The Hidden Power of Photoshop CS: Chapter 2: Color Separations. Pt. 2. By Sybex | 3 | WebReference

The Hidden Power of Photoshop CS: Chapter 2: Color Separations. Pt. 2. By Sybex | 3

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

Multipass Printing

On a press, ink gets put on paper one press color at a time. You can mimic this process printing at home by sending a sheet of paper through your printer multiple times. Use this method to create single copies of your duotone images, both to test the duotone settings on your home printer and to create prints for display that use true duotoning methods.

While in some cases it may be redundant to do this (there is no need to print CMYK in separate passes on a CMYK printer most of the time), this approach may be the best way to get the results you planned using your ink-jet when printing duotones. It will let you imitate press separation and process printing by laying down color one “ink” at a time, though you will actually be processing four inks in each pass to imitate specific tones. By applying colors in this way, you can imitate the result of spot color printing at home (so long as the colors you choose are in the CMYK range). This will probably work best with an ink-jet or liquid ink printer rather than a laser or pigment printer. Inks will mix better than toners.

When working with the multipass technique, keep the tones for each color in separate images, color them using the appropriate fill color, and send the result to the printer one at a time on the same sheet! Print light colors first, and follow with darker. Allow some drying time in between colors. In this way, you will build your print similar to the way it would be created on a printing press. As long as your printer has reasonably accurate paper gripping, you’ll get a pretty refined result. Because each pass can create color using CMYK ink mixtures, you can imitate many Pantone colors.

Blacks will often be richer and darker when you’re printing with multiple passes. You’ll notice a greater tonal range because the blacks will be darker—even though you are dealing with the same number of levels of grays in the image. This works somewhat like putting on a second coat of paint. You may need to allow a few seconds of drying time between passes, and using a good quality photo-print paper is recommended.

Duotone curves can be used to make subtle changes and corrections in difficult black-and-white images. Although the images will not retain the richness of duotoning or the effect of multiple inks when printed in black and white, using duotone may help you visualize the change you want to make by adding more variables to the tone. Creating duotone effects to correct grayscale images can help strengthen subtle detail that already exists in the image.

For example, if you have an image with subtle highlight detail (such as a wedding picture where the dress detail can become somewhat washed out or faded due to harsh flash lighting), you can create a duotone to change the image emphasis in different tones. Open the image in black and white, and then create a duotone. In the case of the wedding dress, you might pick a black and a light-gray ink for your duotone colors. Use the light gray to emphasize the highlight area of the image; this will allow you to make fine adjustments.

With the duotone information separated into channels within the file, you have more flexibility to manipulate the file and get the desired results. For example, if you wanted to add type to your image in pure spot color, or in a specific mixture of colors included in the file, you can do that by manipulating the channels. This is just about impossible to pull off in a Duotone mode file.

To add type that is a specific mix of colors that won’t be a natural mix of the duotone curves (say 100 percent of the light color and 25 percent of the dark), you need to adjust the tone in the channels separately. You can do this by creating a type layer and using that layer as a selection source for manipulating the channels:

1. Using the file from the previous example, choose the Type tool and click on the image to create a type layer.

2. Type in the text you want (to follow the example, type Wagontrain West), and then position the type and size it to fit. The color of the type doesn’t matter at all, but you might want to select a color that comes close to how you imagine the result (to help with your preview).

3. Hold down the z/Ctrl key and click the type layer in the Layers palette to load the type as a selection.

4. Choose the channel/color you want to adjust and fill the selection with the percentage of that color that you want the type to have in grayscale. For this example, choose the Pantone 472 channel and fill with 100 percent black.

5. Choose the other channel/color and fill it. For our example, choose Black and fill with 25 percent black.

The channels that result from these steps are shown in Figure 2.24. While it may be best to handle the type in a separate file (using vectors) to get the sharpest result, this example should help you see how manual duotones allow more potential in manipulating files and results. These changes can also be made in an earlier stage in the layered channel file by directly adjusting the Duotone 1 and Duotone 2 layers using the selection.

Figure 2.24 With the type at 100 percent Pantone 472 (A) and 25 percent Black (B), the channels will look as they do here when adjusted.

Separating CMYK Color

RGB differs from CMYK in that RGB is a reflective light scheme and CMYK an absorptive one. When you add color (light) in RGB, the result gets brighter. When you add more CMYK color (pigment/ink), the result gets darker in what is just about exactly the opposite process. In fact, CMY and RGB have an interesting relationship as opposites, as we saw briefly in the first chapter.

Photoshop provides automated separation to CMYK. To make the separation all you have to do is choose CMYK from the Mode menu, and Photoshop will separate your image according to your specified color preferences (This can be one of the predefined standards from the CMYK Working Spaces in the Color Settings, or a custom definition). The CMYK command can be a godsend, but it almost completely disconnects you from the separation process. Once you can do a CMYK separation manually (few people who use Photoshop can), the reasoning behind the process will help you make good CMYK handling choices, and it will allow you to customize your separation results. Custom CMYK separation in Photoshop is not that easy because the program wrests control from you to simplify the process. In most cases this is handy, but it can certainly hamper your understanding of the color process.

CMYK separation can be a complicated topic, and this topic alone could fill an entire book. Let’s now look at the process of manually separating CMYK right from RGB and examine some additional theory. Initially this discussion will just be a learning tool so that you can see the possibilities and complexity of CMYK separation options.

We’ll walk through the separation process in three sections: the basic CMY separation, the handling of black separation, and the application of black to the image.

Step 1: Making the Basic CMY Separation

In theory, a CMY separation (with no K) is the inverse of RGB. In the RGB color scheme, cyan is a combination of pure blue and pure green, magenta a combination of pure red and pure blue, and yellow a combination of pure red and pure green. You can see color combination in action by opening the RGB.psd file on the CD. With this file open, move the colors around to see how they combine.

Blue is also really the inverse of yellow, green the inverse of magenta, and red the inverse of cyan. Note that they are in directly opposite positions in RGB.psd, CMYK.psd, and in ColorWheel.psd (all on the CD).

You can separate an image into CMY either by using the primary cyan, magenta and yellow colors as a filter or by using the RGB colors that make up those components. Screening an RGB image for blue and green will reveal cyan, screening for red and blue will reveal magenta, and screening for red and green reveals yellow. You can achieve the same effect by creating the RGB channels in layers and inverting each of the channel colors in turn. Completing the separation will give you a very basic CMY separation that works on screen—and in a perfect world. Because pigments (and the surfaces they are used on) are not 100 percent efficient in absorbing light, black is added to CMY to make CMYK. The added black increases the efficiency and dynamic range of the result—similar to the idea of using more than one ink in a duotone to extend dynamic range.

The four steps involved in creating the basic CMY separation are preparing the image, separating the color, converting separations to tone, and applying color for preview. We’ll use the dragonfly.psd from the CD shown in Figure 2.25.


Set up the separation by duplicating the image background once for each of the CMY inks. Once the initial layers are created, you will be ready to start extracting color separations:

1. Duplicate the background layer of the flattened dragonfly.psd image.

2. Change the mode of the duplicate layer to Multiply.

3. Change the name of the layer to Cyan.

4. Duplicate the Cyan layer, and change the copied layer name to Magenta.

5. Duplicate the Magenta layer, and change the copied layer name to Yellow.

6. Activate the background layer.

7. Create a new layer.

8. Fill with white and name it Composite.

Figure 2.25 See the color section for more detail of this dragonfly.

You can complete these same steps by running the CMYK Setup action from the Hidden Power Tools. The image will look awful at this point, but that doesn’t matter. Figure 2.26 shows how the Layers palette should look.

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