Tài liệu Photoshop cs5 by Dayley part 51 docx

n Using the Channels Panel The Channels panel shown in Figure 11.8 provides access to all the channels contained in the images.. Note When working with RGB and CYMK images, the top entry

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 DeleteChannel Save Selection as Channel Create New Channel

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 chanchan-nels 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:

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 adjustdocu-ments 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

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

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

FIGURE 11.13

Selecting the rock and the sky in this image will be easy using the Channels panel

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

the rock, so it is selected as shown in Figure 11.15.

channel, as also shown in Figure 11.15.

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

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

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: