The Hidden Power of Photoshop CS: Chapter 2: Color Separations. Pt. 2. By Sybex | 6 | WebReference

The Hidden Power of Photoshop CS: Chapter 2: Color Separations. Pt. 2. By Sybex | 6

The Hidden Power of Photoshop CS: Chapter 2: Color Separations. Pt. 2

Using the CMYK Separation

Once you have the separation, you’ll want to be able to test it out. You can do this in one of several ways, including splitting out the layers as separate images that you can merge with the Merge Channels command—which is probably the easiest testing method.

1. Merge the Adjustment layers with their associated color layers to accept the changes. This will leave you with the plates and their color preview layers.

2. Change the mode of the image to Grayscale. Don’t merge the layers when offered the opportunity.

3. Duplicate each of the ink layers to a new image in this order: Cyan, Magenta, Yellow, and Black. Name each image according to the layer you are duplicating. You should have a total of five images when you are done, including the original and four others named Cyan, Magenta, Yellow, and Black.

4. Flatten each of the four new images (Cyan, Magenta, Yellow, and Black).

5. Choose Merge Channels from the Channels pop-up menu.

6. Select CMYK from the Mode drop-down list and click OK.

7. Adjust the selections in the Specify Channels list so that Cyan is Cyan, and so forth. Click OK.

Photoshop reassembles the image as a CMYK image. You can use it as is or make additional adjustments. Once you have determined your process and the way you like to separate images, it may require fewer post-conversion adjustments. The file can be saved as any other CMYK file.

Keep in mind that there will be a difference between this RGB-to-CMYK separation and Photoshop’s. Photoshop uses an even more sophisticated procedure to go from RGB to Lab to CMYK. This allows the program to take advantage of Lab during the conversion process to help match and remap RGB to CMYK colors. If you stick with the action as is without making finer adjustments, your results will be mediocre.

On the other hand, you may benefit from the theory we’ve used for other corrections. First, make a levels correction as you would for RGB on the CMY channels. Then make a curve adjustment for one of the midtone colors to balance the gray. Increase the contrast on the CMYK curve—perhaps using a luminosity layer (see the following section). Apply Color Balance; if you do it right, you’ll note that the color won’t be far from the original.

Once you arrive at settings that work for you in adjustments, you can reuse them over and over again for the same output device for a variety of images. For example, if you note that your prints are all a little magenta heavy, you can add a curve to the Magenta layer as part of your process to reduce the magenta influence. At the same time, you can adjust the correction to maximize the potential of the image. In this way, you can make custom corrections to your CMYK content that you simply can’t control otherwise in Photoshop.

We’ve completed our look at the separation process, and you’ve seen the potential for customizing that process. For CMYK conversion, color has to be remapped from RGB to CMY, and black has to be introduced through calculations. You need to optimize these calculations for the output, taking into account how heavy you want the black generation to be, what exactly you want the black to replace, how the device and paper handle color (so you know how to adjust for dot gain), and perhaps even differences in the colors of your ink. These considerations are handled by Photoshop in automated CMYK conversions, based on choices you make in the Color Settings dialog box. With that in mind, let’s have a look at your conversion options.

Automating Customized CMYK Generation

Photoshop offers some advanced tools for adjusting your CMYK output that are probably hidden (intentionally) in the interface within the bowels of the Color Settings dialog box. The purpose of these settings is to allow you to set up your CMYK separations so that you can customize them for output without having to generate them manually every time. Photoshop allows you to create and save custom setups that you can recall when using a particular device or paper, with the aim of optimizing your output.

In the Color Settings dialog box (F/Ctrl+Shift+K), click on the CMYK Working Space drop-down list and scroll up to the top to select Custom CMYK. This will open the Custom CMYK dialog box shown in Figure 2.36.

Figure 2.36 Custom CMYK is just the beginning of potential custom adjustments you can make to CMYK output.

In this dialog box, you enter a name for a custom setup and choose ink and separation options. You can then save the setup you create and reuse it at a later time. First, let’s look at the Ink Options settings.

Ink Options

The basic Ink Options dialog box allows you to define color and absorption behaviors by selecting from a set of color standards from a drop-down list and by specifying the dot gain. You can customize ink colors by choosing Custom from the Ink Colors drop-down list (see Figure 2.37), and you can customize the dot gain by entering percentage right in the field or by choosing Curves from the Dot Gain drop-down list (see Figure 2.38).

Colors in the Ink Colors dialog box can be entered as Yxy coordinates or L*a*b* measure right on the screen. You can open color pickers for each of the nine swatches and enter such values as RGB, HSB, or Lab; or you may choose to sample colors via the Eyedropper (as you might do from a scan). Users can then measure and enter accurate depictions of device response. If you can determine accurate measures, Photoshop can look at these values and make better, targeted conversions.

Figure 2.37 The Ink Colors dialog box allows users to enter colors measured from printed pieces to accurately reflect device reproduction.

Figure 2.38 Dot Gain curves let the user enter a dot gain response of a particular device.

Separation Options

The Separation Options dialog box offers means of controlling generation of black separation. The Separation Type setting can be GCR (Gray Component Replacement) or UCR (Under Color Removal). UCR replaces CMY color with black only in gray areas; GCR replaces grays and some darker areas of color components. As you’ll recall, the manual separation involves removing color under grays and darker saturated areas, so that process is more like GCR.

Black generation lets you determine how closely generation of blacks will match image luminosity. None means no black generation; Light, Medium, and Heavy are mix variations generating less or more total black ink; Maximum generates all tone as black; Custom allows you to adjust a custom generation curve (see Figure 2.39). In the manual separation, you have the opportunity to control black generation by adjusting curves and levels to target tonal range and impact.

Figure 2.39 Customizing black generation curves affects black output.

Assigning a Black Limit means that you tell Photoshop the maximum percentage of black ink that can be used for any color or tone. Often this will be 100 percent (which tells Photoshop to use the entire dynamic range), but lower percentages allow for richer blacks and ink mixes. Total Ink Limit works in conjunction with Black Limit to calculate a result. Percentages can be as high as 400 percent (100 percent of all four inks), but using less ink has advantages such as reduced drying time and less saturation. The manual example uses a 300 percent ink limit. UCA Amount (Under Color Addition) allows you to increase the amount of color inks used in darker areas of the image. This may enhance rich blacks and help retain subtle shadow details.

Any changes in the settings in the Separation Options dialog box affect changes in the Gray Ramp graph. This graph shows how the inks will generate blacks based on the current settings by displaying changes in the CMYK output graphs.

If you have the equipment and are interested in indulging these settings by printing and measuring your results, or if you get recommended settings from your printing service, color and separation adjustments can help maximize the effectiveness of your output and give you intricate control of separations and results. Best of all, the separations will be automated, removing the need for manual efforts—though there may be instances where you will want to combine manually separated components into a CMYK image, such as to achieve totally customized or enhanced black generations based on properties that standard settings don’t provide.

Created: March 27, 2003
Revised: March 1, 2004