Prepress Considerations
There are a number of technical aspects within the Prepress area which have dramatically changed the way the graphic arts community relates to techniques and concepts in the creation of PostScript® color pages. The introduction of page layout software and desktop publishing has made printed output accessible to far more people, but it has also forced the user to be aware of and responsible for correcting problems in the following areas:
Types of Proofing
Halftoning and Banding
Trapping and Overprinting
Dot Gain and Dot Density
Stripping/Imposition
Screening Technologies
Types of Proofing
Proofing is the process of producing an accurate sample or example of how the final output will look. Usually a trial version of a graphic is printed to see how it will look when output in its final form. Laser printers are commonly used to proof monochrome artwork, whereas color artwork is often proofed on thermal color printers. High-quality proofing systems such as Chromalin (DuPont) or Matchprint (3M) can be used to proof color separations. Many of the considerations and techniques for producing good proofs are described at these links.
Composite
In commercial printing, a preliminary output of a design from a printer that includes all image, line art and text elements is a composite. Color composites are often printed on color PostScript® printers to check artwork before it is color separated for four-color process printing. It is also called a comprehensive proof.
Off-Press Proofing
The process of confirming that all elements of an image are present, elements are fitted together in the correct page locations and colors are correct is off-press proofing. In short, you are verifying how will the image look when it is printed. These proofs are made from the imagesetter film by stacking the film in exact registration over a white backing plate to simulate the printed image.
Two types of off-press proofing are in common use, the Integral Proof and the Overlay Proof.
- The Integral Proof is single page of special-base paper that is put through a bath so that it does not feel like paper. It offers a very good quality representation of the image.
- The Overlay Proof uses four separate pieces of acetate and does not result in as good a representation of the image. Trapped air causes internal light reflection that produces gray in white and light-tone areas.
The Press Proof is usually produced on a special press with the client present for a portion of the run. This is not often done because of the expense involved.
A Contact Proof can take the form of any of the proof styles and is used as a method of agreement with the customer as to how the final image will look.
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Halftoning and Banding
Halftones
Halftoning is a method of creating the illusion of shades of gray by using only black ink on a white background. In the past, laser printers and offset printing presses could print only one solid color, as solid spots of ink were placed on the paper. This process was useful when newsletters, newspapers and books were the only things being printed. Its greatest limitation was the inability to reproduce photographic images accurately, and this limitation created the need for the halftone process.
Originally, a halftone screen was an opaque screen with thousands of tiny holes. An image with shading was photographed through this screen using special photographic paper or film. The resulting image would consist entirely of dots. This image was then be used to create printing plates. Now, we create halftoned images with software to simulate the effect of a halftone screen.
Commercial printing presses are unable to produce true shading, but they can create the illusion of shading by printing images made up of tiny dots. The size of the dots determines the different levels of shading (i.e., the larger the dots, the darker the shade). A halftone screen is necessary to convert images with true shading into images made up of tiny dots.
The output device groups together a small grid of device pixels to create a halftone dot. The size of the pixels depends on the resolution currently being used for printing. The size of the grid depends on the lines per inch (lpi) generated by the output device. The pixels are colored or uncolored in a pattern within the grid to create halftone dots. The halftone dots emulate shades of gray throughout the image. The human eye registers a shade of gray when, in fact, it is looking at a fine pattern of black-and-white dots.
To demonstrate this, create an object defined as 50% gray. When printed, half of the dots within one cell of this grid will be turned on, black, and the other half of the dots will remain off, white. The pattern is then replicated throughout the object to make it appear to be gray in color.
 This illustration is a typical halftone dot created by a printer. The small grid represents the device pixels, and the large dot created by filling the grid is the halftone dot. A collection of grids is assembled into a group that creates the image. This is called a halftone screen.
The image will have either the smooth appearance of a continuous tone image or look like a newapaper photograph depending on four factors:
Dot Shape – Although halftone dots can be any shape, the industry has standardized on an elliptical shape because it is less likely to “clog” or become muddy in the midtones. The elliptical shape also means that each halftone dot can be placed closer to the adjacent dot before it contacts its neighbor.
Dot Density – Density is a measure of the proportion of dots in a given area of the image.
Screen Angle – Because each halftone screen consists of a regular pattern of shapes, it creates a pattern on the printed image. When the separations are combined, the patterns created by each separate halftone screen interact. This interaction can create an undesirable effect called a moiré pattern that can be eliminated by changing the screen angle of each color separation. If you were using an actual screen and a camera, you would rotate the screen by hand 15° for each separation. However, because you are using software to create halftone screens, you must change certain print options to change the screen angle. When you print color separations, the screen angles are set automatically. If you make incorrect changes to these settings your image may not print properly.
Screen Frequency – The halftone-screen frequency determines the number of dots used to create the image. The screen frequency is measured in lpi. This measurement refers to the number of rows of dots per inch. When you choose a screen frequency, remember that the higher the screen frequency, the sharper the image. However, there are limits to screen frequency that are determined by the type of printing press on which you are printing and the type of paper you are using. In general, a screen frequency of 85 lpi works on newsprint, and a frequency of 100 lpi works on bond and glossy paper. If possible, consult your service bureau or printing shop to determine the screen frequency you should use.
Banding
Banding in halftones is the appearance of distinct steps across a fountain fill rather than a smooth transition of shades. It is the result of abrupt changes in shades or color in objects with a graduated or “stepped” transition from one color to another.
There are several factors that affect the amount of banding in an object, and you can work with each as follows:
Object Size – Decrease the width of an filled object so that fewer steps are required across the color gradation.
Number Of Fountain Step – Increase the number of fountain steps or bands in the Fountain Fill dialog box.
Percentage Of Gray Change – Decrease the degree of change from one color to another in the fountain fill.
Printing Resolution – Increase the resolution of the print job.
Screen Frequency – Lower the screen frequency as set in the Printer Driver dialog box or the Options\Options\Advanced dialog box of the Corel Print Engine.
Example: A black-to-white color change is from 100% to 0% black. You can determine the optimal number of steps required to make this color change in a fountain fill by dividing the printer resolution (dpi) by the screen frequency (lpi) and multiplying by the desired percentage change in color to determine the number of steps required. For example: (300 dpi/60 lpi = 5) x 1.00 = 5.
The value for Screen Frequency depends on the PostScript® output device, the color being used and the print job. This value should be assigned by your service bureau.
PostScript® Printing
With current PostScript technology, the output device is limited to a total of 256 shades total on a single color or a grid that is based upon 16 X 16 halftone cells = 256 shades of gray. By dividing the desired resolution of your output by 16, you can determine the maximum screen frequency that the device can use to achieve 256 shades of gray. The printer will accept other values, but the grid may be limited. Using a lower number may cause the image to be more coarse than desired. Similarly, using a high value may cause banding to be more prominent in the document.
Example: A 300 dpi laser printer prints at 60 lpi. Using the above formula, 300 dpi/16 = 18.75 lpi. Although grouping 16 device pixels together at 300 dpi will force the printer to produce a coarse image, this value of 18.75 will result in a smooth gradation between colors within the size limitations. Using 60 will produce a reasonable image but the printer will be limited to as many shades of gray as it can actually print. Whether this will be noticeable or not depends on the fountain fills in the document, how they were created and the quality of output desired.
Note: The current PostScript technology limits the maximum number of bands to 256.
By using the method below, you can determine how many shades of gray or bands in a fountain fill are available to the printer based on the current dpi and lpi. It is then possible to determine if banding will be noticeable in certain objects. After the file has been sent to the printer, the PostScript interpreter will make the changes to the values that you have specified.
The human eye can see objects as small as 0.03 inches (1/32") in size, bands in the fountain fill as small or smaller than this will usually appear smooth. To determine the size of a band, divide the number of bands by the length of the object containing the fountain fill.
If the size of the band is smaller than 0.03 inches, banding will not be noticeable.
Number of Bands = [(dpi/lpi)² x (% gray change)]/100 where dpi = resolution and lpi = screen frequency.
The Adobe® PostScript® Language Reference Manual* (the “Red Book”) reads: “…the best choice of screen parameters is often dependent on specific physical properties of the output device itself (e.g., pixel shape, overlap between pixels, and effects of electronic or mechanical noise). …The setscreen operator may make slight adjustments to the requested frequency and angle so as to ensure that the patterns of enclosed pixels remain constant as the screen cells are replicated over the entire page.”
If you are creating black-and-white fountain fills only, this seems fairly straightforward; however, when you are using color, it is necessary to calculate the gray equivalent. The following formula is from the Adobe PostScript Language Reference Manual:
% gray = 0.30 x (% red) + 0.59 x (% green) + 0.11 x (% blue)
CorelDRAW® can display the red, green, blue (RGB) equivalents of the colors you are using if you follow these steps:
- Use the Uniform Fill tool to fill an object with a color.
- Select the Custom Fill tool from the flyout menu.
- In the Cyan, Magenta, Yellow, Black (CMYK) color model, the color is defined as a combination of cyan, magenta, yellow and black.
- By changing to the RGB-color model, you will see that the color is now defined as a combination of red, green and blue.
- Use the formula below to calculate the percentage of gray. We are using the color Orange in this example:
% gray = 0.30 x (100% R) + 0.59 x (40% G) + 0.11 x (0% B)
This works out to be 53.6% gray. Use the same formula to calculate the percentage of gray for the destination color.
You can use these two values to determine the percentage of gray change between the two colors, and then apply the original formula to determine the number of stripes or bands for your fountain fill.
Note: This information is intended as general information only and should not be considered a definitive solution to any specific problem.
* PostScript Language Reference Manual (c) 1985 Adobe Systems Incorporated, Addison-Wesley Publishing Company, Inc.
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Trapping and Overprinting
In commercial printing, the process of adding a slight overlap between adjacent areas of color to avoid gaps caused by registration errors is called trapping. Color trapping is necessary to compensate for poor color registration. Poor color registration occurs when the printing plates used to print each color are not aligned perfectly. Poor registration causes unintentional white slivers to appear between adjoining colors.
Trapping is accomplished by intentionally overlapping colors so that minor problems with alignment are not noticed. Many service bureaus prefer to create color trapping themselves by using a specialized trapping program. Consult your service bureau about trapping if you are unfamiliar with the process.
Color trapping in Corel applications is achieved by overprinting. Normally, portions of an object that are obscured by another object are not printed. However, if the top object is set to overprint, the obscured portions of any underlying objects print anyway, causing an overlap. This makes white gaps between different colors impossible. Overprinting works best when the top color is much darker than the underlying color, otherwise an undesirable third color might result (e.g., red over yellow might result in an orange object).
Depending on the color-trapping options you choose, overprinting can affect an object’s outline or its fill. This means that if an object with a red outline is set to overprint its outline only, then any portions of another object that are obscured by the first object’s outline are printed. This overlap creates a color trap.
Misregistration
 In this example, the misregistration has been exaggerated to demonstrate how the image would appear when the object is printed, if the plates were not registered correctly.
Auto-spreading creates color trapping by assigning an outline to the object that is the same color as its fill and having the outline overprint underlying objects. (Color trapping will be created for all objects in your file that meet three conditions: they don’t already have an outline, they are filled with a uniform fill and they haven’t already been designated to overprint.)
The amount of spread assigned to an object depends on the maximum trap value and the object’s color. The lighter the color, the larger the percentage of the maximum trap value. The darker the color, the smaller the percentage of the maximum trap value.
A trap either spreads (enlarges) the lighter color into the darker color or chokes (reduces) the lighter color into the darker color. A choke trap should be used if the background color is lighter than the object it surrounds. A spread trap should be used if the background color is darker than the object it surrounds.
- Choke – is a type of trap created by extending the background object into the foreground object. Corel PHOTO-PAINT® has an Overprint feature that allows you to create chokes.
- Spreads – is a type of trap created by extending the foreground object into the background object. Corel applications have an Overprint feature that allows you to create spreads and an Autotrapping feature that creates them automatically.
- Overprint – is printing over an area that has already been printed. Overprinting is used to create traps in color-separated artwork. You can also overprint selected spot colors to produce certain visual effects.
Trapping Methods
To Trap by Always Overprinting Black:
- Click File, Print.
- Click Options.
- Click the Separations tab.
- Enable Print Separations.
- Enable Always Overprint Black.
Note: Any object that contains 95% or more black will overprint any underlying objects. This is a useful option for artwork that contains a lot of black text, but it should be used with caution on artwork that has a high graphics content.
The effective use of color trapping requires a solid understanding of the many variables involved in color printing. To ensure satisfactory results, it is important that you seek advice from your service bureau.
Tip: If your service bureau recommends a black threshold value other than 95%, click the Options tab in the CorelDRAW® print engine, choose Overprint Black Threshold from the Special Settings Option list and change the setting as required.
To Trap by Overprinting Selected Color Separations:
- Click File, Print.
- Click Options.
- Click the Separations tab.
- Enable Print Separations.
- Enable Use Advanced Settings.
- Click Advanced.
- Click the color separation to overprint.
- Enable Overprint color.
- Indicate whether you want graphics, text or both to overprint.
To Trap by Auto-Spreading:
- Click File, Print.
- Click Options.
- Click the Separations tab.
- Enable Print Separations.
- Enable Auto-Spreading.
- (Optional) Enter a maximum trap value (if you are unsure, 0.25 is a good default value to use).
- (Optional) Enter a value for Text Above (i.e., the minimum size to which auto-spreading is applied).
If you apply auto-spreading to small text, it can become illegible.
Undercolor Removal (UCR)
Undercolor Removal is a technique in color printing for reducing the amount of cyan, magenta and yellow ink in shadows and neutral areas of an image and replacing them with an appropriate amount of black. This reduces the total area of ink coverage (TAC), which is defined as the sum of dot percentages of all four inks (CMYK) that contribute to a printed color. The net effect of UCR is that less of the more expensive colored ink is required to reproduce the desired colors in the image.
Gray Component Replacement (GCR)
Another approach to the color replacement process is called Gray Component Replacement, which also substitutes CMY ink with black but over a greater color range than UCR.
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Dot Gain and Density
Dot Gain
When an image is printed on a printing press, the enlargement of the dots that make up a bitmap can cause the image to appear darker than intended. Dot enlargement produces the same effect as if the dot size was enlarged or more dots were placed on the halftone screen—it increases the tint value in the image. This can happen both at the imagesetter and during printing. It is especially prevalent when you print to paper with a high absorbency such as newsprint.
Dot gain can also occur within the imagesetter during film exposure. Reflection may take place within the film itself and cause a halo effect around the dot. Film is now available that contains an antihalation backing to prevent this from occurring.
Dot gain is a normal occurrance during the printing process, and it is accepted within measured tolerances of Dmax and Dmin. This value expresses the amount of light absorbed by the paper or imagesetter film. This is why a densitometer scale is printed on each page of a color-separated image to help you gauge the accuracy, quality and consistency of the output. You can print these scales by choosing an option in the Printer’s Marks and Prepress Settings dialog box and use them to measure the dot gain compensation being sent to the Raster Image Processor (RIP) and to maintain accuracy at the imagesetter. This is acheived by calibrating the imagesetter.
Dot Shapes
A halftone screen is a pattern of shapes that is used to simulate shades of color (i.e., darker to lighter) while using a single ink. Some of the PostScript® shapes available in CorelDRAW® are Dot, Line, Diamond, Elliptical and Euclidean. The Elliptical halftone dot is the most widely used dot pattern because, as these halftone dots increase and decrease in size and density, they are less likely to “bunch up” or fill in with ink, This problem is known as “optical jump” and appears in areas of the image where tint is increasing.
This problem has largely been overcome with the introduction of Stochastic or Frequency Modulation screening. Conventional halftone screening places dots of varying size at defined intervals on the screen. Stochastic screening places small halftone dots of uniform size on the screen at densities required to produce the image.
Adjusting Dot-Area Density
Dot-area density can be thought of as the proportion of an image that contains halftone dots. It is directly related to the opacity of film and is used to monitor dot gain. Print a test page, then use the densitometer scale to measure the dot-area density to determine the total dot gain. This test can be performed only on a properly calibrated imagesetter.
Densitometers
A densitometer is an instrument that measures density by reading the amount of light that passes through film or is reflected from the image. Image density can be measured with a transmission or a reflection densitometer. Transmission densitometers measure the amount of light that passes through film. Reflection densitometers measure the amount of light reflected from the surface of the paper.
Transmission Densitometer
A transmission densitometer is used when you are working with film. It measures the difference between a known intensity of light and the amount of light it receives through the film. An area of halftone screen used to produce a solid dark color will produce a density reading of 3.6 – 4.0. This is the Dmax value. Conversely, a lightly tinted area might render a Dmin value of 0.05.
Reflection Densitometer
A reflection densitometer is used to measure density on printed paper. It produces a measurement of how much light is absorbed by the image on the paper compared with how much is reflected from the surface. A solid dark area should absorb almost all of the light it receives (up to 99%) and give a reading of 2.0.
Professionals measure density throughout the prepress and printing process to verify that dot densities remain consistent through to final output. Densitometers are calibrated frequently and are used 2 – 3 times per day during production.
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Stripping/Imposition
Stripping and imposition have traditionally been done manually by film strippers. In both cases, the terms refer to document assembly using a photomechanical process to construct a press layout, or signature. A signature should always contain registration marks, color and density bars, crop marks, center and fold marks and a print-job information sheet.
Stripping is the task of piecing film together in strips on a light table to lay out the finished page. Imposition is the assembly of pages on the press sheet so that the pages are laid out in proper sequence after they have been folded. This is called a flat. Imposition must be considered at the document-planning stage dependent upon factors, such as the characteristics of the output device, and is directly related to printing plates.
With the advent of desktop publishing, electronic imposition has become more common and takes place in the drawing software itself or within a page-layout application such as Corel VENTURA®. However, it still depends on the binding method to be used, the paper size, the number of pages in the document to determine shingling requirements and the limitations of the devices that are being used to output the job (e.g., the output-size limitation of the imagesetter).
Manual Layout or Imposition
The most common types of page-layout arrangement are work-and-turn and work-and-tumble. With both of these methods, the front and the back of the sheet are printed on the same form. In other words, there are two finished layouts on the same sheet. The difference is in how the sheet is turned to print the second side. In a sheetwise layout, each side of the sheet has a different page on it. This method is used when there are enough pages to fill both sides of a sheet. If you are going to assemble the document for final output, it is critical that you consult your service bureau to find out its imposition requirements.
Automated Stripping and Layout Systems
Manual stripping and page layout takes time and labor, and this makes it expensive. Computerized imposition software uses computer-assisted design (CAD) technology to control the movement of a camera and a projection system to compose pages and complete signatures. Some of these systems connect directly to electronic prepress systems. They are extremely accurate and especially effective for CMYK images.
Arranging Images on the Printed Page
In CorelDRAW®, you can set up your print job so that several pages of your document print on a single sheet of paper. This feature might be useful if you want to create a catalog of the images in a file or print relatively small pages on large sheets of paper. Depending on the settings chosen in the Page Setup dialog box and the size of the paper on which you are printing, you have different options when you place several pages on a single sheet of paper. For example, if the paper on which you are printing is much larger than the page size in the Page Setup dialog box, then you may be able to fit several pages on a sheet of paper. If the paper isn’t large enough to fit several pages but you still want more than one page on each sheet of paper, you can shrink the pages to fit on the paper.
Printing Several Pages on a Single Sheet of Paper
- Click File, Print Preview.
- Click Settings, Edit Positioning Style.
- Type the number of rows and columns you want printed on each sheet of paper in the Rows and Columns boxes.
- If you want to change the margins, do one of the following:
- Disable the Auto Margins check box and type the size of the margins in the Left, Right, Top and Bottom boxes. You can change the unit of measure in the Units box on the right.
- Enable the Auto Margins check box.
- If you want the left and right margins to be equal as well as the top and bottom margins, enable the Equal Margins check box.
- If you want to adjust the gutters (space between rows and columns), do one of the following:
- Type the size of the gutters in the Horizontal and Vertical boxes. You can change the unit of measure in the Units box on the right.
- Enable the Auto-Spacing check box.
- Enable the Clone Frame check box if you want all the frames on each sheet of paper to contain the same page. For example, if there are nine frames on each printed sheet of paper, then page one appears nine times on the first sheet of paper, page two appears nine times on the second sheet and so on. In this way you can print multiple copies of one page on a single sheet.
- Enable the Maintain Document Page Size check box if you want each frame to be the same size as the page size specified in the document. For example, if you create a document on an 8.5 X 11 inch page, the frames are constrained to that size. Therefore, if you print on an 11 X 17 inch sheet of paper and specify 2 rows by 2 columns, some of the frames will not fit on the page.
Collating
When you print in CorelDRAW, you can select the collate feature. Collating is useful when you are printing multipage documents. If you enable the Collate check box, CorelDRAW prints a complete copy of each document before it prints the next copy. If collate is disabled, CorelDRAW prints all the copies of the first page before it starts printing copies of the second page, and so on. Corel PHOTO-PAINT® and CorelDREAM 3D include an option to print multiple documents.
Layout Styles
In CorelDRAW, layout styles determine the way the pages of your print job are placed on the printed page. For example, if you are printing a brochure, two pages from your document may appear on a single printed page. The type of document you are printing (e.g., greeting cards or a book) determines the layout style you choose. There are preset layout styles available in the Print Options dialog box or you can create your own custom styles.
If you are printing a specialized type of document, you probably selected a layout style in the Page Setup dialog box when you created your document. If this is the case, the layout style you chose is automatically selected when you open the Print Options dialog box. If you change the layout style in the Print Options dialog box, your work may not print correctly. By default, CorelDRAW uses the Full Page layout style.
Binding Methods
Saddle-Stitching
Saddle-stitching is a binding method in which pages are stapled along a center fold. This is an inexpensive binding that is used for magazines and small books. It has the added advantage of allowing pages to lie flat when the publication is opened. It is not practical for large publications.
Perfect Binding
Perfect binding is used to produce books with a spine. The pages are collated into proper sequence then fastened along the spine with glue. The pages are also stitched if the publication is large enough to require additional reinforcement.
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Screening Technologies
Images can either be created within the computer with a Corel application or may be nondigital and originate from a source external to the computer. These images are translated into digital form by scanning the original using either a drum scanner or flatbed scanner.
The image is placed onto the page layout in CorelDRAW® or Corel VENTURA® and combined with other objects to produce the final document. When the application sends the print job to the printer, it converts the color data in the image into the cyan, magenta, yellow and black (CMYK) color-separation files. You can see an example of this in the sample image of the dog (Conventional Screen Angles).
The separation files are sent to a raster image processor (RIP) that converts the information in the separated files into binary data and sends the data to an imagesetter. The imagesetter contains a laser that combines the laser spots into cells which form halftone dots on the film and create the image. The imagesetter will output four pieces of film, each containing one of the color separations.
Halftoning – A Definition
A halftone screen is a pattern of shapes that is used to simulate various intensities in the image (i.e., darker to lighter) with a single ink. Halftone screening is the process of reproducing a continuous-tone image with an imagesetter by using dots of various sizes and density to emulate shades of color. On laser printers that cannot print different sizes of dots, the halftone is produced by printing different numbers of dots in a given area.
The halftone-screen size corresponds to the size of the image, and the imagesetter places each screen of halftone cells in its location on the image by using a recorder grid. You could compare the recorder grid to a series of addresses that the imagesetter uses to determine where and how to place the halftone screens when it creates the image.
Halftone type refers to the type of dot that is being used to create the halftone. Typically, a halftone screen consists of rows of evenly spaced, round or diamond-shaped dots. However, it is possible to use halftone screens that have dots that are shaped differently. In fact, halftone screens can even use straight lines instead of dots to create an image. You can experiment with different halftone types to create interesting effects.
If you are printing a black-and-white image, the printer will recreate it with black ink only. Screening adjusts the concentration of black dots on the page to reproduce the shades of gray between black and white.
Halftone Dots
The halftone dots are grouped into grids of printer spots to form cells. If the screen frequency is set so that 100 spots (or points on the paper that the imagesetter can define) can be inked or not, then many shades of color can be produced within the cell. When the screen frequency is set to only 10 printer spots, fewer colors can be produced per cell. The capacity of an imagesetter to place printer dots on an image is defined as its resolution and is measured in dots per inch (dpi).
What is Screening?
Originally, a halftone screen was an opaque screen with thousands of tiny holes. An image with shading was photographed through this screen using special photographic paper or film. The resulting image consisted entirely of dots. This image could then be used to create printing plates. Now, halftoned images are created with software to simulate the effect of a halftone screen. The halftone, used in conjunction with the color-separation process, is necessary to convert photographic images into material printed using process colors.
To the eye, a photographic image appears as a continuous-tone image because the chemicals used to produce the image blend smoothly at different intensities to produce shades of color. The printer creates the illusion of all those different colors and shades of color by overlaying patterns of tiny dots in only four colors—cyan, magenta, yellow and black. These colors, called process colors, are used to reproduce full-color images on a printing press. Although your monitor may be capable of producing millions of colors, a printer is capable of reproducing only a few thousand colors.
The screening process combines the technologies of halftone screening and color separation. The screening patterns form the shape and density of the dot patterns required for each of the four-color separations. When the four colors are combined in these patterns, the illusion of all the other colors and shades of color in the image are created.The screen technology should be set to match the type of imagesetter your service bureau will be using. Talk to your service bureau to determine the correct setting. If you are not using an imagesetter or you are unable to speak to your service bureau, use the standard default settings.
Screen Frequency
The halftone-screen frequency determines the number of dots used to create the image. The screen frequency is measured in lines per inch (lpi) or lines per centimeter (lpc). This measurement refers to the number of vertical rows of dots per inch (dpi) or centimeter.
When you choose a screen frequency remember that the higher the screen frequency, the sharper the image. However, there are limits to screen frequency that are determined by the type of printing press on which you are printing and the type of paper you are using. In general, a screen frequency of 85 lpi works on newsprint, and a frequency of 100 lpi or higher works on bond and glossy paper. If possible, consult your service bureau or printing shop to determine the screen frequency you should use.
Imagesetters create screens using differing screen frequency values. When the screen frequency is high, the higher resolution screen produces an image with more detail. A lower screen frequency (the effect is exaggerated in the apple on the right) produces a screen with less detail and the image will look grainy.
Moiré Effect
Because each halftone screen consists of a regular pattern of shapes, it creates a pattern on the printed image. When the separations are combined, the patterns created by each separate halftone screen interact. This interaction can create an undesirable effect called a moiré pattern. These patterns appear when an image is printed from color separations with incorrect halftone screen angles. Therefore, the four screens used in the color-separation process must be properly aligned to avoid undesirable geometric patterns in an image.
Moiré patterns can be eliminated by changing the screen angle of each color separation. If you were using an actual screen and a camera, you would rotate the screen 15° by hand for each separation by hand. However, because you are using software to create halftone screens, you must change certain print options to change the screen angle. When you print color separations, the screen angles are set automatically. If you change these settings incorrectly, your image may not print properly.
 
Here is an example of moireé displaying a rosette pattern in the screening. Consult your service bureau before you change any of these settings.
Conventional Screen Angles
The image is converted to four grayscale separations based on four process inks used to print the image. Conventional screen angles for a CMYK image are demonstrated in the separations below.
Cyan: 15° Magenta: 75° Yellow: 0° Black: 45°
Early printing with CMY devices achieved best results when the screens were offset from each other at 45°, with the yellow screen at 0°. With the introduction of CMYK four-color printing, the screens could no longer be rotated in 30° increments because four rotations of 30° exceeded a total of 90° and a 0° screen would give the same results as the 90° screen. A compromise was achieved by offsetting three of the screens by 30° and the fourth by 15°.
Note: Most manufacturers use proprietary algorithms to generate screen frequencies and angles that may not conform exactly to traditional screening.
Screening Considerations
Screen angles are offset so that the moiré pattern can be avoided; however, when you combine the four-color separations, the slightest misalignment of the separations can lead to moiré. This can also result in color shifting because misalignment of the halftone dots can result in a change of dot density. To correct this problem, the screen frequency or angle must be modified. Some experienced operators will modify default screen angles, but this is probably something that should not be attempted from the desktop.
The screens may be rotated or angled, but the resolution grid of the imagesetter itself always remains stationary at 0° X – Y. This creates a problem when the screen-rotation angle does not allow each halftone dot to correspond to an absolute square on the resolution grid. Some screening compromises have been developed to help overcome the problem.
Proprietary screening algorithms supported by Corel include Agfa Balanced Screening®, Linotronic RT® and HQS screening.
Agfa Balanced Screening®;
The Agfa Balanced Screening supercell algorithm is PostScript® Level 2 compliant and uses conventional screen angles with precalculated screen descriptions for every combination of resolution and frequency. The precalculated screen descriptions are installed on the RIP itself and do not need to be continously recalculated, offering processing speed gains and eliminating the moiré problems associated with calculating the description of each screen.
Rational Tangent Screening
Rational tangent screening is screening technology introduced with the PostScript® page description language in 1985 and patented by Linotype-Hell AG. It places a corner of each halftone cell at the corner of a spot on the recorder grid, which works with precision for the 45° and 90° (or 0°) angles. The dots do not always align properly with the grid at irrational screen angles such as 15° or 75°, and this results in a distortion of the shape of the halftone cell. A compromise is achieved by rounding the irrational screen angle up or down to the nearest rational angle.
Rational tangent screening is best suited to black-and-white reproduction.
Irrational Tangent Screening
When screens are placed at irrational angles, each halftone dot must be calculated and placed individually. This creates an unreasonably heavy load on processors. In many cases, all the dots are generated at the same size to reduce the processing load.
The calculations to find the most precise combination of frequency and angle placement can be infinite, so the irrational tangent screening (IS) algorithm is designed to round off its calculation at a defined limit.
HQS (SuperCell) Screening
SuperCell screening is a form of rational tangent screening that uses thousands of individual halftone cells to create a matrix of halftone cells.The large cells (called a SuperCell) are placed on the recorder grid at the appropriate screen angle. Because of the increased size of a SuperCell, there are more locations where it is geometrically possible to place the corners of the SuperCell onto the recorder grid at screen angles of 15° or 75° and still maintain a rational screen angle.
To minimize errors that may arise from nonintersection areas, the corners of these cells intersect the resolution grid at address points (locations where the imagesetter can place spots).
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