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Halftone is the reprographic technique that simulates continuous-tone imagery through the use of dots, varying either in size or in spacing, thus generating a gradient-like effect.[1] "Halftone" can also be used to refer specifically to the image that is produced by this process.[1]

Where continuous-tone imagery contains an infinite range of colors or greys, the halftone process reduces visual reproductions to an image that is printed with only one color of ink, in dots of differing size (pulse-width modulation) or spacing (frequency modulation) or both. This reproduction relies on a basic optical illusion: when the halftone dots are small, the human eye interprets the patterned areas as if they were smooth tones. At a microscopic level, developed black-and-white photographic film also consists of only two colors, and not an infinite range of continuous tones. For details, see film grain.

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Several different kinds of screens were proposed during the following decades. One of the first attempts was by William Leggo with his leggotype while working for the Canadian Illustrated News. The first printed halftone photograph was an image of Prince Arthur published on October 30, 1869.[6] The New York Daily Graphic would later publish "the first reproduction of a photograph with a full tonal range in a newspaper" on March 4, 1880 (entitled "A Scene in Shantytown") with a crude halftone screen.[7]

The first truly successful commercial method was patented by Frederic Ives of Philadelphia in 1881.[5][7] Although he found a way of breaking up the image into dots of varying sizes, he did not make use of a screen. In 1882, the German Georg Meisenbach [de] patented a halftone process in Germany which he named autotype.[8] His invention was based on the previous ideas of Berchtold and Swan. He used single lined screens which were turned during exposure to produce cross-lined effects. He was the first to achieve any commercial success with relief halftones.[5]

The development of halftone printing methods for lithography appears to have followed a largely independent path. In the 1860s, A. Hoen & Co. focused on methods allowing artists to manipulate the tones of hand-worked printing stones.[9] By the 1880s, Hoen was working on halftone methods that could be used in conjunction with either hand-worked or photolithographic stones.[10][11]

Prior to digitised images, special photographic techniques were developed to break grayscale images down into discrete points. The earliest of these was "screening" where a coarse-woven fabric screen was suspended before the camera plate to be exposed, breaking the incoming light into a pattern of dots via a combination of interruption and diffraction effects. The photographic plate could then be developed using photo-etching techniques to create a printing plate.

Other techniques used a "screen" consisting of parallel bars (a Ronchi ruling), which was then combined with a second exposure with the same screen orientated at another angle. Another method was to expose through a screen-plate with crossing lines etched into the surface. Later, either photographic contact screens were used, or sometimes no screen at all, exposing directly on a lithographic (extremely high contrast) film with a pre-exposed halftone pattern.

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; for example, "150 lpi" or "150#".

The higher the pixel resolution of a source file, the greater the detail that can be reproduced. However, such increase also requires a corresponding increase in screen ruling or the output will suffer from posterization. Therefore, file resolution is matched to the output resolution. The dots cannot easily be seen by the naked eye, but can be discerned through a microscope or a magnifying glass.

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 colors have to remain physically close to each other to fool the eye into thinking they are a single color. To do this the industry has standardized on a set of known angles, which result in the dots forming into small circles or rosettes.

Though round dots are the most commonly used, many dot types are available, each having its own characteristics. They can be used simultaneously to avoid the moir effect. Generally, the preferred dot shape is also dependent on the printing method or the printing plate.

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, and 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 luminance 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. 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. Digital halftoning based on some modern image processing tools such as nonlinear diffusion and stochastic flipping has also been proposed recently.[15]

The most common method of creating screens, amplitude modulation, produces a regular grid of dots that vary in size. The other method of creating screens, frequency modulation, is used in a process also known as stochastic screening. Both modulation methods are named by analogy with the use of the terms in telecommunications.[16]

Inverse halftoning or descreening is the process of reconstructing high-quality continuous-tone images from the halftone version. Inverse halftoning is an ill-posed problem because different source images can produce the same halftone image. Consequently, one halftone image has multiple plausible reconstructions. Additionally, information like tones and details are discarded during halftoning and thus irrecoverably lost. Due to the variety of different halftone patterns, it is not always obvious which algorithm to use for the best quality.

There are many situations where reconstruction is desired. For artists, it is a challenging task to edit halftone images. Even simple modifications like altering the brightness usually work by changing the color tones. In halftone images, this additionally requires preservation of the regular pattern. The same applies to more complex tools like retouching. Many other image processing techniques are designed to operate on continuous-tone images. For example, image compression algorithms are more efficient for those images.[17] Another reason is the visual aspect since halftoning degrades the quality of an image. Sudden tone changes of the original image are removed due to the limited tone variations in halftoned images. It can also introduce distortions and visual effects like moir patterns. Especially when printed on newspaper, the halftone pattern becomes more visible due to the paper properties. By scanning and reprinting these images moir patterns are emphasized. Thus, reconstructing them before reprinting is important to provide a reasonable quality.

The main steps of the procedure are the removal of halftone patterns and reconstruction of tone changes. In the end, it may be necessary to recover details to improve image quality. There are many halftoning algorithms which can be mostly classified into the categories ordered dithering, error diffusion, and optimization-based methods. It is important to choose a proper descreening strategy since they generate different patterns and most of the inverse halftoning algorithms are designed for a particular type of pattern. Time is another selection criteria because many algorithms are iterative and therefore rather slow.

The most straightforward way to remove the halftone patterns is the application of a low-pass filter either in spatial or frequency domain. A simple example is a Gaussian filter. It discards the high-frequency information which blurs the image and simultaneously reduces the halftone pattern. This is similar to the blurring effect of our eyes when viewing a halftone image. In any case, it is important to pick a proper bandwidth. A too-limited bandwidth blurs edges out, while a high bandwidth produces a noisy image because it does not remove the pattern completely. Due to this trade-off, it is not able to reconstruct reasonable edge information. 17dc91bb1f