Tài liệu Photoshop cs5 by Dayley part 51 docx

Chapter 11: Channels 349 On the Web Site The image shown in Figure 11.5 can be found on this book’s Web site as Figure 11-5.jpg. You can open it in Photoshop and use the steps in this section to see the effects of mixing the channels to create a better grayscale image. n Using the Channels Panel The Channels panel shown in Figure 11.8 provides access to all the channels contained in the images. To open the Channels panel, select Window ➪ Channels from the main menu or use the F7 shortcut key and select the Channels tab. Note When working with RGB and CYMK images, the top entry in the Channels panel is a composite channel of the RGB or CYMK channels. When you select the composite channel, all the component channels are selected. n Using the Channels panel, you can view and select individual channels, remove channels, and cre- ate new channels. The following sections discuss using the Channels panel to work with the color channels in an image. FIGURE 11.8 The Channels panel allows you to manage individual color channels in images. Composite channel Visibility Channels panel menu Load Channel as Selection Delete Channel Save Selection as Channel Create New Channel 17_584743-ch11.indd 34917_584743-ch11.indd 349 5/3/10 10:26 AM5/3/10 10:26 AM Part III: Selections, Layers, and Channels 350 Selecting channels A useful feature of the Channels panel is the ability to select one or more of the channels individu- ally. When only one channel is selected, changes that you make by drawing, erasing, and applying filters are made to that channel only. This allows you to create special effects on specific channels or fine-tune adjustments to a single channel. You can select channels in the Channels panel by clicking them with the mouse or by pressing the Ctrl/Ô+# key sequence listed next to the channel name. When you select the composite channel (RGB or CYMK), the image display shows the results of all three channels. When you select an individual channel, the image display shows a grayscale rendition of the levels of the selected chan- nel. To select multiple Channels, use the Ctrl/Ô or Shift keys. When multiple channels are selected. the image display shows the results of the combined channels. You also can specify whether a channel is visible by clicking the visibility box next to the channel in the Channels panel. When the eye is displayed in the box, the channel is visible in the image display. When the eye is not displayed in the box, the channel data is not used to render the image display. Deleting channels Channels can be deleted from the image by selecting the channel in the Channels panel and then clicking the trash can button (refer to Figure 11.8). When you delete a channel, the level data con- tained in that channel is removed from the image. When you delete one of the component channels from an RGB or CYMK image, the composite channel also is removed and the color mode of the image changes to Multichannel. Typically, you are only deleting channels that you add to an image, such as an alpha channel or a spot channel. Duplicating channels A useful feature of the Channels panel is the ability to quickly duplicate one of the existing chan- nels. This allows lots of flexibility when editing channels. After you duplicate the channel, you can make adjustments to the duplicate channel and simply change the visibility between the duplicate and the original to see the different effects. You also can use this feature to save a channel as another document. To create a duplicate of an existing channel, right-click the channel in the Channels panel and select Duplicate Channel from the pop-up menu. A dialog box similar to the one in Figure 11.9 is displayed. You can specify the following settings when duplicating a channel: 17_584743-ch11.indd 35017_584743-ch11.indd 350 5/3/10 10:26 AM5/3/10 10:26 AM Chapter 11: Channels 351 l As: This is the name of the channel. l Document: This selects the document where the duplicate channel will be added. Document defaults to the current image file; however, you can select any open document or a new file. l Name: This specifies the name of the new file if Document is set to new. l Invert: This creates a completely inverted copy of the original channel. The lighter areas of the copy are dark and vice versa. FIGURE 11.9 Creating a duplicate channel in Photoshop Splitting/Merging channels Photoshop also provides the ability to split the channels contained in an image into separate docu- ment files. This is useful when you want to apply lots of adjustments to the individual channels and then merge them later. To split the channels of an existing image into multiple documents, select Split Channels from the Channels panel menu (refer to Figure 11.8). The current document is replaced by a set of docu- ments representing each channel in the image, as shown in Figure 11.10. You can then edit and save each channel separately. To merge the channels back into a color image, select Merge Channels from the Channels panel menu. A Merge Channels dialog box similar to the one shown in Figure 11.11 is displayed that allows you to specify the color mode and number of channels to be included in the new color image. Click OK to bring up the Merge mode dialog box, also shown in Figure 11.11. This dialog box allows you to specify which files to use for each channel of the specified color mode when cre- ating the color image. 17_584743-ch11.indd 35117_584743-ch11.indd 351 5/3/10 10:26 AM5/3/10 10:26 AM Part III: Selections, Layers, and Channels 352 FIGURE 11.10 Splitting color channels of an image results in a new set of grayscale documents being created, each con- taining the data from a separate channel. The filenames are the same as the original with an additional let- ter denoting the channel. FIGURE 11.11 Merging multiple grayscale images back into a color image using the Merge Channels utility 17_584743-ch11.indd 35217_584743-ch11.indd 352 5/3/10 10:26 AM5/3/10 10:26 AM Chapter 11: Channels 353 Note The images that you want to merge into a color image must be monotone images. They also must be open in Photoshop for the Merge Channels option to be active. n Sharing channels between images Photoshop makes it extremely easy to share channels between images. This is useful when you need to move channel data between documents. To move a channel between two documents, simply select the channel in the Channels panel and drag it to another image, as shown in Figure 11.12. A new alpha channel is created in the second image. The contents of the alpha channel are the levels from the selected channel in the original image. FIGURE 11.12 Channels can be shared between documents in Photoshop by selecting a channel from the Channels panel and then dragging it into another image document. Making Channel Selections One of the major advantages of having access to the separate channels in Photoshop is the enhanced ability to make selections. Because each channel provides a different aspect into the detail of the image, each has different contrast in relation to each part of the image. Because you can select each channel individually, you can use individual channels to make differ- ent selections in the image. When you want to select a specific item in the image, use the color channel that provides the greatest contrast around the edges of that item. This example illustrates how to use channels to make better selections: 1. Open the image in Photoshop as shown in Figure 11.13. 17_584743-ch11.indd 35317_584743-ch11.indd 353 5/3/10 10:26 AM5/3/10 10:26 AM Part III: Selections, Layers, and Channels 354 FIGURE 11.13 Selecting the rock and the sky in this image will be easy using the Channels panel. 2. Select Window ➪ Channels to open the Channels panel. 3. After the Channels panel is open, view each channel individually. 4. Select the Red channel. The first selection we want to make is that of the rock the boys are standing on. Notice from Figure 11.14 that the red channel provides the best contrast between the rock and the rest of the image. FIGURE 11.14 Each channel has different contrast levels allowing some to be better for selecting cer- tain objects than others. Red channel Blue channel Green channel 5. Use the Quick selection tool, discussed in Chapter 9 to draw across the center of the rock, so it is selected as shown in Figure 11.15. 6. Then click the Save Selection as a Channel button to save the selection as an alpha channel, as also shown in Figure 11.15. 17_584743-ch11.indd 35417_584743-ch11.indd 354 5/3/10 10:26 AM5/3/10 10:26 AM Chapter 11: Channels 355 FIGURE 11.15 Selecting the rock in the red channel is easy with the quick selection tool because of the higher contrast. Next we want to select the sky in the background. Notice from Figure 11.14 that the blue channel provides the best contrast around the edges of the sky. So we want to select the blue channel this time. 7. Use the Quick selection tool to draw across the sky and pick up the edges around the boys to create the selection shown in Figure 11.16. 8. Again click the Save selection as a Channel to save the selection as an alpha channel. FIGURE 11.16 Selecting the sky in the blue channel is easy with the quick selection tool because of the higher contrast. Using the channel method, we more easily created the two selections than would have been possi- ble using the composite RGB channel. The resulting selections are now channels that can be used as needed for further editing, as shown in the Channels panel in Figure 11.17. 17_584743-ch11.indd 35517_584743-ch11.indd 355 5/3/10 10:26 AM5/3/10 10:26 AM Part III: Selections, Layers, and Channels 356 FIGURE 11.17 The selections created have been added as alpha channels in the Channels panel. On the Web Site The project file used in the example of this section can be found on this book’s Web site as Figure 11-17.psd. You can open it in Photoshop and play around creating selections using different color channels. n The Alpha Channel In the previous section, we discussed storing a selection in an additional channel called an alpha channel. The alpha channel describes additional information used when processing the actual pixel data. The alpha channel does not provide any data about the color content of a pixel but rather how to process the data. The main purpose of the alpha channel is to provide transparency information that can be applied to an image. As with the other channels, each pixel in the alpha channel can have a value from 0 to 255 where 0 represents that the pixel is 100 percent transparent and 255 represents that the pixel is 100 percent opaque. Another useful feature of alpha channels is that you can use them to store and retrieve selections for later use. The alpha channels allow you to have quick access to previous selections and even share them between documents by dragging the alpha channel from one document to another. Creating alpha channels Alpha channels are created from selections. The simplest way to create an alpha channel is to make a selection and then save the selection as an alpha channel using these steps: 1. Open an image in Photoshop. 2. Select Window ➪ Channels from the main menu to open the Channels panel. 17_584743-ch11.indd 35617_584743-ch11.indd 356 5/3/10 10:26 AM5/3/10 10:26 AM . specified color mode when cre- ating the color image. 17_584743-ch11.indd 3511 7_584743-ch11.indd 351 5/3/10 10:26 AM5/3/10 10:26 AM Part III: Selections, Layers, and Channels 352 FIGURE 11.10 Splitting. the copy are dark and vice versa. FIGURE 11.9 Creating a duplicate channel in Photoshop Splitting/Merging channels Photoshop also provides the ability to split the channels contained in an image. adjustments to a single channel. You can select channels in the Channels panel by clicking them with the mouse or by pressing the Ctrl/Ô+# key sequence listed next to the channel name. When you