Image Processing Reference
In-Depth Information
This space is selected to
maximize the gamut coverage
Profile connection space
(PCS)
RGB
RGB
CMYK
X or L *
C
M
Y
K
3 × 3
TRC
3DLUT
TRC
Y or a *
Matrix
[e.g., ProPhoto RGB]
FIGURE 7.1
LUT B2A table converting PCS to device CMYK space in the ICC profile.
RGB
Lab
CMYK
C
M
Y
K
GCR constrained LUT
Gamut
mapping
e.g.,
ProPhoto RGB
TRC
(RGB->Lab -> CMYK)
FIGURE 7.2 LUT B2A table converting PCS to device CMYK space in the ICC pro le
(Note: RGB is an intermediate color space).
7.3 PROFILING—A COMPLEX SYSTEM PROBLEM
While multidimensional pro
les offer opportunity to improve color quality of output
devices, the complexity involved in the creation of these pro
les is high. In general,
the requirements for high quality rendering are very rigid. The simplest requirements
in device owners terms are
get the color right, every time, everywhere, and all the
''
time.
In technical terms, the phrase
every time, everywhere and all the time
''
implies achieving device process stability under all the operating environments,
capturing variation in temperature, humidity, media. However,
''
''
''
getting the color
right
on multiple devices is infeasible because different devices have different color
gamuts and it is physically impossible for a device to produce a color outside its
reproducible gamut. As seen in the discussion below, even with a stable device, and
the best pro
''
les, it is not easy to
''
get the color right
''
so that the color differences are
maintained within the perceptual limits.
7.3.1 T IGHT C OLOR R ENDITION R EQUIREMENTS
Very accurate colorimetrically matched reproduction is desired for high quality
rendering. Colors have to be within perceptual limits in all regions of the color
space that the printer can produce. That means the desired perceptual color
difference for in-gamut colors between the input L*a*b* values to the rendered
L*a*b* values of the pixels must be less than or equal to 1 (
D E a * ,
D E 2000 , etc.) unit.
This is extremely dif
cult to achieve due to (i) 8 bit quantization limits of the imaging
system, (ii) system errors like halftone noise, development noise, etc., (iii) sensor
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