DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Figure 23.9 Linear and
nonlinear coding in computer
graphics standards. In the
PHIGS and CGM standards,
code [128, 128, 128] produces
luminance halfway up the
physical scale, a relative luminance
of 0.5. In JPEG, code
[128, 128, 128] produces luminance
halfway up the perceptual
scale, only about 0.18 in
relative luminance. Values are
denoted RGB in both cases;
however, the values are not
comparable. This exemplifies
a serious problem in the
exchange of image files.
point), performs gamma correction in software, then
writes gamma-corrected values into the framebuffer.
A unity ramp is loaded into the LUT of the framebuffer
associated with the image. This arrangement maximizes
perceptual performance, and produces rendered
imagery without the quantization artifacts of 8-bit
linear-light coding.
Professional video software on Macintosh or SGI platforms
ordinarily loads the output LUT with a ramp function;
code values are then interpreted as in video.
Unfortunately, colors are altered in image data or interface
elements that assume the default gamma of the
platform.
Linear and nonlinear coding in CGI
Computer graphic standards such as PHIGS and CGM
make no mention of transfer function, but linear-light
coding is implicit. In the JPEG standard there is also no
mention of transfer function, but nonlinear (video-like)
coding is implicit: Unacceptable results are obtained
when JPEG is applied to linear-light data. All of these
standards deal with RGB quantities; you might consider
their RGB values to be comparable, but they’re not!
Figure 23.9 sketches two systems displaying the same
RGB triple, [128, 128, 128]. A photometer reading the
luminance displayed by a PHIGS or CGM system is
CHAPTER 23 GAMMA 279