Screening Basics Technology Report

Transcription

1 Screening Basics Technology Report If you're an expert in creating halftone screens and printing color separations, you probably don't need this report. This Technology Report provides a basic introduction and brief history of halftone screening: the technology of reproducing photographs in printed materials. You see what appears to be photographs everywhere you look: in magazines, posters, and newspapers. If you look closely at these "photographs," you'll notice that they're actually made up of tiny dots of four colors: cyan, magenta, yellow and black. These colors, called process colors, are used to reproduce full-color images on a printing press. This Technology Report describes how you convert a photograph -- whether a reflective photograph, a 35mm slide, a 4"x5" transparency, or other type of photograph -- into a form that can be used on a printing press to produce a final product such as a magazine or newspaper. The process combines the technologies of "halftone screening" and "color separation." To help you understand how color separations are produced, this Technology Report: Describes the desktop publishing steps a photograph goes through to be printed on a press Explains how imagesetters create halftones Describes how four halftones combine to make a color separation Describes why the process doesn't always work and the technology developed to deal with the problems Desktop Publishing in Color Using a color prepress workstation, you can take an original photograph or other artwork, combine it with text and other art, and produce final film. The final film is used to make printing plates, which in turn are used on the press to print the job. The job could be a magazine, book, brochure, poster, or just about anything else that can be printed. A photograph is a continuous-tone (or contone) image and contains shades of color. For example, if you look at a color photograph of an apple and a tomato, you see the two are different shades of red. Looking closer, you see that neither the apple nor the tomato is uniform in color. The apple may be dark red in one part, but greenish-red where it is not ripe. The tomato might be orange-red in some places, but slightly yellow-orange in others. Similarly, a black-and-white photograph shows many shades of gray. Sample Black-and-White Contone Because this report is being viewed on a monitor, the above example only simulates a photograph. This Technology Report describes how a real photograph would be processed for printing as a halftone image. The following illustration depicts a typical path a piece of artwork such as the one shown above takes in a prepress system to become final film output.

2 From Picture to Press Using scanning software on your color workstation, you can scan an original using a 35mm slide scanner, drum scanner, or flatbed scanner. The scanner uses filters to convert the colors of the contone into pixels of red, green, and blue, the colors used to illuminate a color monitor. The scanned data is stored on the workstation. You can then use image editing software such as Adobe Photoshop to edit and manipulate the image. When the scanned image is ready, you can use a page layout application such as QuarkXPress to combine the art with text and other art to produce the pages for your job. The page layout application, when it prints the page, converts the red, green, and blue color information into the cyan, magenta, yellow, and black information used to create color separation files. The separation files can be downloaded to an imagesetter such as the Panther RIP. The RIP (Raster Image Processor) converts the information from the separation files into raster data (a series of 1s and 0s). The RIP sends the raster data to an image recorder like the PantherPro, which uses the raster data to activate its laser to image the media. The result is four pieces of film containing the cyan, magenta, yellow, and black information that recreates the original image. The films can be used to create a color proof (using the Pressmatch Color Proofing System available from PrePRESS SOLUTIONS, for example). When the customer accepts the proof, the same films used to make the proof are used to make plates for the printing press. Other types of color proofs exist that do not use the actual films. These proofs can be used to preview the job, but they are no guarantee of what the final job will look like.

3 Halftone Screening A black and white photograph may have hundreds of shades of gray. A color photograph may have upwards of three million colors in it, but most printing presses use only these four process inks: Cyan Magenta Yellow Black Black-and-white printing requires only black ink. To get various shades of gray to reproduce an image, we use a process called screening. Screening breaks an image into a series of dots. Varying the dot sizes approximates shades of color. In a black-and-white photograph, for example, a group of large dots placed closely together appears black. A group of smaller dots with larger spaces between them produces a weaker, gray shade. A group of even smaller dots spaced widely apart appears almost white. Gray Shades In traditional graphic arts, screening was generally done using a screen-like pattern etched into a glass plate. A camera operator had several of these plates, each with a different pattern. The image to be reproduced was projected through a chosen screen onto film, and the resulting image looked like the original except that it was broken into a lot of little dots. Imagesetters create an electronic version of the traditional halftone screen. Screening software in the imagesetter applies an electronic dot pattern to the electronic image. Screening is a result of a combination of these items: Screen frequencies Recorder resolution and halftone dots Dot size and shape Gray levels Dot patterns Screen Frequencies Imagesetters create halftone screens using screen frequencies, measured in lines per inch (lpi). A screen frequency can be represented by a grid like the one shown below. Each square in this grid is a halftone cell, capable of holding one halftone dot. Halftone Grid Higher screen frequencies produce finer halftone screens. Lower screen frequencies produce coarser halftone screens.

4 Example High and Low Frequency Halftones The image on the left simulates a 100 lpi screen frequency; the image on the right simulates a 65 lpi screen frequency. Showing line screens on the monitor isn't an exact duplication of printing the image, but you get the idea. Often, frequency is determined by the type of paper used to print the image: newspapers typically use an 85-to-100 lpi screen to print halftones, while magazines using glossy paper need a finer screen and may use 133-to-150 lpi or higher to print halftones. For very high quality promotional materials or fine art reproduction, frequencies of 180-to-200 or more should be used. To create halftone dots, the halftone grid is superimposed on an image. Superimposing the Halftone Grid on the Image Each halftone cell is assigned a different sized dot to represent the image data for the cell. When looked at together, the dots resemble the original image. In the superimposed image, some cells would be white, some black, and the rest various shades of gray depending on the size of the halftone dot. If the image was broken into an 8 lpi screen (shown by the halftone grid), the wine glass would be virtually indecipherable. However, when the image is printed using a 100 lpi screen as shown you can clearly see the wine glass. Recorder Resolution and Halftone Dots The size of halftone cells is determined by the interaction of the screen frequency with the image recorder resolution. The image recorder resolution setting reflects an image recorder's ability to place imagesetter spots close together. An imagesetter spot is created by the image recorder laser beam when it is focused on a point of paper or film. When the paper or film is developed, the area exposed by the laser beam (imagesetter spot) is black. The closer together the image recorder laser can place the spots, the higher the image recorder resolution. The resolution makes an extremely fine grid (named the resolution grid). The imagesetter spots composing the grid are called printer dots, and, in fact, image recorder resolution is measured in dots per inch (dpi). Recorder Grid When the halftone grid is laid over the resolution grid, each halftone cell is "filled" with imagesetter spots. Combinations of these spots make halftone dots.

5 Halftone Grid with Recorder Grid You can calculate the number of imagesetter spots per halftone cell using the following equation: # of imagesetter spots per halftone cell = (dpi / lpi) In the illustration showing the screen frequency superimposed on the resolution grid, the resolution is 16 dpi (there are 16 imagesetter spots per inch). The screen frequency is 8 lpi (there are 8 lines per inch). The number of imagesetter spots per halftone cell is (16/8) = 4. In a real-world application, there would be hundreds of imagesetter spots per halftone cell. For example, if the output device resolution is 2400 dpi and the screen frequency is 100 lpi, there are 576 imagesetter spots in each halftone cell: (2400/100) = 576 Dot Size and Shape Each of the imagesetter spots within a halftone cell can be turned on (producing a color in your final output) or left off (producing white). The combination of imagesetter spots produces a halftone dot of a specific size and shape. Halftone Cells In reality, the imagesetter images at the intersection of the lines on the grid to make a spot. If the illustrations showed that, you could not see where one dot ended and the next began. For illustrative purposes, the graphics in this Technology Report show the imagesetter spot as the block created by the grid. If the halftone dot needs to be bigger, the image recorder turns on more imagesetter spots. If the halftone dot needs to be smaller, the image recorder turns on fewer imagesetter spots. To create different shapes, the image recorder turns the imagesetter spots on in different sequences. Each sequence is determined by a mathematical equation called a spot function. A separate spot function exists for each dot shape. Common shapes include round, diamond, square, and elliptical.

6 Spot function names can be confusing. For example, there are two types of square spot functions. In one of these, the halftone dots are shaped like squares all the way through the tint scale. In the other, the halftone dots start out shaped like circles, grow to square shapes in the midtones, and then become circular again. In addition, vendors use different spot functions to create their halftone dots. Not everyone's round or square dot is going to grow in exactly the same way. See the Imagesetter Screening Technology Report for a description of the several shapes of spot functions. Gray Levels PostScript generally requires at least 256 levels of gray to properly reproduce an image. Because of this, imagesetter manufacturers have adopted 256 gray levels as a de facto standard. The more imagesetter spots the halftone cells contain, the more shades of gray (also called gray levels) they can reproduce, and the more accurately the output represents the colors in the original picture. To calculate the number of gray levels, use the same equation you used to determine the number of imagesetter spots and add one (for all white) to the result. # gray levels = (dpi / lpi) + 1 In this illustration, each halftone cell contains four imagesetter spots: Grid Frequency The number of gray levels equals: (16 / 8) + 1 = 5. Each halftone cell could be: 0% black (no spots are on, white) 25% black (one spot out of four is on) 50% black (half the spots are on) 75% black (three out of four of the spots are on) 100% black (all imagesetter spots are on) Gray Levels in a Halftone Dot As with everything in the prepress industry, there is a trade-off when you deal with screen frequencies and gray levels. Higher screen frequencies, because they contain more halftone cells, produce finer screens that can capture more detail from the original photo. However, because resolution remains constant, the more halftone cells you have, the fewer imagesetter spots they contain. As the number of imagesetter spots decreases, so does the number of gray levels each halftone cell can reproduce.

7 Dot Patterns Breaking the image into a series of dots solves the problem of how to reproduce tones, but creates a problem of its own. The eye detects patterns quickly. For example, this grid forms a noticeable pattern of alternating black and white "lines" running at 90 angles. When you print your output, you do not want the dot pattern to detract from the image it creates. 90 Screen Angle One way to prevent the pattern from becoming distracting is to rotate the grid. The degree of rotation the eye notices least is 45 as shown in this illustration. 45 Screen Angle The dot pattern still exists, but it is much less noticeable. When a simple black-and-white halftone is created, the halftone screen is rotated 45. The printed output is an image your eyes perceive as a black-and-white photograph, not as a series of dots. Color Separations In the color separation process, because you deal with four screens, dot patterns become a bigger problem. Patterns that are created by the combination of two or more screen grids are called moiré. This illustration shows a moiré pattern created by the combination of the magenta and black plates. Example of Moiré There are acceptable and unacceptable moiré patterns. The above illustration shows an unacceptable pattern. The only acceptable pattern is the rosette. Rosettes are pleasing to the eye and when generated properly generally do not detract from the images they recreate. To form a rosette, the four halftone screens (cyan, magenta, yellow, and black) must be placed at different angles. Otherwise, when the films are superimposed to produce a final image, their dot patterns completely overlap. The resulting printout is a muddy mess that does not resemble the original image.

8 In traditional screening, the conventional angles are: Black: 45 Magenta: 75 Cyan: 15 or 105 Yellow: 0 or 90 Traditional Screen Angles This angle set is called the conventional angle set. The black separation is placed at the angle least noticeable to the eye. This angle set is not "universally correct." If your original image does not contain a lot of black, you may decide to place the separation for a more prevalent color at the 45 angle. Also, black and magenta often are switched if their traditional combination creates moiré in skin tones. Why So Much Fuss About Screening? So far, the rules for producing high-quality output seem clear: Use halftone screens to break images into a series of dots in order to replicate color tones. To avoid moiré, rotate the halftone screens to different angles. As long as we use the conventional angles we should have trouble-free output, right? Unfortunately, electronic screening can not always achieve the conventional angles. Many screening methods, including ESCOR II screening from PrePRESS, have been developed to help reproduce these angles as closely as possible. The trouble stems from the interaction of the screen frequency and resolution grids in the formation of halftone cells. You specify the screen frequency and angle you want when you generate the color separation files using a software application such as Adobe Photoshop. Each separation file contains data that tells the image recorder when to activate and deactivate its laser beam -- creating the imagesetter spots that make up the final image. To continue with our previous analogy, think of each halftone screen as a grid that is superimposed on the image recorder resolution grid.

9 Halftone Dots and the Imagesetter Grid The halftone screen represents the yellow color separation, usually printed at a 90 angle to the resolution grid and so is easy to align with that grid. All the edges of the halftone cells align with the columns created by the resolution grid. In contrast, the other halftone screens (cyan, magenta, and black) are rotated at angles that do not overlap the resolution grid so neatly. The next illustration shows the Magenta screen, usually rotated 75 to the resolution grid. Not all of the halftone dots fall on the intersection of the squares (the areas where the imagesetter can image spots). Magenta Screen Angle The black halftone screen (45 ) is not hard to image. The magenta and cyan screens (75 and 15 respectively) are. Remember drawing angles on graph paper in math class? The conventional angle for the black halftone screen is 45 so we'll use that angle as an example. You align your protractor on the graph paper and mark the origin and the 45 angle. You then use a ruler to connect the two dots.

10 Drawing an Angle The resolution grid resembles a piece of graph paper. The rows and columns are created by fixed imagesetter spot locations. An imagesetter can only place spots where the grid intersects. The large dots in the illustration show where the imagesetter could place spots. The points on the 45 screen fall neatly along the imagesetter spot locations on the resolution grid. The imagesetter can activate the appropriate imagesetter spots to image the halftone dots in the 45 screen. Black Screen Angle By contrast, the 15 or 75 screens are not easy to reproduce. Going back to the graph paper analogy, draw a 15 line on the graph. Cyan Screen Angle The large dots shown would be required for the imagesetter to image at that angle. However, the imagesetter can only image spots at the intersections of the grid. The imagesetter cannot activate the laser at the locations to reproduce the angle precisely. The magenta screen (75 angle) presents the same problem. The imagesetter cannot activate the correct spots to produce the angle. Because the angle cannot be imaged exactly, the placement of the imagesetter spots that make up the final image is not precisely correct. On a single screen, this may not be a problem. However, when you combine four color separations, even minute discrepancies can lead to moiré and color shift.