Pattern, Print

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. more...

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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.

Traditional Halftoning

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.

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).

Read more at Wikipedia.org