Tài liệu Photoshop cs5 by Dayley part 53 potx

Part III: Selections, Layers, and Channels 360 l Selected Areas: When this is selected, the selected areas are highlighted by the overlay color selected in the Color field. Use this option when you want to see the actual pixels of the data that is being affected by the alpha channel. l Spot Color: When this is selected, the alpha channel is converted into a spot color chan- nel and Opacity option is changed to a Solidity option. Spot colors are discussed in the next section. l Color: This specifies the color of the channel overlay. The overlay is displayed in the doc- ument window when the alpha channel visibility is on in the Channels panel. This allows you to view more easily the alpha channel selection because either the selection or the mask is overlaid with the specified color. When you click the color, a color chooser win- dow is displayed, allowing you to specify the color. l Opacity: This specifies the opacity of the overlay channel. Turning up the opacity allows you to better see the pixels behind the overlay. You may need to play around with this set- ting to see enough of the background image to determine the exact selection. l Solidity: This specifies the Solidity of the spot color channel. Turning up the solidity adjusts how much tint of the spot color is applied to the image or other channels when merged with the spot color channel. FIGURE 11.21 Setting the alpha channel options allows you to specify the color of the overlay and whether the selection or the mask is overlaid with color when the alpha channel is visible. Spot Color Channels Spot colors are used for offset printing. A spot color is simply a single color that is impressed on the paper in a single pass through the printer. Offset printing is the process of printing in multiple passes using one spot color per pass. You use spot colors when printing images for two main reasons. First, it is much less expensive to use spot colors when printing than mixing the standard CYMK inks to create the colors needed. Second, you may want to add a color to your image that cannot be created using a mix of the CYMK inks. 17_584743-ch11.indd 36017_584743-ch11.indd 360 5/3/10 10:26 AM5/3/10 10:26 AM Chapter 11: Channels 361 Creating a spot color channel The process of creating a spot color channel is similar to that of creating an alpha channel with a few extra steps. The following steps take you through an example of creating a spot channel: 1. Open the image in Photoshop as shown in Figure 11.22. FIGURE 11.22 A selection in an image in Photoshop can be used to create a spot channel 2. Use the Magic Wand tool to select the area of the image that you want to turn into a spot channel. In the case of Figure 11.22, you would want to select the outside of the martial artist. 3. Press Ctrl/Ô+C to copy the contents of the selection into the clipboard. 4. Select Window ➪ Channels to open the Channels panel. 5. Select New Spot Channel from the Channels panel menu to open the New Spot Channel dialog box as shown in Figure 11.23. FIGURE 11.23 When creating a spot channel, you need to specify the color and solidity to use for the ink tone. 6. Click the Color field to open the Select Spot Color dialog box. 17_584743-ch11.indd 36117_584743-ch11.indd 361 5/3/10 10:26 AM5/3/10 10:26 AM Part III: Selections, Layers, and Channels 362 Choose the color you want to use for the spot color. In this case, Pantone 2717 EC was chosen. 7. Set the Solidity of the spot color. In this case, we used 50%. 8. Click OK to create the spot color channel. The new channel is displayed in the Channels panel, as shown in Figure 11.24. FIGURE 11.24 The spot channel is created from the selection; however, it is a solid spot channel with no detail. 9. Select the new spot channel in the Channels panel. 10. Click the Load Channel as Selection button in the Channels panel to load the selec- tion used to create the spot channel. 11. Press Ctrl/Ô+V to paste the contents saved to the clipboard into the spot channel. The spot channel becomes a tonal spot channel, shown in Figure 11.25, instead of a solid. When printed, the spot color selected in Step 6 is used to print the pixels in that area of the image. 17_584743-ch11.indd 36217_584743-ch11.indd 362 5/3/10 10:26 AM5/3/10 10:26 AM Chapter 11: Channels 363 FIGURE 11.25 Pasting the contents of the original selection into the spot channel selection creates a tonal spot channel that can print the detail in a single ink color. Merging spot color channels The spot channel can be merged into the rest of the image. The solidity value of the spot channel determines how much of the spot channel ink is applied to the pixels in the image. To merge the spot channel with the background data in the image, select Merge Spot Channel from the Channels panel menu. Caution Merging the spot channel into the rest of the image flattens the spot channel layer. This flattens the spot chan- nel into the rest of the image. The spot channel no longer exists, so you cannot get the selection back or alter the color. n Removing ink overlap using spot color channels When printing spot colors, you need to be careful that ink from the image content does not over- lap the ink from spot colors. Overlapping image content and spot colors results in ink mixing, which typically is not a desired result. To prevent the colors from overlapping, you need to create knockout by setting the background color to white and deleting all pixels in the image content under the spot color so the spot color ink is the only ink printed on that portion of the paper. 17_584743-ch11.indd 36317_584743-ch11.indd 363 5/3/10 10:26 AM5/3/10 10:26 AM Part III: Selections, Layers, and Channels 364 Creating a knockout can create another problem. Because there is no ink beneath the knockout, a white border may appear around the spot color if the ink doesn’t print perfectly. To solve this problem, create a trap between the spot color and the image content, shrinking the selection of the spot color by 1 pixel. This allows each of the spot colors to bleed into the image content a bit and eliminate the white border. Note Trapping is a prepress technique that involves creating small overlaps between adjacent colors in an image. Trapping allows the colors to bleed together to avoid white outlines that can occur because of alignment when printing. n Follow these steps to create a knockout and trap to avoid spot colors from overlapping: 1. Create the spot channel as described earlier in this chapter. 2. Select a spot channel in the Channels panel. 3. Click the Load Channel as Selection button to get the selection of the channel. 4. Click the composite channel for the image in the Channels panel. The selection should now be visible in the composite channel of the image. 5. Set the background color to be white. White means that no ink will be printed on that portion of the paper. 6. Create the trap by selecting Select ➪ Modify ➪ Contract to open the Contract Selection dialog box. 7. Set the Contract By option to 1 pixel. This allows the spot channel to overlap onto the image content so the inks overlap, elimi- nating the possible white border between the two. 8. Create the knockout by pressing the Delete key to delete the image content beneath the spot color. If the Fill dialog box is displayed instead of just deleting the pixels, select white and click OK. Summary This chapter discussed using the Channel Mixer to have direct access to mixing, swapping, and combining the color data in your images. Using the Channel Mixer allows you to change the color composition without changing the original detail in the image. This chapter also discussed how to use Channels panel to view and manage channels including alpha channels and spot color channels. Alpha channels are not color channels, but rather they contain information such as transparency about certain areas of images. Spot colors are used when printing specific colors in passes rather than a mix using the standard CYMK ink cartridges. 17_584743-ch11.indd 36417_584743-ch11.indd 364 5/3/10 10:26 AM5/3/10 10:26 AM Chapter 11: Channels 365 In this chapter, you learned the following: l How to mix the color channels to create a better grayscale conversion l That swapping two color channels can drastically change the look of an image without affecting the detail l That you can split channels into multiple monotone documents, edit them, and then merge them back into a single color image l How to share channels between multiple images l That alpha channels can be created from a selection l How to use alpha channels to store and retrieve selection data l How to effectively create spot color channels that can be used in offset printing 17_584743-ch11.indd 36517_584743-ch11.indd 365 5/3/10 10:26 AM5/3/10 10:26 AM . Part III: Selections, Layers, and Channels 360 l Selected Areas: When this is selected, the selected areas are highlighted by the overlay color selected in. example of creating a spot channel: 1. Open the image in Photoshop as shown in Figure 11.22. FIGURE 11.22 A selection in an image in Photoshop can be used to create a spot channel 2. Use the. that portion of the paper. 6. Create the trap by selecting Select ➪ Modify ➪ Contract to open the Contract Selection dialog box. 7. Set the Contract By option to 1 pixel. This allows the spot