Previewing in CMYK


The world of graphic arts reproduction is changing, and many printing firms are now using a fully color-managed work flow in preparation for printing. Those who do so want you to provide your images to them in RGB color , with embedded working space profiles.

Printers request these files because there is no generic CMYK separation that is correct for all types of paper and ink sets. The separation made for sheet-fed offset on uncoated paper is drastically different from the separation made for web-fed glossy paper. Printers want control over this conversion.

When an image is destined for the printed page, it is necessary to preview the image before sending the file to the printer. You also need the ability to preview an image in CMYK without making the conversion to CMYK. Photoshop provides the Proof Colors control, which enables the on-screen preview to simulate a variety of reproduction processes without converting the file to the final color space.

To prepare for and carry out an on-screen proof, first tell Photoshop what kind of proof you want to see. Choose View Proof Setup to select the type of proof to preview (see Figures 15.9 and 15.10). The first option is for Custom setups, which enable essentially any profile to be applied for the proof.

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Figure 15.9: The Proof Setup menu enables you to preview any profile and rendering intent as the proof destination.
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Figure 15.10: The Custom Proof Setup dialog box enables you to select an ICC profile, rendering intent, and whether to simulate Paper White or Ink Black.

After the setup has been completed, you can view a soft proof of the image on-screen by choosing View Proof Colors or pressing /Ctrl-Y. The image will change to preview in the color space you ve chosen in the Setup window. Options include proofing CMYK, each channel individually, or the CMY colors without black. Three RGB display profiles are also available, one for a generic Macintosh monitor, one for a generic Windows monitor, and one for the monitor set as Monitor RGB. Photoshop will look to the operating system to get the assigned profile for the display for this proofing simulation.

Simulating Paper White and Ink Black

One of Photoshop s proofing capabilities is to show the white point of the converted image as either bright white, using the display s white as the target, or paper white, as measured and calculated in the ICC profile. When working with papers such as newsprint and other nonwhite substrates, you can get a better simulation of the actual product if you check the option for Paper White in the View Proof Setup Custom menu. The display will show an image that more accurately represents the appearance of the image on the nonwhite substrate.

Simulating Ink Black in the proof setup will cause the proof to represent the actual measured black of the profile rather than a solid black as dark as the display might make it. When this happens, you will see the darkest black from the measured profile, and the image will usually shift away from deep, solid black to a slightly lighter charcoal image. On most images, the differences are hard to see. Some profiles are able to represent the dark blacks and the paper white with tremendous range, and these will cause less of a shift on the screen during a proof event.

Showing Out-of- Gamut Colors

There are usually colors in an original image (RGB as opposed to CMYK) that exceed the color gamut of the reproducing device. These colors are not going to print correctly when converted to CMYK and put on a press. To preview the colors that will not print accurately, you can ask Photoshop to highlight the out-of-gamut areas on-screen with a special color by choosing View Gamut Warning. Usually these colors are small amounts of relatively unimportant information in an image, but checking is a good idea because the out-of-gamut color might be the most important color in the image.

In the ch15 folder of the CD, I ve included a photo of a yellow truck ( gamwarn1.tif ) that shows color and tonality nicely on-screen.

But when Gamut Warning is turned on, the colors that exceed the currently selected CMYK profile are shown in the color selected to show out-of-gamut values in Photoshop s Preferences palette ”the default is gray (see Figure 15.11). These areas, some of them crucial to this particular image, indicate that the colors on the printed sheet will not look as vibrant as the colors in the original. For more on viewing and selecting out-of-gamut colors, see Chapter 21, Making Difficult Selections.

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Figure 15.11: An image with considerable out-of-gamut color will surprise you. Choose Gamut Warning to show those colors that are in the original image but that cannot be reproduced by the current CMYK working space profile or Proof Setup profile. The out-of-gamut color is set in Photoshop s Preferences palette.
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If the image contains a great deal of gray, so as to make the gamut warning less distinct, you can change the warning color in the Transparency & Gamut preferences (see Chapter 5).

Converting Files

In spite of the color-managed work flow options, many Photoshop practitioners prefer to make their own CMYK conversions. Some printers insist that all files arriving for output be CMYK. To make such conversions, you must set the proper ICC color profile in the Color Settings dialog box, and then make a change of Mode to CMYK. Remember that the quality of a color separation made in Photoshop depends on the quality of the CMYK profile.

Converting Profiles

You can convert your color mode and profile by choosing Image Mode Convert To Profile. This dialog box enables you to select from RGB, CMYK, and other profile types, and also to select the rendering intent for the image you re making (see Figure 15.12). If you choose a profile that requires a change in color mode, Photoshop makes the switch; for example, if you re in an RGB file and select a CMYK profile, your file ends up in CMYK mode.

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Figure 15.12: The Convert To Profile dialog box

Assigning Profiles

To reassign an image from one profile to another without changing color mode, choose Image Mode Assign Profile to display the dialog box shown in Figure 15.13. This is helpful if an image was opened with color management turned off, and you want to assign a profile to the image so it can be processed in an ICC-compliant work flow. When working in RGB, only RGB profiles are available in this menu, and only CMYK are available profiles for those images.

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Figure 15.13: The Assign Profile dialog box
Note  

Assign does not change the current numerical values in the file, whereas Convert will alter the values of the file.

Grayscale Profiles

Converting from color to black-and-white (also called monochrome or grayscale) in early versions of Photoshop was less than ideal. Photoshop s luminance-only conversion was seldom optimal. Converting with the Channel Mixer improved the process, but the ability to use proper grayscale ICC profiles for grayscale conversions makes it even better.

Conversions using Image Mode Grayscale were often too mechanical to have much artistic value, and the methods using the Color Mixer and calculations were too tedious to be effective in production. When using an ICC grayscale profile, the conversion not only is correct for the reproduction process, but results in a more pleasing image than in past versions. Assigning a Grayscale profile in the Color Settings palette causes the Image Mode Grayscale process to use the profile to make the conversion rather than relying solely on the luminance value of the image. Some images will benefit from the use of the Channel Mixer, which allows conversions with an editorial bias, allowing the operator to change a color image into monochrome while asserting tonal changes that improve specific colors in an image.

Spot Color Profiles

Photoshop treats spot color profiles like grayscale profiles and applies them in the same way. There are standard profiles loaded by the program, which are based on various dot-gain values. If

you are inclined to make your own profiles, it s a process of printing a gray ramp scale (it is printed automatically by Photoshop if you choose Calibration Bars when printing) and then measuring the resulting target patches with a reflective densitometer.

What you are measuring is dot area, the ratio of ink to paper for a selected spot on the printed page. If you read the 50% patch, for example, you will get a value of 73% in typical gloss offset environments. To build a profile, you enter the actual measured values of dot area into a custom dot gain table, accessible from the Color Settings dialog box by choosing Custom Dot Gain from either the Gray or Spot pop-up lists in the Working Spaces area (see Figure 15.14). Saving the resulting curve creates an ICC profile.

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Figure 15.14: The Custom Dot Gain menu item is located in the pull-down menu for assigning a profile to grayscale and spot colors in the Color Settings dialog box.

A custom dot gain table (see Figure 15.15) enables you to enter actual grayscale performance curves, which are then used to compensate for the gain you experience. These curves are translated into grayscale and spot color profiles by Photoshop.

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Figure 15.15: The secondary palette in Custom Dot Gain allows the creation of curves that control how the image appears on the computer display.

After you create these profiles in Photoshop, they are available to any application that supports ICC profiles. To save a custom profile, use the Gray or Spot pop-up list in the Working Spaces area of Color Settings and choose Save Gray or Save Spot. By using ICC grayscale profiles on images to be converted for monochrome printing, you create a file that is optimized for reproduction on the measured paper and ink used to make the dot area measurements. It is, in essence, a method for matching the image to the printing capabilities of the chosen process.

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DOT GAIN

When a liquid comes in contact with a piece of paper, the capillary action of the paper s fibers spreads the liquid as it s absorbed. You can easily see this by placing a droplet of water on a piece of tissue .

Ink applied to paper on a printing press produces the same result. The size of the ink dot increases , spread by capillary action, pressure, and several other factors. This phenomenon is known as dot gain . Porous papers such as newsprint absorb more ink than coated stocks, producing greater dot gain (news ink is more fluid, also). Some lower-grade papers will spread an ink dot as much as 40%, whereas glossy coated stocks generally gain 20% or more (measured as the change in dot-area to the 50% dot). The profiles for CMYK, grayscale, and spot color in Photoshop attempt to predict the dot gain based on the qualities of specific paper types. With these profiles, the halftone dots on film output will be made smaller to compensate for the spreading ink when it is applied to paper.

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Photoshop CS Savvy
Photoshop CS Savvy
ISBN: 078214280X
EAN: 2147483647
Year: 2004
Pages: 355

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