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Encyclopedia > Halftone
Left: halftone spots. Right: how the human eye would see this, when viewed from a sufficient distance.
Left: halftone spots. Right: how the human eye would see this, when viewed from a sufficient distance.

Halftoning is the transformation of a grayscale or color image to a pattern of small spots with a limited number of colors (e.g. just black spots on white background), in order to make it printable. Printing is in its bare essence a binary process for each point on the paper: put ink (or toner) on paper (e.g. black) or leave the paper uncovered (e.g. white). This would suggest that only binary images (e.g. black on white or red on yellow) are printable. However, halftoning makes it possible to reproduce so-called continuous tone images, which are images with different shades of grey or color. In the basic case of grayvalue halftoning, the halftone process creates (binary) patterns of small black dots on a white background. When viewed from a sufficient distance, the human viewer will be unable to see the dots themselves, because they are too small. Instead, the human viewer will have the illusion of gray, whose darkness will depend on the amount of black dots on the white background. For example, many black dots or big black dots will create the illusion of a darker gray, while few or small black dots will create the illusion of a lighter gray. Image File history File links Halftoning_introduction. ... This article or section is in need of attention from an expert on the subject. ... An ink is a liquid containing various pigments and/or dyes used for colouring a surface to render an image or text. ... A colour toner bottle Toner is a powder used in laser printers and photocopiers which forms the text and images on the printed paper. ... Piece of Letter paper Paper is a thin material produced by the amalgamation of plant fibres, which are subsequently held together without extra binder, largely by hydrogen bonds and fiber entanglement. ... Grey or Gray (see spelling differences) is the color between white and black. ... Color is an important part of the visual arts. ...

Three examples of color halftoning with CMYK separations. From left to right: The cyan separation, the magenta separation, the yellow separation, the black separation, the combined halftone pattern and finally how the human eye would observe the combined halftone pattern from a sufficient distance.
Three examples of color halftoning with CMYK separations. From left to right: The cyan separation, the magenta separation, the yellow separation, the black separation, the combined halftone pattern and finally how the human eye would observe the combined halftone pattern from a sufficient distance.

Color printing is possible by using a limited set of color inks, for example the well known and widely used CMYK color set with cyan, magenta, yellow and black. Color halftoning generates a halftone pattern for each of these inks. When these patterns are printed over each other, the human viewer will observe a color that depends on the amounts of the color inks. Image File history File links Halftoningcolor. ... Color printing is the reproduction of an image or text in color (as opposed to simpler black and white or monochrome printing). ... Cyan, magenta, yellow, and key (black) CMYK (or sometimes YMCK) is a subtractive color model used in color printing. ...


Some print technologies are not limited to binary output but have multilevel capabilities. This means that the printer is capable of putting an intermediate amount of ink or toner on the paper, beside the traditional full coverage and no coverage. However, these intermediate levels are limited in number and reliability. Therefore, multilevel halftoning techniques are still needed for high quality reproduction.

Contents

Traditional Halftoning

Typical Halftone Resolutions
Screen Printing 45-65 lpi
Laser Printer (300dpi) 65 lpi
Laser Printer (600dpi) 85-105 lpi
Offset Press (newsprint paper) 85 lpi
Offset Press (coated paper) 85-185 lpi
Photograph reduced to a black and white halftone with round spots and a 45° screen
Photograph reduced to a black and white halftone with round spots and a 45° screen
Same photograph in color, reduced to a CMYK halftone
Same photograph in color, reduced to a CMYK halftone

Sample image illustrating a halftone process. ... Sample image illustrating a CMYK halftone process. ... Cyan, magenta, yellow, and key (black) CMYK (or sometimes YMCK) is a subtractive color model used in color printing. ...

Resolution of halftone screens

The resolution of a halftone screen is measured in lines per inch (lpi). This is the number of lines of dots in one inch, measured parallel with the screen's angle. Known as the screen ruling, the resolution of a screen is written either with the suffix lpi or a hash mark. E.g. 150lpi or 150#.


The higher the resolution, the greater the detail that can be reproduced. However, higher resolution requires a better quality printing process, otherwise the result will suffer from posterization. Therefore screen resolution is matched to the selected printing process. An example of a photo in JPEG format (24bit colour or 16. ...


Multiple screens and colour halftoning

When different screens meet, 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). A moiré pattern is an interference pattern created, for example, when two grids are overlaid at an angle, or when they have slightly different mesh sizes. ...


Halftoning is also commonly used for printing colour pictures. The general idea is the same, by varying the density of the four primary printing colours, cyan, magenta, yellow and black (abbreviation CMYK), any particular shade can be reproduced. Cyan, magenta, yellow, and key (black) CMYK (or sometimes YMCK) is a subtractive color model used in color printing. ...


In this case there is an additional problem that can occur. In the simple case, one could create a halftone using the same techniques used for printing shades of grey, but in this case the different printing colours have to remain physically close to each other to fool the eye into thinking they are a single colour. To do this the industry has standardized on a set of known angles, which result in the dots forming into small circles or rosettes.




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 colour drum scanners made by companies such as Crosfield Electronics, Hell and Linotype-Paul.


In the 1980s halftoning became available in the new generation of 'imagesetter' film and paper recorders that had been developed from earlier 'laser typesetters'. Unlike pure scanners or pure typesetters, imagesetters could generate all the elements in a page including type, photographs and other graphic objects. Early examples were the widely used Linotype Linotronic 300 and 100 introduced in 1984, which were also the first to offer PostScript Rips in 1985.


Early laser printers from the late 1970s onward could also generate halftones but their original 300 dpi resolution limited the screen ruling to about 65 lpi. This was improved as higher resolutions of 600 dpi and above, plus dithering techniques were introduced.


All halftoning uses a high frequency/low frequency dichotomy. In photographic halftoning, the low frequency attribute is a local area of the output image designated a halftone cell. Each equal-sized cell relates to a corresponding area (size and location) of the continuous-tone input image. Within each cell, the high frequency attribute is a centered variable-sized halftone dot composed of ink or toner. The ratio of the inked area to the non-inked area of the output cell corresponds to the luminence or graylevel of the input cell. From a suitable distance, the human eye averages both the high frequency apparent gray level approximated by the ratio within the cell and the low frequency apparent changes in gray level between adjacent equally-spaced cells and centered dots.


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. Suppose the smiley face in the top left corner is an RGB bitmap image. ... This example shows an image with a portion greatly enlarged, in which the individual pixels are rendered as little squares and can easily be seen. ...


See also

Screentone is a generic name for a technical drawing technique for applying various textures to surfaces from a specially preprinted sheet. ...

External links

  • Halftone Screens and Dots
  • Article from the early 1900s on halftoning
  • Forward Digital Image Halftoning Toolbox for MATLAB
  • Halftone custom Photoshop shape
  • Learn to create halftone shading with Adobe Photoshop
  • Dr. Daniel Lau's Website about halftoning

  Results from FactBites:
 
Halftone - Wikipedia, the free encyclopedia (643 words)
Halftoning is a method of printing shades using a single color ink.
A pattern of dots that is used to represent a particular shade is known as a halftone screen.
Digital halftoning utilizes a raster image or bitmap within which each monochrome picture element or pixel may be on or off, ink or no ink.
Ted's Photographics - The Science of Photography - Halftone Reproduction (893 words)
Traditionally a halftone is created by placing a glass screen, with a finely ruled grid of lines, close to the emulsion surface in a process camera.
The angle of the halftone screen is usually set at 45 degrees to the horizontal as this appears to be the easiest on the eye.
When two screens are superimposed and are not quite aligned, or when the halftone screening process is applied to images with patterns or previously screened photographs there is the possibility of creating moiré patterns due to the interference of one screen on another.
  More results at FactBites »

 
 

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