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Colorimetric and spectral matching
Prague, Czech republicJune 29, 2017
Marc MahyAgfa Graphics
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Overview
• Modeling color• Color matching• Process control• Profile based color transforms
—Visual effects—Light interactions
• Color matching—Colorimetric matching—Spectral matching—Metameric colors
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Modeling color
• Object has the color of the “light” leaving its surface—Light source—Object—Human observer , )
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Modeling color
• Light source—Electro Magnetic Radiation (EMR)
– Focus on wavelength range from 300 till 800 nm—Different standard illuminants
– Illuminant E (equi-energy)– Illuminant A, D50 , D65 , F11
Spe
ctra
l Pow
er D
istri
butio
n
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Modeling color
• Object colors: Classes object types—Opaque objects
– Diffuse reflection: Lambertian reflector– Specular reflection: mirror– Most objects: diffuse and specular reflection
—Transparent objects– Absorption, no scattering: plexi, glass, …
—Translucent objects– Absorption and scattering: backlit
—Special effects– Fluorescence: substrates– Metallic surfaces: (in plane) BRDF
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Modeling color
• Object colors: Characterizing object types—Opaque objects
– Measurement geometry: 45°:0° or 0°:45°– Colorimetric or reflectance spectra
—Transparent objects– Measurement geometry: d:0° or 0°:d– Colorimetric or transmission spectra
—Translucent objects– In reflection or transmission mode– For reflection mode: White backing, black backing or self backing
—Special effects– Fluorescent substrates: colorimetric data or bi-spectral reflectance– Metallic surfaces: BRDF based on colorimetric or reflectance data
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Modeling color
• Self luminous colors—Displays
– Additive color mixing based on Red, Green and Blue phosphors
• Characterizing self luminous colors—Displays: colorimetric data or radiant
emission spectra
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Modeling color• Human observer
—Color vision: three types of cones—Sensitive to EMR from 360 till 830 nm
• Tristimulus color spaces—Based on color matching experiments—Grassman’s laws (additivity)Color Matching Functions (CMF’s)
• Tristimulus values opaque objects
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Color matching
• Colorimetric matching—Two colors (test and reference sample)
– under the same viewing conditions– under different viewing conditions
—Condition colorimetric match opaque colors– Same viewing conditions: XYZ1 = XYZ2
– Different viewing conditions: Jab1 = Jab2
• Examples—Contract proofing for a given illuminant—Conversion from display (D65) to graphic arts (D50)
• Issues—No match for changes in illumination—No match for observers with different CMF’s
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Color matching• Spectral matching
— Two colors (test and reference sample)– evaluated simultaneously for different illuminants
— Too strict condition spectral match opaque colors:
• Examples— Conventional proofing systems
– Spectral color matching between proof and press– Accurately simulating screening effects
• Issues— Not possible to get a spectral match between display and hard copy— Only supported for object colors
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Process control
• Normal distribution—Continuous probability distribution
,2
with x: variableμ : averageσ : standard deviation
—Propertiesnormal distribution φ ,
=> ̅ normal distribution φ , /=> Reducing uncertainty &
increasing accuracy0
0.5
1
1.5
2
-5 -4 -3 -2 -1 0 1 2 3 4 5
n=1n=5n=10n=20
0
0.5
1
1.5
2
-5 -4 -3 -2 -1 0 1 2 3 4 5
(μ=0, σ=1)
area = 1
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Process control
• Normal distribution (cont’d)—Confidence intervals
– +/- 1σ: 68.3 % 3170 errors per 10,000 measurements– +/- 2σ: 95.5 % 460 errors per 10,000 measurements– +/- 3σ: 99.7 % 27 errors per 10,000 measurements
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Profile based transforms
• ICC.1 approach
• ICC.2 approach (iccMAX)
Source DestinationPCS
Source DestinationPCS
Exchange color data:Colorimetric only
Exchange color data:Both Colorimetric and spectral
Profile connection spaces- XYZ, LabViewing conditions- D50, 2° observer- CAT
Profile connection spaces- XYZ, Lab or spectralViewing conditions (PCC): - Illuminant, observer andlighting surround
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Profile based transforms: Visual effects
• Visual effects• Appearance based color matching
• Needed if viewing conditions source and destination are different• For ICC.1: CAT’s are used• For ICC.2: Support provided by modified CIECAM02
• Local effects such as lightness and color induction, crispening, ...• Not simulated as they are present in the image
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Profile based transforms: Light interactions
• Print substrates—Large variety of substrates used in ink jet applications
– Plastics, backlit, plexi, glass, wood, …– Special light objects interactions Not always easily to measure
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Profile based transforms: Light interactions
• Measurement instruments—Both available for reflectance and transmittance—Support M0, M1 and M2 measurement conditions for reflectance
– M1 either for D50 or a custom illuminant—Typical characteristics
– Different apertures to measure local non-uniformities– Spectral data provided– Black and white backing available
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Profile based transforms: Light interactions
• Effect thickness substrate—Plexiglass—Reflectance measurements are not accurate or meaningfull
Exp
ecte
dre
flect
ion
DetectorLight source
Act
ual
refle
ctio
n
Plexi
Air
Lambertian reflector
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Profile based transforms: Light interactions
• Transparent and translucent substrates—Interaction light in substrate
—Measurement dot gain based on ISO-12233 (Slanted edge method)
edge witha small angle
Edge SpreadFunction
Line SpreadFunction MTF
DerivativeFourierTransformPre processing
substrateink
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Profile based transforms: Light interactions
• Transparent and translucent substrates—Based on edge spread function
– Patch size on translucent substrates 20 mm– Patch size on transparent substrates 5 mm
0
10000
20000
30000
40000
50000
60000
-10 -8 -6 -4 -2 0 2 4 6 8 10
Edge
spr
ead
func
tion
D (mm)
Translucent substrate
0
10000
20000
30000
40000
50000
60000
-10 -8 -6 -4 -2 0 2 4 6 8 10
Edge
spr
ead
func
tion
D (mm)
Transparent substrate2.5 mm 8 mm
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Profile based transforms: Light interactions
• Transparent and translucent substrates—Effect illumination size
– Effect white ink on transparent substrate– Effect cyan ink on translucent substrate
0
0.1
0.2
0.3
0.4
0.50.6
0.7
0.8
0.9
1
380 430 480 530 580 630 680 730
Tran
smitt
ance
wavelength
White on transparent substrate
unlimited size ofilluminationsmall size
Large aperture
Small aperture
0
0.1
0.2
0.3
0.4
0.50.6
0.7
0.8
0.9
1
380 430 480 530 580 630 680 730
Tran
smitt
ance
Wavelength (nm)
Cyan on translucent substrate
Only box
Medium aperture
Small aperture
Large aperture
Small apertureMedium aperture
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Profile based transforms: Light interactions• Transparent substrates
—Tunneling effect (inter-reflections in substrate)
Interreflections
Incident light
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Profile based transforms: Light interactions
• Textile—Non-uniform surface—Effect directional geometry
-5
-4
-3
-2
-1
0
1
2
3
4
5
0 45 90 135 180 225 270 315 360
Textile substrate
Angle
-5
-4
-3
-2
-1
0
1
2
3
4
5
0 45 90 135 180 225 270 315 360
Proofing substrate
Angle
Z-va
lue
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Colorimetric matching
Colorimetric match
Opaque Trans-parent
Trans-lucent
Display Fluorescent substrate
OpaqueTransparentTranslucentDisplayFluorescent substr.
• Conventional color management—Supported both by ICC.1 and ICC.2 (iccMAX)
• Use cases—Perceptual match=> typically viewing in isolation—Proofing => side by side evaluation
• Object type combinations for ICC.2 (iccMAX)
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Colorimetric matching• Handling mismatch in viewing conditions
—ICC.1: CAT’s– Perceptual: CAT’s used– Proofing: undo CAT’s, recalculation destination profile is needed
—ICC.2: Profile Connection Conditions (PCC’s)– Perceptual: mechanism in place to handle different PCC’s– Proofing: inverse table cannot be reused
DigitalPrintOffset
Viewing conditions- D50 simulator- 2° observer
Viewing conditions- D65- 2° observer
PCSOffsetprofile
Digital printprofile
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Colorimetric matching
• Handling mismatch in viewing conditions (con’d)—Effect illuminants on gamut —Spectral measurement file for Fogra51
C*
D50-D65 L*=50
a*
b*
L*
Hue = 50°L*=50
D50D65D93Illuminant A
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Colorimetric matching• Closed loop
—Iteratively improving device model (forward transform)—Closed loop approach limited by
– Accuracy measurements – Stability printer
—Results in– Increased profile accuracy w.r.t. behavior printer
Digitalprint
PCSOffsetOffsetprofile
Digital printprofile
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• Accurate linking—Reduction interpolation errors for in-gamut and out-of-gamut colors
L*
C*
L*
C*
Table based linking Accurate linking
Colorimetric matching
DeltaE due tointerpolationerrors
DeltaE = 0
Explicitgamut
mappingDesaturated mapping
out-gamut-colors
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Colorimetric matching
• Stability neutrals—Proofing: Absolute colorimetric intent—Neutral check target—RGB target consisting of 11 different neutrals repeated per line (x26)
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Closed loop + Accurate linking
-1.5
-1
-0.5
0
0.5
1
1.5
20 40 60 80 100
-1.5
-1
-0.5
0
0.5
1
1.5
20 40 60 80 100
-1.5
-1
-0.5
0
0.5
1
1.5
20 40 60 80 100
Conventional link
-1.5
-1
-0.5
0
0.5
1
1.5
20 40 60 80 100
-1.5
-1
-0.5
0
0.5
1
1.5
20 40 60 80 100
-1.5
-1
-0.5
0
0.5
1
1.5
20 40 60 80 100
Colorimetric matching: Stability neutrals
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Spectral matching• Not explicitly supported by conventional color management
—Dedicated support provided by ICC.2 (iccMAX) only• Use cases
—Similar appearance prints from different CMYK presses—Packaging: spot color processing
• Object type combinations for ICC.2 (iccMAX)
—Focus on matching between opaque objects, CMYK printed output
Spectral match Opaque Trans-parent
Trans-lucent
Display Fluorescent substrate
OpaqueTransparentTranslucentDisplay IdenticalFluorescent substr. Identical
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Spectral matching• Spectral match preserving transformations
—Repurposing transformation– From Fogra51 to CGATS TR 006– Preserves GCR from source Acceptable spectral match
—Identity transform– Repurposing transform between
identical profiles Preserves spectral match
Original Repurpose ICC convert
CM
YK
CM
Y
K-o
nly
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Spectral matching• Spot color mixing
—Dynamic rendering of spot colors– Spectra step wedge on white and black background (CxF/X-4)– Output profile defines substrate and viewing conditions
—Rendering characteristics– Realistic rendering of ink combinations based on spectral mixing– Continuity between different subsets of ink values
—Spectra substrate and process inks: spectral reflectance estimation
100
75
50
25
0
0 25 50 75 100Process cyan
Spo
t Yel
low 100
75
50
25
0
0 25 50 75 100Process cyan
Spo
t Yel
lowSpectral
mixing
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Spectral matching• Spot color mixing (cont’d)
—Spectral reflectance estimation– Given discrete version of the tristimulus values XYZ
- with reflectance spectrum objectspectral power distribution illuminantcolor matching functions
N dimension spectral space– Invert tristimulus values XYZ and solve for with constraints
» Reflectance:» Smoothness criteria » Typical aim curves
0 Ri 1 :1i N
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• Illuminant discriminating metameric colors—Idea:
—Calculation illuminant discrimination metameric color pair:– Look for a set of CMYK values that
- are the same for a first illuminant- have a maximum E*ab for a second illuminant
Lab for D50
Color matching: Metameric colors
CMYK
Lab for Illuminant A
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Thank you for your attention Any questions?