20050611(Hybrid ing A Useful Methdod for Flexography

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School of Electrical Engineering and Computer Science

Kyungpook National Univ.

HybridHybrid HalftoningHalftoning, A Useful Method for, A Useful Method for

FlexographyFlexography

JOURNAL OF IMAGING SCIENCE AND TECHNOLOGYJOURNAL OF IMAGING SCIENCE AND TECHNOLOGYvvol. 49, no. 1, January/February 2005ol. 49, no. 1, January/February 2005

Sasan Gooran


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Hybrid halftoning

Existing problem in the flexography using just an

AM halftoning method

Not producing dots sufficiently small to handle the

highlights and the shadows of the original image

Overcoming this problem FM method in the highlights (and the shadows)

AM method in the rest of the image

AbstractAbstract


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IntroductionIntroductionHalftoning

AM halftoning

Being variable in the size of the dots while their spacing isconstant

FM halftoning

Being kept constant in the size of the dots while their spacingvaries

Replacement of the terminology

Not using the term halftone cell but using the term dpi

Flexography

Modified form of letterpress printing method

Usage of the packaging industry on the most varied materials


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Lower print quality than that in offset printing

Printing method that can print on very thin, flexible, and

solid films, thick card boards, rough-surface packaging

materials and fabrics

Problem

Not producing the dots sufficiently small

Handling the very light and dark parts of the original image

by just using an AM method

Solution

Using AM halftoning in the mid-tones

Being lower dot gain than that of the FM halftoning Performing very well in the highlights by FM halftoning

Optimized dot positioning and the choice of the minimum dot size


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Previous workPrevious workPresent hybrid method

Not giving any detailed description on how the

methods should be implemented

Lin and Allebach

Using a stochastic dispersed-dot texture in highlights

Utilizing a periodic clustered-dot pattern in mid-tones

Not using the flexography but being electrographic

devices

Reducing the noisy impression in mid-tones


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FM methodFM methodProcess

Initial halftoned image

No dots

Solution of the problem of halftoning a grayscaleimage

Placing a number of dots on this empty initial image

Impression of lightness/darkness of the image

Being determined in advance for the number of dots to beplaced

Sum of the pixel values in the original image rounded to thenearest integer


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Where to place 1s (black dots) Being placed for the dots iteratively

Decreasing the difference between the original image andthe halftoned image

Being placed at the position of the darkest pixel in theoriginal image for the first dot

Position of the maximum in the original image

Being low-pass filtered for the original image at first

Feed-back process

Being placed for the first dot

Being subtracted from the filter version of the originalimage for the filtered version of the current halftoned

image

Being founded for the position of the maximum

Placing the second dot there

Performing the feed-back process


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Continuing until the predetermined number of dots areplacing and the final image is achived

Not being reproduced for gray tones in the highlight

and shadow regions Overcoming the problem

Controlling the number of dots to be placed in a number of

gray tone regions

Using more control regions in these area Choosing seven control regions between 0 and 0.1

Choosing eight control regions between 0.1 and 0.9

Choosing seven control regions between 0.9 and 1

Totally 22 control regions

Being determined for the number of dots in advance

Sum of pixel values in the corresponding region


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Problem in the extreme highlights

Not being placed homogeneously for the dots

Cause

Size of filter that is chosen dependent on the gray tone The lighter the gray tone, the bigger the filter

Fig. 1.A constant image with a gray value of 1% is halftoned by the proposedFM method. a) The filter size is b) The filter size is.1111 .2121


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HybridHybrid halftoninghalftoning ppi, lpi and dpi

ppi (pixels per inch)

Scanning resolution

The number of samples taken per inch

lpi (lines per inch)

Line screen frequency The number of halftone cells per inch

Halftone cell

Being divided into small areas for the original image

dpi (dots per inch) Print resolution

The number of micro dots per inch

Micro dots which consist of halftone cell


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Size of the halftone cells

The number of gray levels

( ) 12 +lpidpi (1)

Fig. 2. Two halftone cells are shown. The halftone cell to the left represents agray value of and the one to the right .%75.186412 = %256416 =

lpidpi


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The Hybrid halftoning method

Using Adobe Photoshop in order to AM halftone our

images

Representing black and white

1 and 0 respectively

Method

Being explained by both words and Matlab pseudo codes

Being known in advance

Critical dot ( its size and shape)

The smallest possible reproducible dot

Gray value for which AM halftoning method produce this

dot


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Independent components

Shape of halftone dots and halftone angle

Testing environment

Original test image g

Independent components

Halftone angle and the critical dot shape

600600 200=ppipixels with , dpi600 lpi100, , dot shape round

halftone angel o45, critical dot size 22 micro dots

,

,

gray tone that corresponds to this dot: 125.0

Fig. 3. The original test image. Pixelswith and printed at

66200=ppi .200dpi


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AM halftoning implemented process

Using Adobe Photoshop in order to AM halftone

Building a mask using in order to locate the

highlights

Meaning that mask is equal to 1 in positions where is

smaller than and elsewhere

Being AM halftoned exactly the same values as were used

for AM halftoning the original image for mask

( )ghalftoneAMgAM _= (2)

where lpippi 2= and halftone cell is ,66AMg is 18001800 pixels

( )3.Eq

125.0


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Using mask we can locate the parts of where the dots

are bigger than the critical size (bigger than )

Fig. 4. The mask printed at 600 dpi.The parts that are black, i.e., 1, are theparts of the image that are going to beFM halftoned.The parts that are white,i.e.,0, are being AM-halftoned.

AMg

22

AMmaskAM gMASKg &~_ =(5)

where ~ means a logical NOT and ~MASK is a matrix whose elements are

1s where MASK has zero elements, and 0s where MASK has non-zero elements. & denotes the element-by-element logical ANDbetween the matrices


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FM halftoning implemented method

Only using the critical dot, a square dot

Simplest way

Being first FM halftone the parts that should be FM halftoned,

and then double the size of the result

Fig. 5. An enlarged version of a part ofis shown. There are some dots in the transitionare that have been cut and are smaller than

Apart from these unwanted dots, the imagealso shows an enlarged version of a part of thetest image that would be obtained in print if theprint press were not able to print the dotssmaller than This image is printed at 150

dpi.

maskamg _

.22

.22

22


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Being resized to pixels before being FMhalftoned

Only problem being MASK and

Areas of the original image that are black in the mask thatare supposed to be FM halftoned

Resizing again

Removing the unwanted dots at boder between AM and

FM halftoned parts in Adding AM and FM parts together to get final hybrid

halftoned image

900900

( )5.1,gimresizegnew = (6)

18001800 900900newg

( )5.0,MASKimresizemasknew = (7)

( )newnewfm

maskghalftoneFMg &_= (8)

)2,fmfm gimresizeG = (9)

amG

amfmhyb GGG = (10)


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Fig. 6. Our test image and a grayscale ramp

are halftoned by the proposed hybridhalftoning method. In the test image the partsthat were black in MASK, shown in Fig. 4, areFM halftoned and the rest of the image AMhalftoned. The FM method is non modifiedError Diffusion. Image are printed at 600 dpi.

Fig. 7. Our test image and a grayscale rampare halftoned by the proposed hybrid halftoningmethod. In the test image the parts that wereblack in MASK, shown in Fig. 4, are FMhalftoned and the rest of the image AMhalftoned. The FM method is the methodpresented in this article. Images and printed at

600 dpi.


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How to remove the unwanted dotsHow to remove the unwanted dotsUnwanted dots

Separating the AM part from the part being FM

halftoned Generating some dots in the transition

Being cut and consequently smaller than the critical dot

(a) (b)Fig. 8. The left half of the images is AM-halftoned and the right half is FM-halftoned. (a)the unwanted dots are not removed, hence a visible vertical dashed-line structure isobserved in the border between AM and FM pattern. (b) The unwanted dots are removed

and the visible vertical dashed-line has disappeared.


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Reason for removing the unwanted dots

Degrading the quality of the final image

(a) (b) (d)(c)

(e) (f) (g)Fig. 9. How to remove the dots smaller than The two dots to the right in are goingto be removed. The other two dots will remain as they are. (a) The original, called (b)The image in Eq. (11). (c) The image in Eq. (12). (d) The image in Eq. (13). (e) Theimage in Eq. (14). (f) The image in Eq. (16). (g) The final image in Eq.(17).

.22 a.a

b c de f

newa


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Method Using the convolution function in the Matlab

Process

Locating the dots that are equal to the critical dot Convoluting a with kernel that represents the critical dot

=

11

11,2 aconvb (11)

where

the kernel is not rotated if it has a symmetrical shape

2conv denotes the two-dimensional convolution and the

matrix represents the critical dot22


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Holding 4 The number of black micro dots in the critical dot

Indication of a dot equal or bigger than the critical dot size

Meaning that matrix c is equal to 1 in those positions where b

equals 4, and equal to 0 elsewhere

Convoluting c with the kernel

Not rotated version of the kernel

( )4=== bc (12)

= '',

11

11,2 samecconvd (13)

where same makes the result to be the same size asd c


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Rebuilding dots Not some dots in the d but some dot in the original image

Finding connected micro black dot in for

Black micro dots or connected to at least one black

micro dots

Final image

( ) ( ) ( ) ( )1~&~201~& ++= daddae (14)

d e

= '',

111

111

111

,2 sameeconvm (15)

1&10 ==>= emf (16)

dfanew = (17)


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Critical Dots with General ShapeCritical Dots with General ShapeProcess

Making a critical dot shape that is not square

Assuming to want the critical dot to be the dot in the upper leftof Fig. 9(a)

Being a dot plus another three black micro dots

Making image p First assuming the dot shape is Assuming that is constant image with 1%

coverage

Consisting of 7 black micro dots FM halftoning

Resizing it in order to

22

22

( )newnew maskg &

( )newnew maskg &74

22


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Detailed method Finding the dots

Replacing a by p in Eq.(11) and Eq.(12)

Obtaining c Computing the new dot

22

= '',

0100

1110

0111

,2 samecconvq (18)

(a) (b)

Fig. 10. (a) The dots are , and(b) the dots are the new critical dot.The fractional are covered withblack dots is the same as in a.

22


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Test printTest printProcess

Testing environment

Being , printing resolution: ,

screen frequency: , gray levels=197, halftone angle= ,

and critical dot =

Comparison Presented method and SambaFlex

Result

Looking better in the highlights because the FM dots are placed

more homogeneously

o

37

44

300=ppi dpi2100

lpi150


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(a) (b)

Fig. 11. An enlarged version of a part of thetest print, printed at SambaFlex

raster method was used.

.300dpi

Fig. 12. An enlarged version of a part of thetest print, printed at The hybrid

method proposed in this article was used.

.300dpi

Fig. 13. (a) An enlarged version of a part of the image in Fig.11, printed at 150 dpi. (b)An enlarged version of a part of the image in Fig. 12, printed at 150 dpi.


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Discussions and future workDiscussions and future work Quality of halftoning method

Depending on the original images for real printing

Studying the mask before preparing the halftoned image

Result

Not suffering from any evident distortion in the

transition area Being used good FM method, the proposed method works

very well


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Very simple method Needing only 17 lines for using Matlab

Well performing FM method

Future work Extension of this hybrid halftonig method method to

color

Halftoning the color separations dependently for the colorextension of the FM method

20050611(Hybrid ing A Useful Methdod for Flexography

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