. The combination of the default
LUT and the usual 2.5-power monitor function results
in a 1.72-power function that relates QuickDraw R’G’B’
values (such as the values stored in a PICT file or data
structure) to displayed tristimulus values.
If a desktop scanner is to produce QuickDraw R’G’B’
values that display relative luminance correctly, then
a 1.72-power function must be loaded to the scanner
LUT. In the typical Macintosh situation, the 1 ⁄1.72,
1 ⁄1.45, and 2.5 exponents combine to achieve an endto-end
exponent of unity. This is suitable for scanning
photographs or offset printed matter, where a suitable
rendering intent is already incorporated into the image.
For QuickDraw R’G’B’ values originated by application
software, part of Macintosh gamma correction must be
effected by application software prior to presentation of
R’G’B’ values to the QuickDraw graphics subsystem; the
remainder is accomplished in the output LUTs. When
scanning, part of Macintosh gamma correction is
effected by the LUT in the scanner driver, and the
remainder is accomplished in the output LUTs.
Halftoned printing has a builtin nonlinearity, owing to
the phenomenon of dot gain. Reflectance from the
printed page is approximately proportional to the
1.8-power of CMYK code values. QuickDraw R’G’B’
values are not perceptually optimum; however, apparently
by serendipity, QuickDraw R’G’B’ coding is nearly
perfectly matched to the dot gain of halftone printing.
This has led to the dominance of Macintosh computers
in graphic arts and prepress, and has made “gamma
1.8” image coding a de facto standard.
CHAPTER 23 GAMMA 275