1. EE 638: Principles of Digital Color Imaging Systems
Module 32

2. Recall: Primaries Additive systems (display devices)
Ideal block model for primaries
Subtractive color model
- Reflectance

3. Consider ideal block subtractive primaries
Let
Consider ideal block subtractive primaries
Assume spatially non-overlapping primaries
i.e. at any point on paper have either Cyan, Magenta, Yellow, or no colorant; can write for locally average absorptance:
C=B+G
M=B+R
Y=G+R
c, m, y – fractional area coverage of each colorant

4. Printing technologies
Problem:
Real colorants have non-ideal overlapping absorptances
Are spatially non-overlapping colorant realistic?
Printing technologies
Traditional analog printing technologies
 plate-based technologies
(1) letter press (oldest)
The information about the VanderCook press was provided by Steve Brune, HP Inc., Boise, ID (6/26/17)

5. Where is letterpress today?
text

6. (2) gravure (3) intaglio ink is contained in wells ink is goes here
tone is controlled by locally varying density on ink
Used for National Geographic
(3) intaglio
printed under great pressure
Used for currency
ink is contained in wells
ink is goes here
plate

7. (4) offset lithography - ink - water Printing plate
Most common high speed/high volume printing technology
- ink
- water
Printing plate
Blanket transfer roller
improves quality of transfer
paper

8. Traditional halftone printing
Rotate screens with respect to each other by 15º
Recall M-D equation for one colorant C
Typical
halftone
print
(2 color)
Cyan
Magenta
Average reflectance

9. text
From work of
Alty Jumabayeva

10. text
From work of
Alty Jumabayeva

11. Let be total fractional area coverage of Cyan
Neugebauer primaries
Let be total fractional area coverage of Cyan
Let be total fractional area coverage of Magenta
Unknown and
depends on
half-toning algorithm
known

12. Want an expression for in terms of
In general, this will depend on the color halftoning algorithm.
Assume uniform, independent placement of colorant dots
Pick an arbitrary point in square and compute probability that each of four different Neugebauer occurs there:
{ Cyan is printed at &
magenta is not printed at }

13. Assume independent, random placement of colorants
Assume independent, random placement of colorants. For example, for a_c the probability that a pixel is covered by cyan only is probability that it is covered by cyan times the probability that it is not covered by magenta. Note that this does not reflect the structure of any particular halftoning algorithm.
De Michel coefficient

14. Assume independent, random placement of colorants
Assume independent, random placement of colorants. For example, for a_c the probability that a pixel is covered by cyan only is probability that it is covered by cyan times the probability that it is not covered by magenta. Note that this does not reflect the structure of any particular halftoning algorithm.
De Michel coefficient

15. is spectral reflectance of a solid patch of cyan  Next step:
Find CIE XYZ coordinates corresponding to our printed patch with colorant amounts C & M and spectral reflectance
Assume illuminant power spectral density
is spectral
reflectance of a
solid patch of cyan 

16. For example, are the Tristimulus coordinates for solid cyan printed on a particular media and viewed under illuminant I.
What do we have?
Have model for
Note: are measurements of appropriate colorant combinations made when they are printed on a particular media (substrate)
(Forward printer model)
C, M, Y, K
Printer
XYZ
by extension

17. What about dot gain? Mechanical dot gain adjust, C, M, Y, K
or more directly adjust etc.
Optical dot gain apply Yule-Nielsen model:
N – empirically determined parameter of the model

18. Extension to 3-color printing Neugebauer primaries
C, M, Y CM, CY, MY CMY, W
B G R Kc
For four color printing, just extend model straight forwardly by adding a fourth primary Kp process black
Why use 4-color system?
Kp is cheaper than Kc
Minimizes mis-registration impact (important for text)
Bigger gamut (specifically darker or graphics colors can be printed. CMYK yields rich black) || || || ||
primaries
secondary
tertiary
pros

19. cons No longer have unique 1-to-1 mapping from
printer-colorant-in to CIE-XYZ-out
More expensive
cons

20. Y M C W Neugebauer model Assume random placement of colorant
De Michel coefficient