8-Halftoning by xiuliliaofz

VIEWS: 109 PAGES: 25

									By: Mahsa Nasibi
    Roghaye Musavi
    Mina Malekmohammadi

Advisor: Mr.Samsonchi

•   History
•   Resolution of halftone screens
•   AM and FM halftoning
•   The Rule Of Thumb
•   Multiple screens and color halftoning
•   Digital Halftoning
•   Dot Gain
Halftone is the reprographic
technique that simulates
continuous tone imagery
through the use of dots,
varying either in size or in
Where continuous tone imagery contains an infinite
range of colors or grays, the halftone process
reduces visual reproductions to a binary image that
is printed with only one color of ink.
What is Half-toning

                                            • A halftone of an
                                              image simulates a
                                              continuous tone
                                              image using
                                              “dots” which vary
                                              in size and/or
                                            • Important when
                                              output gray level
                                              is limited
  Half-toning Image       Original Image

1/13/2011             VIP Sensor Forenics                         5
Tone Similarity

 • Gaussian filter
 • G(I,Ih) , difference (MSE) of filtered I and Ih


              Original (I)
         Filtered original (I)   G(I,Ih)       Halftone (Ih)
                                           Filtered halftone (Ih)
Short History
The history of halftoning technology goes back to the middle of
19th century. The first halftoning technologies
were so called optical. The binary halftone was obtained by
projecting the negative of the original image through
a mesh screen .
Bright light, as it passes through the mesh screen, would form a
large and round spot on the plate. Dark light would on the other
hand form a small spot.
The idea of halftone printing is due to William Fox Talbot. In
the early 1850s
The first printed photograph was an image of Steinway Hall in
Manhattan published on December 2, 1873
The first truly successful commercial method was patented by
Frederic Ives of Philadelphia in 1881
The use of halftone blocks in popular journals became regular
during the early 1890s.
Resolution of halftone screens

totally it is possible to represent 65 (82+1) different gray tones
by 8 x 8 halftone cells.
•The number of halftone cells per inch is called line screen
ruling or screen frequency and is denoted by lpi, lines per
Studies have shown that the halftone dots are not detected by
the eye from the normal viewing distance at screen frequencies
above 200 lpi
The number of the micro dots per inch is called the print
resolution and is denoted by dpi (dots per inch).
AM and FM halftoning

    The halftoning methods can mainly be divided into two main
    types, namely AM (Amplitude Modulated) and FM
    (Frequency Modulated). In the AM methods the size of the
    halftone dots vary, while their spatial frequency is
    constant. This means that the size of the halftone dot becomes
    bigger as the tone gets darker. In the FM methods,
    on the other hand, the dot size is constant while the frequency
    (the number of micro dots) varies.
The Rule Of Thumb

    If the digital image is supposed to be displayed by for example a
    computer screen it isunnecessary to scan the image with a ppi
    higher than the resolution of that computer screen, if the
    reproducedimage is supposed to be the same size as the
    However, when it comes to print one should consider the
    screen frequency of the print before deciding the scanning
    resolution. There is a rule of thumb that says that the
    scanning resolution (ppi) should be about 1.5 to 2 times the
    screen frequency (lpi) used in the press at 100%
    scale .
           printed size
    ppi =                 * 2 * lpi
           original size
Multiple screens and color halftoning
  When different screens are combined, a number of
  distracting visual effects can occur, including the
  edges being overly emphasized, as well as a moiré
  pattern. This problem can be reduced by rotating the
  screens in relation to each other. This screen angle
  is another common measurement used in printing,
  measured in degrees clockwise from a line running to
  the left (9 o'clock is zero degrees).
Multiple screens and color halftoning
  Halftoning is also commonly used for printing color
  pictures. The general idea is the same, by varying the
  density of the four primary printing colors, cyan, magenta,
  yellow and black (abbreviation CMYK), any particular
  shade can be reproduced
Color Halftone

     Original image     halftoning
                      Color diffusion
Color Halftone

  Original image   Halftoning   Color diffusion
Digital halftoning

 Digital halftoning has been replacing photographic halftoning
 since the 1970s when 'electronic dot generators' were developed
 for the film recorder units linked to color drum scanners made by
 companies such as Crosfield Electronics, Hell and Linotype-Paul
Digital halftoning
 Digital halftoning uses a raster image or bitmap within which
 each monochrome picture element or pixel may be on or off, ink
 or no ink. Consequently, to emulate the photographic halftone
 cell, the digital halftone cell must contain groups of monochrome
 pixels within the same-sized cell area. The fixed location and
 size of these monochrome pixels compromises the high
 frequency/low frequency dichotomy of the photographic halftone
 method. Clustered multi-pixel dots cannot "grow" incrementally
 but in jumps of one whole pixel. In addition, the placement of that
 pixel is slightly off-center. To minimize this compromise, the
 digital halftone monochrome pixels must be quite small,
 numbering from 600 to 2,540, or more, pixels per inch. However,
 digital image processing has also enabled more sophisticated
 dithering algorithms to decide which pixels to turn black or white,
 some of which yield better results than digital halftoning
Halftoning Methods
•Table Halftoning
•Threshold Halftoning

Table Halftoning
Probably one of the simplest halftoning methods is table halftoning. In
this method different areas in the original image are replaced by their
corresponding tables (halftone cells). If we want to follow the rule of
thumb discussed earlier at least each 2 x 2 pixel area in the original
image should be replaced by a table
Threshold Halftoning
This technique is quite similar to table halftoning and
also results in images with similar quality. However, in
this technique instead of having a pre-decided set of
screen tables, a threshold matrix is used. Depending
on thecontent of the original image the result will vary
due to the form of the threshold matrix. This technique
cansimply be described by following equation:
B(i,j) = 1 if g(i,j) > = t(i,j)
         0 if g(i,j) < t(i,j)
    Dot Gain

The printed dots normally appear bigger than they are in the
bitmap. This is partly because that the dots become
physically bigger due to for example the ink spread and other
distortions produced by the printer or print press.
This is what we call the physical (mechanical) dot gain. Another
reason why the printed dots appear bigger than
their real physical size is the diffusion of the light in the paper or
substrate. This is called the optical dot gain. In
this section these concepts are briefly described.
More Results
•Digital Halftoning by Robert Ulichney
• http://en.wikipedia.org/wiki/Halftone
Thank you

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