Adobe Photoshop CS5 for Photographers - Martin Evening [199]
Alpha channels
An alpha channel is effectively the same thing as a mask channel and if you choose Select Save Selection…, you can save a selection as a new alpha channel. These are saved and added in numerical sequence immediately below the main color channels. Just like normal color channels, an alpha channel can contain up to 256 shades of gray in 8-bits per channel mode or up to 32,000 shades of gray in 16-bits per channel mode. You can select a channel by going to the Channels panel and clicking on the desired channel. Once selected, it can be viewed on its own as a grayscale mask and manipulated any way you like using any of the tools in Photoshop. An alpha channel can also effectively be viewed in a quick mask type mode. To do this, first select an alpha channel and then click on the eyeball icon next to the composite channel, which is the one at the top of the Channels panel list (see Figure 9.3). You will then be able to edit the alpha channel mask with the image visible through the mask overlay. There are several ways to convert an alpha channel back into a selection. You can go to the Select menu, choose ‘Load Selection…’ and then select the name of the channel. A much simpler method is to drag the channel down to the Make Selection button at the bottom of the Channels panel, or -click the channel in the Channels panel.
Adding to an image selection
1.
Let's add an extra step to the example shown on page 455. Here, I started off with the same elliptical selection. I then selected the rectangular marquee tool and dragged across the image with the key held down in order to add to the elliptical selection.
2.
I did actually refine the selection a little more than that, because I also selected the polygon lasso tool and again, with the key held down, clicked a few more times to add the outline of the building on the left. As with the previous example I added a Curves adjustment and clicked on the Invert button in the Masks panel so that the adjustment darkened the areas outside the selection. I then added a 2 pixel feather to the selection edge to obtain a smoother edge blend.
Selections, alpha channels and masks
As was pointed out at the beginning of this chapter, there is always an interrelationship between selections, quick masks and alpha channel masks. This also extends to the use of vector paths and vector masks (vector paths are discussed towards the end of this chapter). The accompanying diagram in Figure 9.7 illustrates these relationships in detail.
Figure 9.7 This diagram illustrates the interrelationship between pen paths, vector masks, alpha channels, selections and quick masks. For example, you can convert a selection to a mask and a mask back into a selection, and a selection can temporarily be displayed and edited in Quick Mask mode and switched back to Selection mode again. The red arrows indicate that some data loss is incurred during the conversion from one state to the other.
Starting at the top left corner, we have a path outline that has been created using the pen tool in Photoshop. A pen path outline can be saved as a path and an active path can be used to create a vector mask, which is a layer masked by a pen path mask (see pages 532 and 536–538). A vector mask can also be rasterized to make a layer mask (a layer that is masked by an alpha channel). Meanwhile, a pen path can be converted to a selection and a selection can be converted back into a work path. If we start with an active selection, you can view and edit a selection as a quick mask and a selection can also be converted into an alpha channel and back into a selection again.
When preparing a mask in Photoshop, most people will start by making a selection to define the area they want to work on and save that selection as an alpha channel mask. This allows you to convert the saved alpha channel back into a selection again at any time in the future. The other way to prepare a mask is to use the pen tool to define the