rgb inks in a cmyk world

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RGB inks in a CMYK world

Dr. Ján MorovičHewlett–Packard CompanyFogra Colour Management Symposium26th February 2010, München

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OverviewBackgroundCMY and CMYK printingPotential benefits of multi–primary printingChallenges of using multiple primariesConclusions

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What ismulti–primary printing?

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Terminologyprimary – a colorant (ink, toner, dye, wax, ...)

chromatic primary – primary with significant colorfulness as opposed to a neutral, achromatic primarysecondary – the result of using two primaries simultaneously

e.g. cyan and yellow, giving a green result

multi-primary – using more than three/four primaries (cyan, magenta, yellow + black)

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Background

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Digital printing on the upmove from B&W to CMYK+

2007: CMYK+ 45%; multi-primary 10%2015: CMYK+ 75%; multi-primary 15%

growth of WW digital print market 2005: €32 billion2015: €125 billion

strong benefits for customers (differentiation, flexibility) and print service providers (value added, cost)market share highly desirable

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HP portfolioHP Professional Displays

HP DreamColor LP2480zxProfessional Display

HP Photosmart Pro Printers

HP Photosmart Pro B9180 Printer

HP SmartStream Workflows

HP LF Design Printers

HP Designjet Z3200Photo Printer series

HP LF Commercial Printers

HP Designjet L65500 Printer series

HP LF Industrial Printers

HP Scitex TJ8300/TJ8500 Printer series

HP Indigo Digital Presses

HP Indigo 7000 Digital Press

HP Light Production MFP

HP CM 8000 Series Color MFP

HP Web Press

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CMY printing

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The simplicity of CMYideal CMY primaries – each controls one of eyeʼs color receptor (cone) typese.g., changing amount of cyan (C):

changes amount of light absorbed in low–frequency part of visible electromagnetic spectrum L cone is sensitive to it – response controlled by C

each color can be matched using exactly one CMY combination

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Color inputs to ink amounts

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CMY limitationsunwanted stimulations

cone sensitivities overlaptrichromatic methods cannot match all color experiences

e.g., C affects not only L cones but also M and S onesprinting black text or line art using CMY

can result in artifacts from miss–registrationunwanted absorptions

actual inks are not like ideal ʻblock dyesʼ – further gamut droppoor color constancy

prints change appearance with illumination

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CMY limitations

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CMYK printing

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What can adding blacK do?

increase lightness rangeprinting black line art and text using single inklower cost

amounts of more expensive CMY inks replaced by smaller amount of cheaper K ink

improve color constancyusing more K (spectrally flat) gives prints that change less with changes in lighting

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What can adding blacK do?

increase lightness rangeprinting black line art and text using single inklower cost

amounts of more expensive CMY inks replaced by smaller amount of cheaper K ink

improve color constancyusing more K (spectrally flat) gives prints that change less with changes in lighting

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What can adding blacK do?

increase lightness rangeprinting black line art and text using single inklower cost

amounts of more expensive CMY inks replaced by smaller amount of cheaper K ink

improve color constancyusing more K (spectrally flat) gives prints that change less with changes in lightingco

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eric

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Color inputs to ink amounts

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Cost of adding Kredundancy

same color can be achieved using multiple, alternative primary combinations – e.g. K=10 and C=M=Y=10 may be equivalent

trade-offsK=10 is cheaper and more color constant; C=M=Y=10 is less grainy

consequencesidentify which print attributes matter – use primaries appropriatelybalance likely to depend on application – in photography grain is more objectionable than in technical illustrations

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Multi–primary benefits

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most obvious benefitadding carefully chosen new inks to CMY allows for reduction of unwanted absorptions

therefore spectra that allow for greater cone response ratiostherefore greater resulting colorfulness

e.g., adding red, green and blue inks to CMY can increase color gamut on a given paper by a quarter

More gamut

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Prolonged lightfastnesscolorfulness and longevity are natural enemiessome materials tend to have greater gamut at expense of longevity

some dyes versus more lightfast but less colorful pigments

trade-off is most marked at same number of primaries though → adding more primaries can deliver both attributes

e.g. gamut of six primary dye printer can be matched by ten primary pigment printer, which has far greater longevity

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Prolonged lightfastness

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Prolonged lightfastness

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Other benefitsBetter color constancy: colorants resulting in ʻflatterʼ spectra are usedLess grain: colorants with more similar density / lightness are combinedSpectral reproduction: match under wider range of illumination enabled by greater spectral gamutReduced ink volume: primaries close to traditional secondaries

e.g. M=Y=10 can be replaced by R=10 in hypothetical printer – halving the ink amount; 15% overall saving typical

Küppers side–by–side printing: enabling opaque inks, lower ink use, greater predictability (e.g., Océ CPS700 printer)

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Challenges

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Color inputs to ink amounts

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Multi–primary color gamutdetermining color gamut of 12 ink printer requires sampling 12 dimensional space

coarse, uniform sampling of 10 steps per colorant – 1 trillion primary combinationspredicting the color of each of these combinations – decades of computation

gamut is needed when:designing and choosing primaries color separations are developed for them

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Twisting transitions

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Sneaky secondaries

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Sneaky secondariesY+R=R

M+B concave

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Spectral gamut mappingwhen spectral reproduction is the aim, what is done for spectra that cannot be matched – spectral gamut mappingunderstanding still in infancymost popular approach (e.g. Chau & Cowan, 1996):

represent spectra in 3+n dimensional space (Wyszecki, 1953)

first three account for colorimetry under chosen light source – ʻfundamental basesʼ remaining are ʻmetameric blacksʼ - only account for spectral and not colorimetric properties

match original in fundamental basesminimize distance in metameric blacks

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P. Morovič ʼ07

original reflectance

fundamental bases

metameric blacks

all metamers

+single set ofblack weights

all possibleblack weights

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Other challengesAmplitude modulated halftoning: moiréColorant capacity: same limit applies for all ink sets – only combinations below it are valid

Chen ’06

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RGB and CMYK accessn–primary systems most directly addressed via n channelsbut, there is need for RGB and CMYK interfaces to multi–primary systems:

ease of integrationvast majority of content defined in three dimensions (RGB or CMYK with a specific black generation)

challenge: design virtual RGB and CMYK spaces for addressing multi-primary system

the two also need to be consistent with each other

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Self–calibration and profilingcomplex, multi–primary printing systems benefit from embedded spectral reflectance measurementself–calibration and profiling result in

more stable and predictable outputeasier and fasterintegration of printerinto existing workflows greater flexibility ofusing them to printonto wider rangeof substrates

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Conclusions

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Futurology• Move towards more primaries in

production (benefits seen in photo & fine art + potential for new benefits)

• Move towards some spectral reproduction (but not across the board – colorimetric originals need no spectral reproduction)

• Cost likely to be strong driver (but at increasing image quality)

• Commoditization of color gamut (with differentiation in others; underlining importance of standardization)

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Conclusions1. multi–primary printing has potential to deliver more

value both to its providers and recipients and is on the increase

2. having more than three chromatic inks opens door to host of benefits (gamut, constancy, spectral reproduction, grain, ink volume, non–overprinting)

3. delivering potential benefits is not low hanging fruit but requires tackling currently unsolved problems

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AcknowledgmentsMichel EncrenazJohan Lammens

Albert SerraJordi ArnabatVirginia PalaciosMarti MariaJay GondekXavi Bruch

Rafa GimenezAleix OriolRamon PastorPeter MorovičEduard Garcia

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Thank you!