Halftone
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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.
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.
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.
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[edit] 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 |
[edit] 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.
[edit] 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).
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.
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.
[edit] Digital halftoning
Digital halftoning has been replacing photographic halftoning since 1986 when the Apple LaserWriter printer incorporating Adobe PostScript software was 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.
