Halftone is the reprographic technique that simulates continuous tone imagery through the use of dots, varying either in size or in spacing.[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 Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors. Color categories and physical specifications of color are also associated with objects, materials, or greys Complementary colors are defined to mix to grey, either additively or subtractively, and many color models place complements opposite each-other in a color wheel. To produce grey in RGB displays, the R, G, and B primary light sources are combined in proportions equal to that of the white point. In four-color printing, greys are produced either by, the halftone process reduces visual reproductions to a binary The binary numeral system, or base-2 number system, represents numeric values using two symbols, 0 and 1. More specifically, the usual base-2 system is a positional notation with a radix of 2. Owing to its straightforward implementation in digital electronic circuitry using logic gates, the binary system is used internally by all modern computers image that is printed with only one color of ink. This binary reproduction relies on a basic optical illusion An optical illusion is characterized by visually perceived images that differ from objective reality. The information gathered by the eye is processed in the brain to give a percept that does not tally with a physical measurement of the stimulus source. There are three main types: literal optical illusions that create images that are different—that these tiny halftone dots are blended into smooth tones by the human eye. 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 Film grain or granularity is the random optical texture of processed photographic film due to the presence of small grains of a metallic silver developed from silver halide that have received enough photons.

Just as color photography Color photography is photography that uses media capable of representing colors which are produced chemically during the photographic processing phase. It is contrasted with black-and-white photography, which uses media capable only of showing shades of gray. It does not include hand colored or Photochrome photographs either. Some examples of evolved with the addition of filters Optical filters, generally, belong to one of two categories. The simplest, physically, is the absorptive filter, while the latter category, that of interference or dichroic filters, can be quite complex. Optical filters selectively transmits light having certain properties , while blocking the remainder. They are commonly used in photography, in and film layers, color printing is made possible by repeating the halftone process for each subtractive color A subtractive color model explains the mixing of paints, dyes, inks, and natural colorants to create a full range of colors, each caused by subtracting some wavelengths of light and reflecting the others. The color that a surface displays depends on which colors of the electromagnetic spectrum are reflected by it and therefore made visible—most commonly using what is called the "CMYK color model The CMYK color model is a subtractive color model, used in color printing, and is also used to describe the printing process itself. CMYK refers to the four inks used in some color printing: cyan, magenta, yellow, and key black. Though it varies by print house, press operator, press manufacturer and press run, ink is typically applied in the order". [2] The semi-opaque property of ink Ink is a liquid that contains pigments and/or dyes and is used to color a surface to produce an image, text, or design. Ink is used for drawing and/or writing with a pen, brush, or quill. Thicker inks, in paste form, are used extensively in letterpress and lithographic printing allows halftone dots of different colors to create another optical effect—full-color imagery.[1]

Contents

History

The first printed photo using a halftone, December 2, 1873.

The idea of halftone printing is due to William Fox Talbot William Henry Fox Talbot He was born on February 11, 1800 and died on September 17, 1877. He was the inventor of calotype process, the precursor to most photographic processes of the 19th and 20th centuries. He was also a noted photographer who made major contributions to the development of photography as an artistic medium. His work in the 1840s. In the early 1850s, he suggested using "photographic screens or veils" in connection with a photographic intaglio Intaglio is a family of printmaking techniques in which the image is incised into a surface, known as the matrix or plate. Normally, copper or zinc plates are used as a surface, and the incisions are created by etching, engraving, drypoint, aquatint or mezzotint. Collographs may also be printed as intaglio plates. To print an intaglio plate, ink process.[3]

Several different kinds of screens were proposed during the following decades. One of the well known attempts was by Stephen H. Horgan while working for the New York Daily Graphic. The first printed photograph was an image of Steinway Hall in Manhattan Manhattan is one of the five boroughs of New York City. Located primarily on Manhattan Island at the mouth of the Hudson River, the boundaries of the borough are identical to those of New York County, an original county of the state of New York. It consists of Manhattan Island and several small adjacent islands: Roosevelt Island, Randall's Island, published on December 2, 1873.[4] The Graphic then published "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.[5]

The first truly successful commercial method was patented by Frederic Ives of Philadelphia Philadelphia is the largest city in Pennsylvania and the sixth-most-populous city in the United States in 1881 1881 was a common year starting on Saturday (link will display the full calendar) of the Gregorian calendar (or a common year starting on Thursday of the 12-day slower Julian calendar).[3][5] 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 1882 was a common year starting on Sunday (link will display the full calendar) of the Gregorian calendar (or a common year starting on Friday of the 12-day slower Julian calendar) the German George Meisenbach patented a halftone process in England. 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 A relief print is an image created by a printmaking process, such as woodcut, where the areas of the matrix that are to show printed black (typically) are on the original surface; the parts of the matrix that are to be blank (white) having been cut away, or otherwise removed. Printing the image is therefore a relatively simple matter of inking the halftones.[3]

Shortly afterwards, Ives, this time in collaboration with Louis and Max Levy, improved the process further with the invention and commercial production of quality cross-lined screens.[3]

The relief A relief print is an image created by a printmaking process, such as woodcut, where the areas of the matrix that are to show printed black (typically) are on the original surface; the parts of the matrix that are to be blank (white) having been cut away, or otherwise removed. Printing the image is therefore a relatively simple matter of inking the halftone process proved almost immediately to be a success. The use of halftone blocks in popular journals became regular during the early 1890s.[3]

The development of halftone printing methods for lithography Lithography is a method for printing using a stone (lithographic limestone) or a metal plate with a completely smooth surface. Invented in 1796 by Bavarian author Alois Senefelder as a low-cost method of publishing theatrical works, lithography can be used to print text or artwork onto paper or another suitable material 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.[6] By the 1880s Hoen was working on halftone methods that could be used in conjunction with either hand-worked or photolithographic stones.[7][8]

Traditional screening

The most common method of creating screens—amplitude modulation Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent. For example, changes in the signal strength can be used to specify the sounds to be reproduced by a—produces a regular grid of dots that vary in size. The other method of creating screens—frequency modulation In telecommunications, frequency modulation conveys information over a carrier wave by varying its frequency (contrast this with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant). In analog applications, the difference between the instantaneous and the base frequency of the carrier is—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.[9]

Resolution of halftone screens

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

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 Posterization of an image occurs when a region of an image with a continuous gradation of tone is replaced with several regions of fewer tones, resulting in an abrupt change from one tone to another. This creates an effect somewhat similar to that of a simple graphic poster. Therefore file resolution is matched to the output resolution.

Multiple screens and color halftoning

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. This close-up of a halftone print shows that magenta on top of yellow appears as orange/red, and cyan on top of yellow appears as green.

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 In physics, 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. 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 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 The CMYK color model is a subtractive color model, used in color printing, and is also used to describe the printing process itself. CMYK refers to the four inks used in some color printing: cyan, magenta, yellow, and key black. Though it varies by print house, press operator, press manufacturer and press run, ink is typically applied in the order), any particular shade can be reproduced.[10]

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.

The dots cannot easily be seen by the naked eye, but can be discerned through a microscope or a magnifying glass.

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.

In the 1980s halftoning became available in the new generation of imagesetter An imagesetter is an ultra-high resolution large-format computer output device. It exposes rolls or sheets of either photographic film or bromide paper to a laser light source. Once the film or paper is developed, a very high quality black and white image is revealed. Development usually occurs in a unit separate to the imagesetter, as does raster 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 PostScript is a dynamically typed concatenative programming language created by John Warnock and Charles Geschke in 1982. PostScript is best known for its use as a page description language in the electronic and desktop publishing areas RIPs A raster image processor is a component used in a printing system which produces a raster image also known as a bitmap. The bitmap is then sent to a printing device for output. The input may be a page description in a high-level page description language such as PostScript, Portable Document Format, XPS or another bitmap of higher or lower in 1985.[11]

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 Dither is an intentionally applied form of noise used to randomize quantization error, preventing large-scale patterns such as "banding" in images, or noise at discrete frequencies in an audio recording, that are more objectionable than uncorrelated noise. Dither is routinely used in processing of both digital audio and digital video 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 In computer graphics, a raster graphics image or bitmap is a data structure representing a generally rectangular grid of pixels, or points of color, viewable via a monitor, paper, or other display medium. Raster images are stored in image files with varying formats image or bitmap within which each monochrome picture element or pixel In digital imaging, a pixel is a single point in a raster image. The pixel is the smallest addressable screen element; it is the smallest unit of picture that can be controlled. Each pixel has its own address. The address of a pixel corresponds to its coordinates. Pixels are normally arranged in a 2-dimensional grid, and are often represented 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 Dither is an intentionally applied form of noise used to randomize quantization error, preventing large-scale patterns such as "banding" in images, or noise at discrete frequencies in an audio recording, that are more objectionable than uncorrelated noise. Dither is routinely used in processing of both digital audio and digital video to decide which pixels to turn black or white, some of which yield better results than digital halftoning.

See also

Significant academic research groups

References

  1. ^ a b c Campbell, Alastair. The Designer's Lexicon. ©2000 Chronicle, San Francisco.
  2. ^ McCue, Claudia. Real World Print Production. ©2007, Peachpit Berkeley.
  3. ^ a b c d e Twyman, Michael. Printing 1770–1970: an illustrated history of its development and uses in England. Eyre & Spottiswoode, London 1970.
  4. ^ LIFE. "100 Photographs That Changed the World". Time, Inc. August 25, 2003, p 18.
  5. ^ a b Meggs, Philip B. A History of Graphic Design. John Wiley & Sons, Inc. 1998. p 141. ISBN 0-471-291-98-6.
  6. ^ August Hoen, Composition for etching stone, U.S. Patent 27,981, Apr 24, 1860.
  7. ^ August Hoen, Lithographic Process, U.S. Patent 227,730, May 15, 1883.
  8. ^ August Hoen, Lithographic Process, U.S. Patent 227,782, May 18, 1880.
  9. ^ Gaurav Sharma (2003). Digital Color Imaging Handbook. CRC Press. p. 389. ISBN The International Standard Book Number is a unique numeric commercial book identifier based upon the 9-digit Standard Book Numbering (SBN) code created by Gordon Foster, now Emeritus Professor of Statistics at Trinity College, Dublin, for the booksellers and stationers W.H. Smith and others in 1966 9780849309007. http://books.google.com/books?id=AkByHKRGTsQC&pg=PA389&dq=halftone+amplitude-modulation&as_brr=3&ei=rPI2S8DGEpWQlQSHoIXBAQ&cd=2#v=onepage&q=halftone%20amplitude-modulation&f=falsehttp://books.google.com/books?id=AkByHKRGTsQC&pg=PA389&dq=halftone+amplitude-modulation&as_brr=3&ei=rPI2S8DGEpWQlQSHoIXBAQ&cd=2#v=onepage&q=halftone%20amplitude-modulation&f=false.
  10. ^ Halftone Line Screens in Printing "Use of halftone line screens for printing digital images on press". (last checked on 2009-04-20)
  11. ^ Linotype History - 1973–1989

External links

Categories: Printing terminology | Photographic techniques

Personal tools
Namespaces
">
Variants
Views
">
Actions
Search">
Hurricane Grace was a short-lived Category 2 hurricane that contributed to the formation of the powerful 1991 "Perfect Storm". Forming on October 26, Grace initially had subtropical origins, meaning it was partially tropical and partially extratropical in nature. It became a tropical cyclone on October 27, and ultimately peaked with
Navigation
Interaction
Toolbox
Print/export
Languages

 

The above information uses material from Wikipedia and is licensed under the GNU Free Documentation License The purpose of this License is to make a manual, textbook, or other functional and useful document "free" in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a.
Some facts may not have been fully verified for accuracy. [Disclaimers Wikipedia is an online open-content collaborative encyclopedia, that is, a voluntary association of individuals and groups working to develop a common resource of human knowledge. The structure of the project allows anyone with an Internet connection to alter its content. Please be advised that nothing found here has necessarily been reviewed by]
This page was last archived by our server on Sat Jul 31 11:58:59 2010. [ refresh local cache ]
Displaying this page or its contents does not use any Wikimedia Foundation's resources.
The owners of this site proudly support the Wikimedia Foundation.

[Hide]▼
[Hide]▲