DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Video, PC
TRISTIM.
Computergenerated
imagery
TRISTIM.
SGI
TRISTIM.
Macintosh
TRISTIM.
GAMMA
CORRECTION FRAMESTORE (implicit)
MONITOR
0.5
(implicit) FRAMEBUFFER
RAMP
8-bit Bottleneck
SCANNER
LUT FRAMEBUFFER
1 ⁄1.29
≈0.775
SCANNER
LUT FRAMEBUFFER
1 ⁄1.72
≈0.58
QuickDraw RGB codes
RAMP
FRAMEBUFFER
LUT
1 ⁄2.2
274 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES
2.5
MONITOR
2.5
FRAMEBUFFER
LUT MONITOR
1 ⁄1.7
≈0.59
≈0.69
2.5
FRAMEBUFFER
LUT MONITOR
1 ⁄1.45
2.5
1.25
1.14
1.14
Figure 23.7 Gamma in video, CGI, SGI, and Macintosh are summarized in the rows of this
diagram. Tristimulus signals enter from the left; the columns show the transfer functions of
(respectively) a camera or scanner; the image storage device (framestore or framebuffer); output
LUT; and the monitor.
In video, sketched in the top row, a transfer function that mimics vision is applied at the camera
(“gamma correction”); the signal remains in perceptual space until the encoding is reversed by the
monitor. (PCs have comparable signal encoding.) In computer graphics, sketched in the second row,
calculations are performed in the linear-light domain, and gamma correction is applied in a LUT at
the output of the framebuffer. SGI computers take a hybrid approach: Part of the correction is accomplished
at the camera or scanner, and part is accomplished through a 1 ⁄1.7-power function that is
loaded into the LUT. Macintosh computers, sketched in the bottom row, also take a hybrid approach:
The camera or scanner applies a 1 ⁄1.72 power, and a 1 ⁄1.45-power function is loaded into the LUT.
Using γ E ≈ 1⁄ 1.72 is appropriate for prerendered imagery, to produce an end-to-end exponent of 1.0.
The end-to-end power function exponent, or rendering intent (see page 81), is shown for each
row by the number at the extreme right. This number is the product of the exponents across the
system. Some people call this “system gamma,” but that term is so widely misused that I reject it.
1.0