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Revisiting Spectral Printing: A Data Driven Approach P. Morovič, J. Morovič, J. Arnabat and J. M. García–Reyero Hewlett Packard Company, Sant Cugat del Vallés, Catalonia, Spain

© Copyright 2012 Hewlett-Packard Development Company, L.P. The information contained herein is subject to change without notice.

© Copyright 2012 Hewlett-Packard Development Company, L.P. The information contained herein is subject to change without notice.

Q: Is the extra effort of spectral printing worth it in practice?

-  What are the benefits of spectral reproduction?

-  What are the spectral constraints of multi-ink printing systems?

-  How can we tell whether one spectral reproduction is better than another?

-  How do the spectral matches of a spectral and colorimetric reproduction compare?

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Light source 2 Light source 2

Light source 2

Light source 1

Spectral printing: what are we all waiting for?

Spectral printing has clear benefits! -  Match original under any light source -  Match for all observers (human, animal) -  à reproduction changes with illumination

in the same way as the original does, for all observers (‘I can see what the Mona Lisa would look like in my living room’)

What is needed? -  Spectral content (little point in matching RGBs) -  Multi-ink printers (metamerism is needed)

-  Spectral control of output (print properties need to be chosen based on expected spectra)

-  Printers that span sufficient spectral gamut (on a chosen substrate) to expected content

O P

Light source 2

O P

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Metameric/Colorimetric printing: where are we today?

What are colorimetric matches like? -  Designed to match color under single light source (typically D50)

-  In high-end printers accurate to within noticeable differences* (< 1DE)

-  Repeatable, consistent (via calibration to keep systems stable)

BUT: -  Even under chosen light source there are color differences

-  Even under other light sources there are (uncontrolled) color differences

-  Assumption: for a spectral print, color differences under variety of illuminants are lower than for colorimetric (metameric) print

* for in-gamut colors – same applies in spectral case

Light source 2 Light source 2

Light source 2

Light source 1

O P

Light source 2

O P

P ?

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How good is a print spectrally?

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The spectral use case

Many metrics defined in the literature -  Purely spectral, taking into account CMFs, focusing on metamerism between specific light

sources, looking at sets of color difference statistics per light source + combinations of these

BUT: how does an observer experience the goodness of a spectral match? -  Original – print ß observer

-  Observer views original-print-pair under many light sources (not arbitrary à database of measured lights)

-  Under each light source they see certain levels of color difference (à most accurately predicted by ∆E2000, with JND units)

-  Surveying their experiences from under multiple light sources gives rise to a variety of color difference magnitudes (à choice of relevant statistics)

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MIPE: an experience–based metric

Paramers (>0 ∆E even under ‘reference’ illuminant)

Non-parametric descriptive statistics

Median: how close a match can be expected for an arbitrary, but realistic, light source

95th percentile – median: how much this match varies

Maximum: how far apart the two can get at worst

Light sources / illuminants CIE standard and recommended illuminants + measured light sources

Color difference with visually meaningful units (~JND)

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How good a spectral match is a metameric print?

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Test setup

Printer: HP Designjet Z3100 -  10 inks: cMmYnNKRGB + glossy/matte black + gloss enhancer -  substrates: Hahnemühle Smooth Fine Art (matte), HP Premium

Instant-dry Photo (glossy) -  BUT: inks not optimized for spectral reproduction!

Measurement -  XRite i1 spectrophotometer, 400 nm to 700 nm at 10 nm intervals,

45°/0° geometry

Original data sets -  Spot color: PANTONEs on three substrates: uncoated, matte and

coated, 1224 patches per substrate (3672 total samples; mixtures of Pantone system’s 15 base inks)

-  Fine art: 1168 measurements taken from multiple paintings

matte

glossy

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How does MIPE relate to spectral dimensionality?

N=3, 99.83%, 24.4∆E"

N=4, 99.92%, 17.6∆E"

99.95, 11.3"

99.97, 5.9"

99.99, 3.1"100, 2.4"100, 1.4"100, 1.4"0"

5"

10"

15"

20"

25"

30"

35"

40"

97" 98" 99" 100"

MIP

E (∆

E200

0)"

% spectral variance accounted for"

Glossy print spectra"

Median"

95th percentile"

Maximum"

Degree of spectral variance accounted for does not relate well to visual consequences.

99.8% coverage still has 95th percentile MIPE of 5 ∆E2000 and max. of 24 ∆E2000.

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How good a spectral match is the result of matching colorimetry?

Results: colorimetrically derived print

Colorimetric ‘goodness’ Spectral ‘goodness’

-  Broadly similar performance under D50 (for which colorimetry was matched) and all other 172 illuminants (color gamut differences a big contributor already)

-  MIPE errors higher than D50 ∆Es (sanity check for maximum)

-  Gap between MIPE and D50 ∆Es indicates room for improvement from spectral matching

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How about a real spectral print?

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Spectral printer control

HANS -  Take advantage of greater solution space with HANS

(kn linear v. n non-linear)

-  Yule-Nielsen modified CIE XYZ space for convexity

-  Full convex hull of Neugebauer Primaries (in YNN) – i.e. set of arbitrary within-ink-limit convex combinations, is addressable à printable spectra

Simulation -  In spectral-gamut samples: zero error

-  Out-of-spectral-gamut samples: spectral gamut mapping (min. Euclidean distance to YNN convex hull)

-  Original reflectance à linear model basis à convex hull à MIPE

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

CIE x

CIE

y

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

CIE x

CIE

y

matte

glossy

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Results: bases and projections

400 450 500 550 600 650 7000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

wavelength (nm)

rela

tive

refle

ctan

ce (%

)

PCA Basis 1PCA Basis 2PCA Basis 3PCA Basis 4PCA Basis 5PCA Basis 6

400 450 500 550 600 650 7000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

wavelength (nm)

rela

tive

refle

ctan

ce (%

)

PCA Basis 1PCA Basis 2PCA Basis 3PCA Basis 4PCA Basis 5PCA Basis 6

mat

te

glos

sy

337 OOG

3D

483 OOG

3D

More color gamut ≠ greater spectral coverage at same number of bases.

Greater spectral variety à more bases needed to characterize to same level

Not same 3 bases!

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Results: MIPE

Spot colors - spectral Fine art - spectral

Colorimetric reproduction Decreasing gamut coverage

Increasing similarity

Gamut mismatch is limit – improvements

in-gamut

Error reduced to between quarter

and half (BUT: repeatability)

Some maxima increased (MIPE v.

YNN min. dist.)

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Conclusions

Spectral printing has clear benefits in some cases (e.g., Fine art on glossy), but: -  Spectral matching of colorimetric prints is often as good as or close to it (e.g., no benefit for

spot colors in this case)

-  Maximizing the benefit of fully spectral printing requires highly accurate matching and high repeatability

-  Further improvements could be expected from ink–sets optimized for spectral printing (although constrained options using manufacturable materials)

-  Biggest benefits will be in specialty applications (e.g., Hersch ’11 security printing)

Is spectral going to replace colorimetric printing? Not anytime soon …

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Acknowledgements

Carlos Amselem Óscar Martinez

África Real Rafael Giménez

Johan Lammens

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Thank you!