From the most appearance of adobe Photoshop it was addressed to the users who are engaged in preparation of printing issuings. Therefore developers always paid huge attention to adjustments of the press and its modern technologies. Many innovations in a color separation and color management have been implemented for the first time in Photoshop. The new version of the program is not an exception of this tendency.

Processes of a letterpress printing demand special handling of illustrations. Half-tone monochromatic illustrations should be растеризованы, and color also are subjected a color separation. More often in printing house the breadboard model is delivered on the transparent film full-scale. The last years all is more widely used technology СТР (Computer to Plate) which allows to receive at once offset forms, passing the intermediate stage of films. The document output is carried out by means of phototypesetting (imagesetter) or exhibiting (platesetter) automatic machines. In the most rough approach they are similar to laser printers, only the laser lights not a photosensitive drum, and a film or the photoform of the offset machine.

All phototypesetting automatic machines use language PostScript and have several times more high resolution of the press, than the best printers (on the average 3600 dpi). These are rather expensive and difficult devices which purchase is presumed to itself far not by each publishing house. Therefore for an output of dummy layouts, as a rule, address to services of studios допечатной preparations.

Though procedure of an output to the phototypesetting automatic machine is absolutely similar to an output to the PostScript-printer, the subsequent typographical process makes some specific demands. We should consider them before to address to reviewing of possibilities of adobe Photoshop in preparation of images for the press.

You already repeatedly faced the linear rasters both by preparation of images, and in an everyday life. Rasterization is applied by almost all numeral output devices — from monitors to printers. The rasterization essence consists in an image partition on small cells a so-called raster grid. Thus each cell has continuous filling. The raster grid of the monitor breaks the image on пикселы, representing group of points of phosphor, and the laser printer or phototypesetting — into black points of the different size. Color printers and offset machines operate with several monochrome rasters simultaneously. At an output to these devices rasterization parameters have crucial importance as they are tightly connected to their hardware possibilities. They define possible parameters of rasterization and superimpose the specific restrictions.

пособ transmissions of halftones in analog (photo) and numeral (printers, typographical machines) processes it is in essence distinguished. Though in both cases the image is formed of very small elements, the difference consists both in elements, and in their layout. The Black-and-white photo is under construction of the points scattered randomly and consisting of a different amount of molecular silver in a layer of an emulsion. They can have various optical density, i.e. the shade of gray. In numeral methods of the press usage of separate colors for transmission of each shade of gray is impossible. Only imagine, how many works it is necessary to expend on adjustment приводки and the press of two hundred with superfluous colors, and what type would be had by a paper already after first ten running! In practice only one color — black is used, a halftone are thus transferred at the expense of a raster. If to look on printed in printing house or on the laser printer at the image, it is easy to note that it consists of set of small points which are called as raster. Most often points are allocated regularly, on identical distance from each other, forming the linear raster or a raster grid. The irregular layout of raster points then the raster formed by them is called as irregular or stochastic, and a rasterization method — FM-rasterization (frequency-modulated) is sometimes applied.

Unlike a photo, the raster point cannot have any shade, — it always black. For transmission of shades raster points of the different sizes are formed. "Fatter" of them, being printed in adjacent cells of a raster grid, leave among themselves not enough white space. It creates illusion of dark color hue. On the contrary, the small points printed with the same interval, leave white the paper most part in space in between. It causes sensation of a light shade (fig. 10.1).

Thus, playback of halftones by a typographical method is entirely constructed on an optical deceit. The eye perceives gradation gray on the image at which press "black color, but not by continuous filling, and in the form of separate points was used only.

The image in laser printers and phototypesetting automatic machines forms by a laser ray. The ray cannot have the variable size that is necessary for creation of raster points of the changing size. Therefore rasterization process consists in join of the "real" points created by a laser ray, in the groups forming raster points.

Such raster represents set of square cells into which the image is broken. Each cell is led out for one raster point. The raster point, in turn, consists of group of "real" points of the identical size created by the output device. The the cell most part is filled by printer points, the больший the raster point formed by them has the size and the more so it transfers a dark shade of gray. For example, to achieve filling of a section of the image 50 percent gray, the rasterization program (растеризатор) fills this section with raster points, each of which will represent half filled cell of a raster. If needed to render 25 percent gray color of a cell of a raster are filled black only on a quarter.

Printer points in a raster cell can occupy various positions. The form of a raster point will depend on it. In polygraphy various forms of raster points (fig. 10.2), but the most traditional and widespread — round are used. To approach the form of a raster point to a circle, at filling of a cell of a raster растеризатор tries to put "real" points as it is possible is more subcentral cells.

What device fulfills functions растеризатора? In overwhelming majority of cases the rasterization task is laid to the output device: the printer or the phototypesetting automatic machine. Растеризатор "is built in" the interpreter of the page description language used by the printer. The most perfect растеризатором, applied in all high-quality output devices, is interpreter PostScript. The second popular page description language created by Hewlett Packard for the printers, — PCL (Printer Control Language), — does not allow in a measure demanded for polygraphy is thin to control rasterization parameters.

Note that the digit specified in the passport of the printer as resolution (for example 600 dpi), represents an amount of "real" points which the printer can print of 1 inch on an individual segment in length or 1 the Amount of the raster points having on unit of length see (i.e. the raster slot pitch), is called as a lineature and is measured in lines/inches (lpi) or lines/sm (1рс). As for formation of a raster point real points are required some, the bitmap image lineature always appears below printer resolution.

The lineature of a raster for printing of halftone images can be set arbitrarily. Its upper value which cannot be above printer resolution is restricted thus only. About the correct setting of a lineature we also talk in following section.

The relation of resolution capability of the output device to a raster lineature gives the size of the side of a cell of the raster, measured in printer points. The maximum quantity of points of the printer forming a raster point, is equal to a square of the side of a cell. So, for example, if the raster lineature is installed equal 100 lpi, and printer resolution — 600 dpi the side of a cell of a raster will be equal 600/100 = points. Under these conditions the raster point is formed from 6х6 = 36 points of the printer. This value has very much great value for adequate transmission of the photographic image since defines an amount of shades of gray which the raster with the given parameters is capable to transfer. Generally the amount of shades N transferred by a raster, settles up so:

Where dpi — output device resolution, and lpi — a press lineature. Added unit corresponds to white color when the raster cell is not filled absolutely not.

Practice shows that adequacy of transmission of halftones depends on an acoustical range of the image, but even in the opportunity at least 150 shades of gray for this purpose are necessary. The quality printing demands still большего an amount of transmittable halftones. Most often rasterization parameters settle up proceeding from necessity of transmission of all shades which are given by the computer original, that is 256 shades at 8-bit half-tone channels.

For an example we try to calculate a lineature of a raster which is required for transmission of 150 shades of gray on office laser printer having resolution of 600 points/inches. Simple conversion resulted above the formula leads us to following expression:

If to substitute in it digits lpi from our task it turns out:

Big it is value or small? That was with what to compare, say, that the majority of newspaper illustrations is printed by a raster in 75 lpi, journal — 133 lpi, and illustrations in good albums can have a lineature to 170 lpi. Visible quality of illustrations also depends On a raster lineature. The above a lineature, the points forming a raster and the more close a print to the photographic original are less noticeable. As you can see, 50 lines/inches received by us — very small value. If to print with such lineature an illustration in size with a stamp hardly you can define that on it it is represented. After all each line of a raster grid will consist all of fifty points.

At the press on office printer for magnification of a lineature of a raster it is necessary to sacrifice an amount of transmittable shades of gray. In the majority of 600-pointwise printers value of a lineature of a raster by default equally 85 lpi. At such lineature the amount of shades of gray on images does not exceed (600/85 2 + 1 = 50. It three times is less than the minimum defined in 150 shades. From here low printing quality of images on office laser printers.

If you well understood a material of this section now it is obvious to you, why the phototypesetting automatic machines providing a quality printing, are obliged to have such high resolution. It is caused by the mechanism of numeral rasterization and necessity of transmission of a considerable quantity of halftones. It is possible to calculate that for transmission of all 256 shades of gray on a cover of the book printed with a lineature in 150 lpi, are required the resolution of the phototypesetting automatic machine equal

Now we can formulate the general rule, a defining ratio of a lineature and the resolution necessary for transmission of all (256) shades of gray. That the raster grid was capable to transfer such amount of shades, the side of its one cell should make

Thus, for transmission of all tones of the image, press resolution should exceed a lineature in 16 times. The limited number of the shades transferred by a raster, is especially noticeable on graduated fills. Creating a color extension, you assume that on a print the smooth passage from an initial halftone gray (or chromatic color) to the finite will be made. But that passage of halftones is implemented by mechanical means, instead of the smooth passage it turns out graduated. In one case it is almost imperceptible, in other — strongly is evident.

The aliasing level depends on a lineature of a used raster, resolution of the laser printer or the phototypesetting automatic machine, distinction of initial and finite values of an extension and length of an extension. Quite probably such combination of values of these parameters at which the amount of passages of tone will be explicitly insufficient. In this case each of passages it becomes distinct we distinguish from adjacent, there is "banding". At reduction of resolution the raster of the same lineature is capable to transfer less shades of gray.

Fortunately, now the majority of graphic applications the necessary amount of passages, therefore "banding" of extensions automatically calculate became less widespread defect. However the understanding of the mechanism of formation of color extensions helps you to avoid this undesirable effect completely.

It is necessary to mean that "the banding" shown on color extensions in control prints, received on the laser printer, can disappear at an output to the phototypesetting automatic machine with high resolution. On the other hand, if the final dummy layout is issued on the laser printer with resolution 300 or 600 dpi, to forget about possibility of origin of this defect it is impossible.

"Banding" also arises, if an amount of the shades transferred by the output device, is restricted. In this case the extension represents a dial-up of bands, each of which corresponds to the following intermediate gradation. The more the length of an extension, the more widely these bands and that they is more noticeable. Especially explicitly they are visible at the press on the laser printer. Above in the given section we calculated an amount of the shades transferred by a raster with a lineature 85 lpi on the 600-pointwise laser printer. It. Makes 50 shades. If the page contains an extension from black to white in the length 200 mm on one shade transferred by a raster it is necessary 200/50 = 4 mm. Thus, the extension will be printed not in the form of the smooth passage, and in the form of 50 bands in width on 4 mm. At an output of the same page to the phototypesetting automatic machine with resolution 2400 dpi the amount of the shades transferred by a raster 85 lpi, makes (2400/85 2+1 == 798. On one shade in this case it is necessary 200/798 == 0,25 mm. Bands of such thickness of the close shade will not be noticeable any more and the extension makes impression of the smooth.

Generally the amount of gradation in an extension is defined under the formula:

Where dpi — output device resolution; lpi — a press lineature; L, — brightness of finite color of an extension; L, — brightness of initial color of an extension.

The first factor represents the amount of shades transferred by a raster, and the second — a share of the gradation of color used in an extension from their general amount.