1.3.3.3. Halftoning

With image sizing problem being solved there had appeared the facility of direct output the halftone transparences as far as scaling of previously halftoned ones is inadmissible because of effecting on the screen ruling. Initially the image was used for such purpose to be recorded through the contact screen fixed over a film exposed by the laser or halogen lamp whose beam intensity was controlled by the output signal of a scanner.

Operational inconveniences inherent in this technique were comprised of the need to provide the tight contact between emulsion sides of a screen and exposed film as well as of the proper fixation of the both on a recording drum. Each scanner had to be completed by rather expensive set of a contact screens with different parameters (frequency, structure, halftone dots geometry…). However, the most principle disadvantage was called forth by the use of photo screening effect conflicting with idea of electronic reproduction itself. Due to this effect, as it’ll be shown lower, the connection between the resulting signal value and dot area wasn’t hard enough but depended on multiple collateral factors involved by chemical processing of a super contrast film.

This problem was solved by introducing the electronic screening where the halftone dots were formed in optical channel of film recorder under direct control of said signal. Galvanic mirrors, mechanic and electronic diaphragms were used to form on a sensitive film layer the exposing spot - image of a variable area halftone dot [1.7 - 1.9].

In case of relief plates electronic engraving the dot area was depended on the depth of pyramidal stylus immersion in a plate material. Electromechanical engraving machines, such as Vario Klischograph of Hell, were for some period making the halftone transparences for lithography too by means of selective withdrawal of an opaque layer from the surface of a transparent plastic or film [1.10].

Another technical problem was comprised of forming the angled halftone structure to avoid the moiré in multicolor printing. That’s why in electronic screening of Graphic Arts laboratory of the Leningrad Institute of Electric Communication there was for the first time used for this purpose non-periodic (stochastic in current terminology) dots placement as shown in figure 1.11 [1.11].

Figure 1.11 Copy of multicolor print produced with the use of electronic screening and non periodic, stochastic halftone dots placement to avoid moiré (1969).

As digital techniques being developed the analogue electronic screening with continuous dot area variation were replaced by matrix methods. Halftone dots, as well as a text characters became being formed of the tiny discrete sub-elements - microdots. That had involved the second, after image capturing and digitizing, process of tone range quantization with the number of levels corresponding to finite amount of such microdots in a matrix. The continuous dot area variation remained just in some Computer-to-Plate (CtP) devices for gravure printing.

At approximately same time the masking at analogue signals was replaced by the digital Color Look-up Table (CLUT) technique.

Vast technical parameters overview for the electronic reproduction equipment of 1980ies was given in [1.12].

Meanwhile, it isn’t out of place to point out the certain trade off inherent in transfer from camera prepress to electronic reproduction. It was stipulated by the use of scanning itself which, contrary to photography, limits the image definition due to picture separating onto, at least, individual lines. The use of a finite size scan spot is accompanied by the so called aperture distortion once again limiting the sharpness of an image.

There was described above how the task of an image sizing in fast scan direction of an analogue scanner was solved with help of single stroke signal digitizing. Meanwhile, the image scaling and rotating problem, unfamiliar for camera reproduction, has once again arisen with further complete image numerical presentation in computer and should be finally overcome by the cumbersome interpolative recalculating procedures of huge image data volumes.