Another use for the channels is as a way to make a selection of an object on the image. For example, if you encounter problems isolating an object, check each of the three channels in turn to see whether any of them provide good contrast between the object and its background. If so, display that channel and make your selection on the channel itself before switching back to the image to continue working (see Figure 5).
Figure 5 When you need to make a selection of an object from its background, often one channel will provide the desired contrast between them to make the job easier.
Channels have another application in relation to selections: You can save a selection into a channel so it will be saved with the image:
To see how this is done, do the following:
- Make a selection of an area on an image and choose Select > Save
Selection. Type a name for the selection, leave the New Channel option selected,
and notice that the current document is named as the place to which the
selection will be saved (see Figure 6). Click OK.
Figure 6 When you make a selection, you can save it as a channel in the image so you can use it again later on.
- View the Channels palette and notice that your newly saved channel appears
in the Channel list. This channel is called an alpha channel, it is a saved
mask—the black area is the area you have selected (see Figure 7).
Figure 7 When you save a selection as a channel, it appears as a new channel in the Channels palette.
Of course, you need to save your photo in a format that supports channel data, such as PSD, so that you can retrieve it later on. Avoid using any format that flattens the image, such as BMP or JPG, or else you will lose the selection.
To retrieve your selection later on from an alpha channel, select the alpha channel in the Channels palette and click the Load Channel as Selection button at the foot of the Channels palette. Make sure to click to select the composite RGB channel before returning to your image so any changes or adjustments you make are made on the image itself.