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Screen Ruling, Print ResolutionAM, FM and Hybrid Halftoning
Sasan Gooran
Linköping University
LiU-Norrköping
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DIGITAL IMAGES (pixel based)
Scanning
Photo Digital image
ppi (pixels per inch): Number of samples per inch
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ppi (pixels per inch)
• ppi (scanning resolution): Number of samples per inch
• The higher ppi the better the representation of the con-tone image (Photo)
• Higher ppi requires more memory
• ppi should not be unncessarily high
• Choice of ppi????
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ppi = 72
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ppi = 36
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ppi = 18
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DIGITAL IMAGES Memory
• Grayscale 8 256 tones
• RGB 3*8=24 256^3=16.7
bits/pixel
million colors
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DIGITAL HALFTONING
• Since most printers and image setters are not able to reproduce different shadows of gray the original digital image has to be transformed into an image containing white (0’s) and black (1’s)
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Halftoning
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DIGITAL HALFTONING
Prepress Halftoning PrintCon-tone
Image
Halftoned
Image
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DIGITAL HALFTONING Example
Periodic and clustered dots
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DIGITAL HALFTONING Example
Non-periodic and dispersed dots
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HALFTONE CELL Pixel (/a number of pixels)
Halftone cell
The fractional area covered by the ink corresponds to the value of the pixel (or the area)
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HALFTONE CELL
Original image Halftoned image
Halftone cell
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SCREEN RULING/FREQUENCY
• lpi (lines per inch): Number of halftone cells per inch
• The higher lpi the better the print (?!)
• High lpi requires more stable print press etc.
• Does a higher lpi always lead to a better print? (to be answered later)
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RULE OF THUMB
lpisizeOriginalsDppi *2* ize esired
Ex. A 10 x 15 cm2 photo that is supposed to be 20 x 30 cm2
when printed at 150 lpi has to be scanned with a ppi about 2*2*150 = 600.
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HALFTONE CELLMicro dot
dpi: Number of micro dots per inch
This halftone cell represents at most 82 + 1= 65 gray tones
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HALFTONE CELL
Halftone cellHalftone cell Resolution: number of micro dots per inch (dpi)Resolution: number of micro dots per inch (dpi)
Micro dotMicro dot
In this case:17 gray tonesIn this case:17 gray tones
Screen ruling: number of halftone cells per inch (lpi)Screen ruling: number of halftone cells per inch (lpi)
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lpi & dpi
• lpi: Number of halftone cells per inch
• A halftone cell consists of micro dots
• dpi: Number of micro dots per inch
• The ratio dpi/lpi decides the size of the halftone cell
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lpi & dpi
gray tones of n12
lpidpi umber
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lpi & dpi (Example)
• Assume that dpi is fixed at 600
• lpi = 150 only gives 17 gray tones
• lpi = 100 only gives 37 gray tones
• lpi = 50 gives 145 gray tones
• Does a higher lpi always lead to a better print? Not necessarily!
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High lpi, few gray tones
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Lower lpi, more gray tones
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Low lpi, more gray tones but large halftone dots, (not satisfying)
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AM & FM HALFTONING
• AM (Amplitude Modulated)– The size of the dots is variable, their frequency
is constant
• FM (Frequency Modulated) 1st generation– The size of the dots is constant, their frequency
varies
• FM (Frequency Modulated) 2nd generation– The size of the dots and their frequency vary
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AM & FM (1st & 2nd Generation) Halftone
AM FM, 1st FM, 2nd
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AM & FM Halftone
AM FM
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FM Halftone, 1st and 2nd generation
First Second
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Hybrid Halftoning
AMFM_1FM_2
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THRESHOLDING
),(),( if ,0
),(),( if ,1),(
nmtnmg
nmtnmgnmb
g and b are the original and the halftoned image, respectively.
t is the threshold matrix.
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THRESHOLDING
0.6
0.1
1
0.3
0.2 0
Original Image Halftoned imageThreshold matrix
This threshold matrix represents 10 gray tones
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THRESHOLD MATRIXExample: Line
1 2 3 4 1 2 3 4 5 6 7 8 5 6 7 8 9 10 11 12 9 10 11 12 13 14 15 1613 14 15 16
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THRESHOLD MATRIXExample: Spiral
1 2 3 4 1 2 3 4 12 13 14 5 12 13 14 5 11 16 15 6 11 16 15 6 10 9 8 710 9 8 7
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THRESHOLD MATRIX Clustered & Dispersed, 45 degrees
14 12 13 16 19 21 20 1714 12 13 16 19 21 20 17 5 4 3 10 28 29 30 23 5 4 3 10 28 29 30 23 6 1 2 11 27 32 31 22 6 1 2 11 27 32 31 22 9 7 8 15 24 26 25 18 9 7 8 15 24 26 25 1819 21 20 17 14 12 13 1619 21 20 17 14 12 13 1628 29 30 23 5 4 3 10 28 29 30 23 5 4 3 10 27 32 31 22 6 1 2 1127 32 31 22 6 1 2 1124 26 25 18 9 7 8 15 24 26 25 18 9 7 8 15
1 30 8 28 2 29 7 271 30 8 28 2 29 7 2717 9 24 16 18 10 23 1517 9 24 16 18 10 23 15 5 25 3 32 6 26 4 31 5 25 3 32 6 26 4 3121 13 19 11 22 14 20 1221 13 19 11 22 14 20 12 2 29 7 27 1 30 8 28 2 29 7 27 1 30 8 28 18 10 23 15 17 9 24 16 18 10 23 15 17 9 24 16 6 26 4 31 5 25 3 32 6 26 4 31 5 25 3 32 22 14 20 12 21 13 19 11 22 14 20 12 21 13 19 11
Clustered Dispersed
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TABLE HALFTONING
0.6
0.2 0
Original Image Halftoned image
MeanMean
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TABLE HALFTONING
Clustered Dispersed
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FM HALFTONING Error Diffusion
0.7
Threshold0.5
Original image Halftoned Image
0.3
error-Filter
7
3 5 1(*1/16)
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COLOR PRINT
Additive color mixing, TV, Computer screen
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COLOR PRINT
Subtractive color mixing, Printers
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COLOR PRINT
• Three primary colors
• CYAN (C)• MAGENTA (M)• YELLOW (Y)
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COLOR PRINT
• Three secondary colors
• And Black
• RED (R, MY)• GREEN (G, CY)• BLUE (B, CM)
• BLACK (K, CMY)
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COLOR PRINT
Original
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COLOR PRINT
AM
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COLOR PRINT
FM
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AM HALFTONE same angle for C, M, Y & K
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Conventional Color HalftoningSame raster angle
Error in position can cause color shift
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Conventional Color HalftoningSame raster angle
Error in raster angle can cause Moiré
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Conventional Color Halftoning Different raster angle, 0, 15, 75 and 45 degrees
AM different anglesRosette patterns
FM
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AM HALFTONE different angles for C, M, Y and K
15, 75, 0 and 45 degrees
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ROSETTE PATTERN
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ROSETTE PATTERN