Screen Ruling, Print Resolution AM, FM and Hybrid Halftoning

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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
bits/pixel

• Grayscale 8 256 tones
• RGB 3824 256316.7

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 (0s) and black (1s)

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Halftoning
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DIGITAL HALFTONING
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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
Halftoned image
Original image
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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
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 22150 600.
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HALFTONE CELL
dpi Number of micro dots per inch
This halftone cell represents at most 82 1 65
gray tones
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HALFTONE CELL
Screen ruling number of halftone cells per inch
(lpi)
Micro dot
Resolution number of micro dots per inch (dpi)
Halftone cell
In this case 17 gray tones
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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
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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
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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
AM
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(No Transcript)
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(No Transcript)
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THRESHOLDING
g and b are the original and the halftoned image,
respectively. t is the threshold matrix.
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THRESHOLDING
This threshold matrix represents 10 gray tones
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THRESHOLD MATRIXExample Line
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THRESHOLD MATRIXExample Spiral
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THRESHOLD MATRIX Clustered Dispersed, 45
degrees
Clustered
Dispersed
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TABLE HALFTONING
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TABLE HALFTONING
Dispersed
Clustered
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FM HALFTONING Error Diffusion
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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 angles Rosette 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