Multi-ink Color-Separation Algorithm Improving ... - Semantic Scholar

Multi-ink Color-Separation Algorithm Improving Image Quality Journal of Imaging Science and Technology vol. 52, No. 2, 2008 Yongda Chen, Roy S. Berns, Lawrence A. Taplin, and Francisco H. Imai Presented by Dae Chul Kim

School of Electrical Engineering and Computer Science Kyungpook National Univ.

Abstract ‹ Proposed

─ Novel color separation algorithm • Optimize color look-up tables for improved image quality – Perceptual metrics » Color constancy » Graininess » Color gamut » Color look-up table smoothness

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Introduction ‹ Digital

color printing

─ For three-ink printer • Mapping from ink amounts to colorimetric coordinate is unique • Various interpolations or fitting techniques

─ For four-ink printer • Depending on the black ink amount – Under-color removal(UCR) – Gray-component replacement(GCR)

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─ For high-fidelity color printing • Additional chromatic inks – Color gamut extension – Considerable redundancy » Ink combinations can produce the same color and even the same spectra

• One strategy – Separating the color gamut into sub-gamut » CMYKRGB ink set » Color gamut was separated into six sub-gamut produced by ink combination - CGK, GYK, YRK, RMK, MBK, and BCK

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─ New color separation method • Use of perceptual metrics – Choice of samples for the construction of look-up table from candidate ink combinations that have similar CIELAB values • Perceptual metric – Color inconstancy – Gamut volume – Smoothness

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‹ Algorithm

overview

Fig. 1. Flowchart of creating color look-up table for multiple-ink printers. 6 /25

‹ Multi-ink

printer and its spectral printing model

─ Use of epson pro 5500 ink jet printer • Six-ink printer – CMYKGO ink set

Fig. 2. CMYKGO ink positions projected onto the a*b* plane.

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─ Cellular Yule-Nielsen modified spectral Neugebauer model(CYNSN) • Additive-mixing model – Each printed color type positioned at the vertices of each cell

─ Accuracy of the printer model Table I. Accuracy of the printing model.

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‹ Creation

of virtual sample set

─ First phase of the proposed algorithm • Creation of virtual sample set of ink combinations

─ Original method • By combining area coverages of different inks from 0% to 100% in 10% intervals for each inks – High proportion of dark samples – Non-uniform distribution of samples in CIELAB space

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─ Quad-tree decomposition • By dividing the colorant space into equal-sized square blocks – Use of threshold criterion – Repeated iteratively until each block met the criterion – Small threshold create more samples » Extremely slow processing speed » System failure

Fig. 3. A demonstration of two-dimensional quad-tree decomposition. 10 /25

• Lightness distribution of new samples

Fig. 4. Histogram of CIE lightness of virtual sample set. 11 /25

‹ Perceptual

metrics

─ Selection of seed sample set • Color constancy, grainness, color gamut, smoothness

─ Color inconstancy index(CII) • Evaluation of color variation under different illuminant 1/2

⎡⎛ ΔL* ⎞2 ⎛ ΔC * ⎞2 ⎛ ΔH * ⎞ 2 ⎤ ab ab CII = ⎢⎜ ⎟ +⎜ ⎟ +⎜ ⎟ ⎥ , ⎢⎣⎝ 2 S L ⎠ ⎝ 2 SC ⎠ ⎝ S H ⎠ ⎥⎦ where

(1)

ΔL* = L*refer _ c − L*test _ c ΔC *ab = C *refer _ c − C *test _ c 2 2 2 ΔH *ab = ⎡⎢( ΔE * ) − ( ΔL* ) − ( ΔC * ) ⎤⎥ ⎣ ⎦

1/2

2 2 2 ΔE * = ⎡⎢( ΔL* ) + ( Δa* ) + ( Δb* ) ⎤⎥ ⎣ ⎦ SL = 1

1/2

SC = 1 + 0.045C *refer ,test S H = 1 + 0.015C *refer ,test * * * Crefer Crefer ,test = , c ⋅ Ctest , c

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• CII histogram of the virtual sample set

Fig. 5. Color inconstancy index CII histogram of the virtual sample set note logarithmic scale. 13 /25

─ Grainness index(GI) ∑ GI =

n pixel =1

( CIEDE 2000( Lab

blured , pixel

, Labmean ) )

(2)

n

─ Graininess indices for black and yellow ramp

Fig. 6. Graininess Index for black and yellow ramps. 14 /25

─ Color gamut extension

Fig. 7. Gamut comparison between the CMYK ink set dashed line and the CMYKGO ink set solid line on the a*b* slice with L*=55. 15 /25

─ Chroma metric • One of selection criteria for the cells on the edge (3)

* Cab = a*2 + b*2

─ Smoothness index(SI) SI =

∑ ( ac 6

i =1

i , orig

− aci , smooth )

2

(4)

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Experimental ─ Experimental environment • Spectral measurements – Use of GretagMacbeth spectroscan spectrophotometer – Colorimetric values » Calculation of illuminant D50 and F11 and CIE1931 2° standard observer

• The selection criteria – Color gamut, color constancy, grainness, and smoothness of the look-up tables

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─ Initial selection criterion * Cab CII GI Q = − k1 + k + k 2 3 * max(Cab ) max(CII ) max(GI )

(5)

─ In implementation * Cab CII GI 0.5 Q = −4 + + * max(Cab ) max(CII ) max(GI )

(6)

─ Selection of a sample with the minimal Q value in each cell ─ Creation of a uniformly spaced each plane

64 × 64 × 64

CLUT for

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─ Look-up table based on initial selection

Fig. 8. The interpolated area coverage for cyan ink at L*=44 without the smoothness application. 19 /25

─ The final look-up table • Trade-off between larger gamut, look-up table smoothness, color constancy and grainness * Cab CII GI SI Q = − k1 + k + k + k 2 3 4 * max(Cab ) max(CII ) max(GI ) max( SI )

(7)

where SI represents the smoothness index of the look-up table, and k1 , k2 , k3 , and k4 were set to 2, 1, 0,5, and 1.

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─ Creation of look-up table • By selected samples with the addition of smoothness index

Fig. 9. The interpolated area coverage for cyan ink at L*=44.0 with smoothness applied. 21 /25

─ The selection metrics for three look-up tables • Look-up table with minimum color inconstancy index Q=

CII max(CII )

(8)

• Look-up table with minimum graininess Q=

GI max(GI )

(9)

• Four color(CMYK) look-up table with all metrics * Cab CII GI SI 0.5 Q = −2 + + + * max(Cab ) max(CII ) max(GI ) max( SI )

(10)

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‹ Result

and discussion

─ Performance of look-up tables with different metrics Table II. Performance of look-up tables with different metrics.

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─ Statistical CII of four-ink and six-ink samples Table III. Statistical CII of four-ink samples and sixink samples in the same four-ink gamut.

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Conclusions ‹ Proposed

algorithm

─ Deal with one-to-many mapping problem ─ Unique feature • Combination of color constancy and graininess as selection criteria among ink combination yielding a similar color

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