RGB 2 CMYKAn AppleScript was used to explore the forward and reverse tables of the ISO Coated v2 (ECI) ICC profile. The 1617 patch ECI visual target was chosen because the profile was built using the same data points and therefore we should expect a close match.
Round tripping from Lab to CMYK (backward) with perceptual intent and then from CMYK to Lab (forward) with absolute intent. The high dE values are an indication of where, and how, the profile has been manually tweaked to accommodate wide gamut images:
Average dE = 4.83
Ten worst colour values:
70 100 0 0 = 14.29 dE
55 100 0 0 = 14.12 dE
70 0 100 0 = 13.36 dE
55 0 100 0 = 12.87 dE
70 100 0 20 = 12.56 dE
40 0 100 0 = 12.23 dE
70 100 10 0 = 12.21 dE
85 100 0 0 = 12.10 dE
70 0 100 20 = 11.86 dE
The purples and greens above are going to be impossible to hit when converting to CMYK and impossible to maintain if you need to convert back through the PCS -- including just going to Lab and back to CMYK again.
Round tripping Lab to CMYK and then CMYK to Lab using absolute intent in both directions. An indication of how closely the forward and backward tables match each other.
Average dE = 0.24
Ten worst colour values:
100 0 0 100 = 4.82 dE
100 40 0 100 = 4.05 dE
100 0 40 100 = 4.01 dE
100 100 0 100 = 3.35 dE
100 0 100 100 = 3.23 dE
40 100 0 100 = 3.02 dE
0 100 0 100 = 3.02 dE
100 100 100 100 = 2.82 dE
100 85 85 100 = 2.40 dE
0 100 40 100 = 2.35 dE
