4.3.7 Screening in low-resolution outputs

The resolution of the “on-drum” and “in-drum” imagesetters as well as of some CtP systems achieves 5000 dpi. This allows for providing a smooth tone rendition by forming halftone dots in the matrices up to 30x30 microdots. Despite such a large size of the weight function period, the screen frequency and, accordingly, its visibility remain at the same level as in the previously used photomechanical systems. However, getting a high enough screen ruling was a technical problem at the outputs with relatively low resolution. In the roll fed, capstan devices it is about 1000 dpi and even lower in digital printing, as shown in the table 2.

For technical and patent reasons, the outputs of printing devices of various manufacturers use rather different ways for the stepwise distortion reduce and screen ruling increase making it less noticeable. One of them is to increase the number of possible references to the control numerical data (bitmap) on the given recorded area [4.22]

Table 2 Approximate resolution values of image output devices

Type of device	Resolution dpi (lines/mm)	Dot size (µm)
In-drum, on-drum scanners, CtP	5000 (200)	5
Roll fed, capstan devices	1200 (48)	20
Liquid toner printers	800 (33)	30
Dry toner printers	600 (25)	40
Ink jet printing	480 (20)	50
Screen printing	240 (10)	100

.

For example, if it is possible to turn on - off the recording spot of 25 µm only once during the time of its move at the distance equal to its size, in a matrix of 4 x 4 subelements (screen ruling of 40 Lpi or 100 lines per cm), there are only 16 values of the reflection implemented (Figure 4.18, a). This is clearly not enough for smooth tone rendition.

Increasing the size of the matrix will lead to a greater screen visibility due to the ruling decrease. However, with doubling the frequency of spot control for each of the coordinates, there are provided 64 reflection values within the same cell (Figure 4.18, b), referring to the screen function having a dimension of 8x8.

Figure 4.18. Within unit print area, it is possible to get quarterly more dots of different size, if to double the control frequency of exposing spot horizontally and half overlap the recorded lines vertically

In some of printers the switching frequency is controlled by a full eight-bit image signal. Due to this, the laser beam can be interrupted by an optical modulator at any moment of its movement in the horizontal scan direction at 1/256 of its diameter. For a similar increase of the control addressability in the transverse direction, it is necessary to ensure the overlap of the adjacent scan lines.

Accordingly, the technical characteristics of a number of such devices indicate the possibility of greater or lesser quality of recording, i.e. with different resolutions, for example, 600 dpi and 1200 dpi. The second of these figures, strictly speaking, does not characterize the actual resolution. Despite the improvement of the tone rendering, reducing the contours stepwise distortion and screen visibility, the device remains capable of recording separate lines with a thickness of only 1 / 600, but not in 1/1200 of an inch.

The same problem is solved in Scitex imagesetters by changing the intensity of the spot, exposing the peripheral microdots of the halftone dot. Figure 4.19 (a, b, c) shows the illumination profiles in the latent image on a film for three dots of different sizes as the sum of the distributions of three, five and seven single microdot exposures. The exposure spot size corresponds to the scan pitch. Therefore, the halftone dot can be reduced or increased to the right or left only by a multiple of such pitch. At the same time, as shown in figure 4.19 (c), the intensity variation in the outer spot gives an additional offset (1) of the front of the total illumination profile and, accordingly, of dot edge on the developed film on just a part of a scan pitch.

Figure 4.19. In halftone dot recording on sensitive layer the total exposure depends on the number of its constituent microdots; the dot size can be changed by a value (1) smaller than the scan pitch by the control beam intensity for outer microdot

Another method of forming the halftone structure of acceptable, unobtrusive ruling in the low resolution printer is the periodic, inside screen function matrix, grouping of weight values in a set, for example, of twenty-five clusters, as was shown in figure 4.13.

In reproduction of line matter or text, such measures can improve the geometric accuracy of counters and edges by improving the recorder frequency response. However, it does not allow for reducing the width of scan lines themselves to increase the image definition. The resolution of the system, as already noted, may not exceed the reverse size of the scan spot. In this sense, the manufacturer claims that the resolution is 2000 or 3000 dpi (80 or 120 lines per mm) in the output device with scan spot of 27 microns, are only promotional in nature. However, such measures allow, albeit indirectly, to improve the definition of tone illustrations by increasing the screen ruling. As follows from the first example above (Figure 4.18), 64 gradations can be provided with half less (4x4) screen period.

In a number of printing methods, there is a possibility of some intermediate effect on the material between its full on and off. For example, if in an inkjet printer for the same substrate point both single and repeated deposition of an ink drop is possible, then not two, but three of its resulting states are available. It can be not printed at all, partially marked with one ink drop and completely covered as a result of additional spreading due to the deposition of the second one.

In the absence of the ability to control the drops number at given location, some ink jet printers use the additional print heads, fed by the same, but twice as less intensive CMY inks. Combining the states (no ink, weak, normal, and weak+normal ink) allows to quarterly increase the number of responses in the same spatial copy period, without increasing the acuteness of its structure. Such methods are referred in the literature to multi-level halftoning [4.23].

If, in spite of all the measures taken, the dot alphabet is insufficient for smooth tone rendition, the banding is diluted using a mixture of neighboring characters of such an alphabet. The first digital scanners, due to the inherent technical limitations of the time, worked on a 6-bit signal, which corresponds to the tone quantization scale of just 64 levels. Figure 4.20 (a) shows the jumps between three adjacent tone values m - 1, m and m+1 represented by three neighboring symbols of dots alphabet [4.24]. Smooth transition between these gradations in figure 4.20 (b) is provided by the mixture of dots corresponding to the adjacent gradations.⁷

Figure 4.20. At limited bit depth the quantization noise (banding) is eliminated by "error diffusion" – stochastic mixing the adjacent symbols of a halftone alphabet for smooth tone variation

Some principle theoretical aspects of halftoning technique are disclosed in works [4.1; 4.25-4.27].

Summary issues

Levels of tone quantization scale of the original pixel are dispersed for halftoning among the sub-elements (microdots) of some spatial copy period.

To implement the photomechanical screening effect, a high-contrast film and non-П-shaped exposure distributions in the screen cells are required.

In the projection method, the necessary exposure distribution is provided by the selection of geometric relations between the camera components, accounting also the density range of an original.

It allowed for setting only the effective range of tone values, but not the law of their distribution within it.

In the contact method, according to the interval of original, the exposure and screen parameters are selected.

Correction properties (halftone dot sharpness) were relevant for manual color retouching in photomechanical reproduction.

In electronic halftoning, the dot size is uniquely set by the signal, and the photomechanical screening effect is not used.

The halftone dot shape should provide minimal distortion of its area in tolerances on the stability of the plate making and printing processes.

Print elements can be specified by the symbols (graphemes) of halftone alphabet, microdots weights, analytically and in other ways.

The ink coverage (tone) values correspond to the absorbance (reflection), not to visually uniform parameters such as lightness or optical density, that’s why the dot alphabet should significantly exceed the quantization scale of a perceptually uniform image signal.

Screen structures obtained by the error diffusion method make greater use of the printing resolution with a sufficient amount of the original signal.

The relatively low resolution of "digital" printing creates a problem of providing acceptable screen ruling, which is solved in various ways.

Tests

4.1 Prerequisite photomechanical screening effect:

a) use of projection screen;

b) ensuring non- П -shaped distributions of the dot exposure;

c) formation the dot of required size in the optical path.

4.2 Another condition for obtaining this effect is the presence of:

a) film with small exposure range;

b) film with wide photographic latitude;

c) pre-screened photo layer;

d) contact screen.

4.3 At the specified image magnification and the projection screen ruling, the density range of halftone negative was controlled by changing the:

a) screen cell size;

b) film – screen distance;

c) camera diaphragm;

d) screen - diaphragm distance.

4.4 Shooting with "partial" exposures at two aperture values in projection screening was used to provide the:

a) sharpness of halftone dots;

b) effective tone range on the negative;

c) control of TRC shape.

4.5 If the density intervals of the gray contact screen and original were different, then for the formation of dots in the entire range there was changed:

a) contact screen;

b) exposure;

c) duration of film development.

4.6 Purple contact screens were used for:

a) color separation;

b) control of the density range;

c) variation the form of TRC within its interval.

4.7 In contact screening, the shape of the gradation curve of the process was determined by the:

a) choice of exposure;

b) density distribution in a screen cell;

c) ratio of screen and original density ranges;

d) sensitivity curve of the film used.

4.8 The contact screen is characterized by all of the following parameters except of:

a) ruling;

b) format;

c) optical density range;

d) color;

e) film sensitivity;

f) forms of elements;

g) shape of the density distribution within its element;

h) angle and geometry.

4.9 Photographing a pictorial original through a projection screen does not allow for obtaining a transparency:

a) color separated;

b) with desired TRC form;

c) negative;

d) with the acceptable tone range.

4.10 Correction properties of the halftone mechanicals are usually estimated by the:

a) magnitude of the possible optical density reduction in etching;

b) halftone dot edge sharpness;

c) degree of possible change in the tone value.

4.11 The principal advantage of electronic dot generating in relation to its photomechanical counterpart is the increase in:

a) print definition;

b) productivity;

c) tone rendition stability;

g) screen ruling.

4.12 The weight values distribution in the screen period does not affect the:

a) screen angle;

b) screen ruling;

c) TRC;

d) image definition.

4.13 If the size of the weights matrix in the LUT of the EDG (see Fig. 4.12) is 16 x 16, then to encode the address signals for this memory, it is necessary:

a) 2;

b) 4;

c) 6;

d) 8;

e) 16 binary digits.

4.14 Two-level quantization of the original eight-bit sample with a value of 128 at the threshold of 127 requires diffuse the error equal to:

a) 1;

b) 128;

c) -128;

d) 0.

1 This product approximately reflects the reciprocity effect (constancy of blackening) with the counter change of these factors.

2 Photosensitive layers are related to the high contrast ones when their contrast, defined by the curve inclination tangent in this range, exceeds 3.

3 Illustrations of multi-stage processes by the system of interrelated characteristics of their individual stages are used in a number of subsequent sections of this publication.

4 In electronic engraving, such a connection is uniquely determined by the degree of stylus immersion in the forme material.

5 From a comparison of figures 4.11 (a) and (b) it can be seen that the difference between the "hill" and "crater" is only in the phase shift of the function by half of its period at both coordinates.

6^[1] - Under standard viewing conditions, the wedge of 100 or even less equal contrast patches is perceived as a smooth tone variation from white to black.

7 The method proposed later by Floyd and Stemberg [4.18] differs from this one in that the error diffusion is extended in it not by the one step, but by the half of quantization scale.

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