Talk:Halftone
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While I very much enjoy and benefit from the technical discussion, below, in my confusion I'd thought that this talk page's purpose was for writers to discuss the merits of the current wiki page. This writer, for example, longs for a definition or three of "halftone," a definition that will precede the descriptions of "halftoning" that dominate the wiki page. Walter Dufresne 20:21, 3 January 2007 (UTC) walterdufresne Brooklyn, New York, 3 January 2007
The effect has its limits, when the dots get too small or spaced too far apart, the eye starts seeing dots again.
How so? Obviously, when dots are too far apart, they will look like individual dots, but when they are too small? I'd think the smaller they get (that is, the greater the lines-per-inch of the halftone screen), the more smooth they would appear to the human eye. Can someone clarify this? -- Wapcaplet 01:28, 10 May 2004 (UTC)
When the halftone frequency is too high for the reproducing medium's resolution (be this the Laser Printer or Image Setter dpi resolution or the repro film's grain size) an effect of posterization occurs. Maybe this is what the comment is getting at. Another possibility is due to the flaw that this article refers to 'Screened halftoning' only and not other forms of halftone such as dithering. Under Floyd-Steinberg or Error-Diffusion dithered halftoning (halftoning processes used in inkjet printers) areas of low intensity (ie where the dots are spaced wide apart) have the dots much clearer to the eye (due to high contrast). This is the reason that reverse grey component replacement is used. It is also the reason for the new generation of 6 or 7 colour inkjet printers (often called photo printers). One day I will explain this clearly in the article itself, but it's a big ask as I'm new to wiki... --Bb3cxv 7 July 2005 14:15 (UTC)
I've always been fascinated to learn how this process actually took place before digital. How did a newspaper in the 70s take a photograph and turn it into this dots pattern? Can someone add this in - a brief descrption of the actual process? TeamCoachingNetwork 06:43, 30 November 2006 (UTC)
It was originally done in about the 1880s by using closely-spaced parallel lines etched onto glass - hence the 'lines-per-inch' measurement and the 'screen ruling' term. 2 sheets of ruled glass were glued face to face at 90 degrees at first. This was called the 'screen.' Dropping an expensive glass screen was a disaster. Aligning the rules diagonally rather than at right angles would give diamond-shaped cells that generated elliptical dots, which were useful for printing certain types of image. By the 1970s when I was doing this at print college, the 'contact screens' were single cross-ruled sheets of thin clear plastic.
The screen was placed in a very large 'process' camera. It was positioned ruled-side down in close contact with a sheet of unexposed film, and the whole sandwich held flat by a vacuum plate. A photograph was then taken of the continous tone photographic original. A high contrast 'lith' film was used, which gave essentially pure black or pure white images, ie close to binary. As the original greyscale image was projected by the lens, it fell onto the screen where it was split up by the cells into small discrete areas which formed the basis of individual dots.
The dots were round despite the square screens because of diffraction effects around the cell edges, coupled with the high contrast film/paper which ignored the grey tones due to light fall-off around the edges of the cells. The dots were therefore sharp-edged, with different sizes depending on the light intensity of the particular area of the original that they represented.
The haltoned film image was a negative, which was then placed face-down in contact with a photo-sensitive surface on the printing plate (which might be letterpress, lithography, flexography or gravure). Placing the film face-down was vital for maximum image transfer quality, but meant that the image was mirror-image reversed. To allow for this, a reversing mirror set was placed in front of the lens of the process camera, so the initial screened negative image was reversed to start with. This meant that the final plate was correct.
So actually the halftone dots were originally formed by a relatively simple optical effect and the challenge of early electronic systems was to recreate this effect digitally for scanners and laser film recorders. Simoneccles 03:48, 3 January 2007 (UTC)
[edit] seeking a working definition of "halftone"
The page does a wonderful job of defining, at length and in detail, such compelling processes as "halftoning." This seems good. I miss reading any kind of definition of a slightly different thing, the "halftone" of the page's title.
What is a halftone? How might we define such a thing? Is it possible to write a working definition of "halftone"?
Walter Dufresne 17:21, 6 February 2007 (UTC)Walter Dufresne, Brooklyn, NY, USA
