HANS: Enabling CMY Metamers
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Transcript of HANS: Enabling CMY Metamers
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2011 © Hewlett–Packard Company
HANSEnabling CMY Metamers
Ján Morovič, Peter Morovič & Juan Manuel García-ReyeroHewlett–Packard CompanyBarcelona, Spain
Presented at ICC/HP Digital Print Day, 15th June 2011
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2011 © Hewlett–Packard Company
What makes printed colors?
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2011 © Hewlett–Packard Company
Print color formation
Side view
70% W13% C10% K 6% M 1% CM
Neugebauer prim
aries
Relativ
e area
cov
erag
es
Subtractive
Additive
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2011 © Hewlett–Packard Company
Analog from digital
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2011 © Hewlett–Packard Company
From color to halftone pattern
CIE L*a*b*sRGB
SWOP CMYK...
color managementcolor appearance modeldevice characterization
color enhancementgamut mapping
color separationlinearizationcalibrationhalftoning
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2011 © Hewlett–Packard Company
Controlling print color – a first principles approach
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How do we get from color to halftone pattern?
20% W30% C20% M 0% Y20% CM 0% CY 0% MY 0% CMY
Source color Color management Printable color
Halftone pattern’s Neugebauer Primary
statisticsHalftone pattern
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2011 © Hewlett–Packard Company
How does this differ from traditional color control?
Traditional New
Color separation
‘How much of each ink should I use for each
color?’Output: ink amounts
How much area should I cover with each
Neugebauer Primary’Output: Neugebauer
primary area coverages
Halftoning
Decides where to place ink drops based on color separation constraints.Controls: spatial and
overprinting properties
Decides where to place ink drops based on
color separation constraints.
Controls: spatial properties only
Ink amounts v.
patterns1:1 1:many
Specifying Neugebauer Primary area coverages provides access to vastly greater space of printable patterns.
kn v. n(for system where up to k-1 ink drops per pixel can be specified for n inks)
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2011 © Hewlett–Packard Company
From theory to practice
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2011 © Hewlett–Packard Company
A minimal Halftone Area Neugebauer Separation setup (CMYK, 1bpp)
Print & measure Neugebauer primary
(NP) CIE XYZs
Compute convex hull & tetrahedralize
hull NPs
Find printable color’s enclosing
tetrahedron
Printable color20% W30% C20% M 0% Y20% CM 0% CY 0% MY 0% CMY 0% K 0% KC 0% KM 0% KY 0% KCM 0% KCY 0% KMY 0% KCMY
Barycentric coordinates are vertex NP areas
Select one NP per pixel & diffuse NPac-NP error
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2011 © Hewlett–Packard Company
Optimizing HANS
• Form all polyhedra using the kn NPs
• Find all colors for which NPacs are to be optimized enclosed by each polyhedron
• Compute barycentric coordinates
• Evaluate each candidate (grain, ink use, color constancy, ...)
• Assign combined score
• Select best NPac for each color
CMY 1dpp: kn=23=8 → 163 polyhedraCMYK 1dpp: kn=24=16 → 64839 polyhedraCMYKcm 2dpp: kn=36=729 → 2.8x10219 polyhedra
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2011 © Hewlett–Packard Company
Reductio ad absurdum – the CMY case
• With traditional, colorant space control: no metamers
• each in-gamut color matched by one CMY colorant amount combination
• Even at max. 1 drop per pixel, CMY colorant set has 23=8 Neugebauer Primaries
• 3D colorimetric space to 8D NP space results in 1:many mapping
• 163 polyhedra span gamut and provide metamers
• mid–gray: 115 metamers
• mean 13% ink use difference between metamer using most and least ink for each color
• More metamers still if more than one drop per pixel is used
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2011 © Hewlett–Packard Company
Does it work?
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Test setup: ‘Can we find NPacs that use less ink?’
• Printer: HP Designjet L65500• Inks: CMYKcm latex
• Substrate: Avery Self-Adhesive Vinyl
• Color samples: 748 Lab-uniform ISO coated v. 2 samples
• Color workflows compared:
• Ink space separation, GCR optimized for low grain, ink space halftoning (current default)
• Ink space separation, maximum GCR optimized for low grain, ink space halftoning (current optimal)
• NPac space separation (optimized for minimum ink use) and halftoning (HANS)
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2011 © Hewlett–Packard Company
Results – ink use
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2011 © Hewlett–Packard Company
What next?
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2011 © Hewlett–Packard Company
Challenges and benefits
• Challenges:
• printer model accuracy (the more accurate the better the optimization)
• computational efficiency (weeks of computation per substrate)
• optimization (efficient models of print attributes, efficient traversal of NPac space)
• Benefits:
• greater & direct optimization (more from the same printer-ink-substrate)
• explicit trade–off among print attributes (grain v. ink use v. color constancy)
• inkset agnosticism (same process for CMY 1bpp and CMmYKkNnRGB 2bpp)
• More detail at this year’s IS&T/SID Color and Imaging Conference(subject to acceptance)
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Acknowledgements
– Dudi Bakalash
– Lahav Langboim– Shay Maoz
– Amir Sheinman– Alex Veis – Igor Yakubov
– Gary Dispoto– I-Jong Lin
– John Recker– Ingeborg Tastl
– Bob Ulichney
– Lluis Abello
– Carlos Amselem– Xavi Bruch
– Michel Encrenaz– Eduard Garcia– Oriol Gasch
– Rafa Gimenez– Josep Girlat
– Rafael Goma– Andrés Gonzalez– Jacint Hument
– Johan Lammens
– Alan Lobban
– Óscar Martinez– Scott Norum
– Aleix Oriol– Ramon Pastor– Yvan Richard
– Aurora Rubio– Utpal Sarkar
– Albert Serra– Jep Tarradas– Joan Uroz
– Jordi Vilar