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8/3/2019 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