The Hidden Power of Photoshop CS: Chapter 2: Color Separations. Pt. 2. By Sybex | 5 | WebReference

The Hidden Power of Photoshop CS: Chapter 2: Color Separations. Pt. 2. By Sybex | 5

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

Making the Luminosity Mask

A luminosity mask helps you target image tone based on light and dark. You use it to isolate the dark areas of the image where adding black will matter most. To create the mask:

1. Add a layer above the Saturation layer. Name the layer Black and fill with gray (50 percent gray or RGB 128, 128, 128).

2. Activate the Luminosity layer.

3. Open the Levels dialog box by pressing z/Ctrl+L. Make the adjustment by moving the white slider to 128. This changes the mask so that only 50 percent grays or above will appear as gray (0–50 percent gray input will appear as white). This adjustment is optional. A smaller change allows black to substitute in lighter gray areas of the image; a greater change confines black substitution to only the darkest parts of the image. The Levels dialog box should look like Figure 2.32.

4. Merge the Luminosity layer with the Black layer. The layer should be named Black after the merge.

5. Move the Saturation layer above the Black layer and change the mode of the Saturation layer to Screen. This will brighten (dodge) areas of the image that are saturated.

6. Merge the Saturation layer with the Black layer. The result is your Black (K) plate; it should still be named Black.

Several interesting things happen here. The black is generated to substitute in areas where the image is darker than 50 percent gray. In the lighter area, saturated color is spared the effect of black mixing in. Overall, this allows black to work where it is most effective: pure gray and dark colors.

While you have generated the black from existing image information, you have done nothing to apply it. You still have to introduce the black back to your original and make adjustments. You can complete the black separation following these same steps using the CMYK Black action after completing the CMY separation and preview. (This same action completes the steps outlined in the section “Copying the Black Separation to the Original.”)

Figure 2.32 The dark portion represents the darkest half of the image (50–100 percent black).

Step 3: Applying the Black Separation

Having the black separated completes only part of the process. You could just overlay the black on the other colors, but more than one thing could go wrong. One of these problems is over-saturating the inks. Black is really added to replace some of the other color; the black needs to be used to target and remove some of the color to balance the result.

First, you need to copy the black layer back to the original image you were working on. Next you will want to remove some of the CMY ink in areas where the black will take over the responsibility (similar to duotone work, applying black where it is most efficient and reducing the effect of other ink). This serves the additional purpose of reducing the amount of ink used in black areas—which in turn can shorten ink drying time and lower the possibility of streaking or other over-inking effects.

Copying the Black Separation to the Original

This is the easy part. All you have to do here is copy the Black layer back to the CMY layered image you were working on:

1. Copy the Black layer that was the result of completing steps 1 through 6 in “Making the Luminosity Mask” back to the original CMY image using the Duplicate command from the Layer palette pop-up menu.

2. Close the image used to generate the Black layer.

3. Move the Black layer to the top of the layer stack. Set the layer mode to Multiply.

Removing Color Under Black

To reduce ink use and to get better results on press and in your prints, decrease the amount of ink printed in the darkest areas of the image in accordance with the amount of ink you are adding. This can keep the ink from over-saturating, streaking, and drying poorly. Ideally, it might seem that you would want only a maximum of 300 percent ink: The darkest areas of the image would have 100 percent each of cyan, magenta, and yellow if no black were introduced. You have to remove CMY inks to make room for the black to maintain the 300 percent maximum.
We take a simple route here, but you may want to fiddle with this result to achieve different ends.

Figure 2.33 Adjusting the Levels determines how much of the color below the black you will be taking out of other color plates— and that constrains the total ink.

To use the black ink information in removing the color below it, use the black layer information as a mask. Inverting the black will help you use it to lighten the CMY inks. A simple adjustment lets you control exactly how much total ink you will have.

1. Duplicate the Black layer and invert it by choosing z/Ctrl+I.

2. Open the Levels dialog box (z/Ctrl+L) and change the white output slider to somewhere between the 128 and 191 levels. The 128 setting is used by default in the action supplied. For the example, the Levels dialog will look like Figure 2.33.

Figure 2.34 With the Black Copy layers in place, you’ve essentially completed the look of the separation.

3. Move the Black Copy layer to just above the Yellow layer and name it Yellow Adjust. This should place it between the Yellow layer and the Yellow Fill layer.
4. Change the mode to Screen. This reduces the gray values in the Yellow layer by the intensity/density of the black ink.

5. Repeat steps 1 and 2, and move the new copy above the Magenta layer. Change the mode to Screen and name the layer Magenta Adjust.

6. Repeat steps 1 and 2, and move the new copy above the Cyan layer. Change the mode to Screen and name the layer Cyan Adjust. Your layers should look like Figure 2.34.

That’s it. What you have now is a complete separation that shows the cyan, magenta, yellow, and black with color removed under the black to reduce the density of inks so there won’t be over-inking on press. The results appear here in Figure 2.35 as gray components, and you’ll find the printed result in the color section.

Figure 2.35 Your completed CMYK separation should look something like this in separated channels.

You may see the potential here for manually adjusting the performance of your images. That can be valuable in some rare instances. What we have really taken a look at is something similar to what goes on behind the scenes during creation of your CMYK separations in Photoshop. This particular calculation is perhaps not complete in that there are many other variations you can make. For example, cyan ink is usually reduced less than yellow and magenta when removing color under black. The color is also often reduced using curves rather than levels as we did here. Any separation requires a little testing to achieve the best output.

With the separation layers in place, you can make adjustments to the image in CMYK, just as you would adjust an RGB image or layered duotone. That is, if you feel there is a little too much or too little of any color, you can reduce the amount by insinuating a curve or levels layer just above the plate color layer and making adjustments. You can preview the changes directly in the image as you make them.

Actions are included in the Hidden Power Tools so you can experiment with this process of separation. There are actions for each segment and one that plays the whole CMYK series. Encompassed in this process are most of the behind-the-scenes functions that go on in Photoshop when you convert to CMYK. You can adjust this procedure to your liking, and automate the result by duplicating the CMYK action and adjusting it to fit your needs. It will also help in looking at custom settings for generating automated CMYK separations, which we will look at more in a little bit.

Created: March 27 2003
Revised: March 1, 2004