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Getting Started with Photoshop: Alpha Channels and Selections

Corrie Haffly
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Alpha Channels and Selections

You can use alpha channels to create selections and save them for later use. If you open the Channels palette, you’ll see several channels, displayed in a similar way to layers in the Layers palette. By default, you’ll see the color channels, which represent how much of each color is represented in the document. You can click the Create New Channel icon at the bottom of the palette to create your own alpha channel.

Creating a new alpha channel

You can then use any of Photoshop’s painting or drawing tools to create a grayscale image that will represent your selection — white areas represent selected areas, black areas represent deselected areas, and grays represent the levels of transparency in the selection.

Creating a grayscale image

To turn your alpha channel masterpiece into a selection, simply hold down Ctrl and click the channel’s thumbnail (hold Command and click if you’re on a Mac).

Creating a channel-based selection

To return to the normal image view, click on the Layers palette tab, and select any layer. Your selection will still be visible.

Returning to the Layers palette

You can also create your own alpha channels from existing selections — a capability that can be very useful! For example, let’s say you’ve created a selection of an island silhouette like the one shown in the example below. You have a feeling that you’ll be reselecting this island pretty often, but you’d rather not recreate the selection each time. No problem! Once the selection has been made, use Select > Save Selection. Name your selection (in this example, Land), and click OK.

Saving the selection to a channel

If you go to the Channels palette, you’ll see a new selection at the bottom of the list, named Land in the following image — that’s your saved selection. Now you can reload your Land selection as many times as you need to!

New channel in the Channels palette

The History Palette

The History palette is your key to time travel (in Photoshop, anyway). It lists the most recent steps that you’ve made, and allows you to undo your actions by rolling your image back to a previous state. You can set the number of steps that are stored in the memory by selecting Edit > Preferences > Performance (Photoshop > Preferences > Performance on a Mac) and changing the value in the History States text box.

Like most of Photoshop’s other tools, the History palette has a set of useful keyboard shortcuts for quick access:

  • Ctrl-Z (Command-Z on a Mac) lets you undo and redo the previous step.
  • Ctrl-Alt-Z (Command-Option-Z) steps back through the History palette.
  • Shift-Alt-Z (Shift-Option-Z) steps forward through the History palette.

Creating a history snapshot

As only a limited number of history states are available, there may be cases in which you want to save a”snapshot” of your document so that you can revert back to it later if required. To do so, click on the small triangle on the top-right of the History palette and choose New Snapshot…. You can save a snapshot of the whole document, the current layer, or merged layers.

When Photoshop Stops Working

Woah! Photoshop stops working? That certainly doesn’t sound too promising! Before you panic, let me explain. Given the multitude of powerful features and fantastic tools it offers, it’s no wonder that, on occasion, Photoshop can exhaust itself. It may start behaving a bit erratically, and might even freeze, crash, or automatically exit during startup. If you find yourself in this situation, the first thing to do is reset the preferences file. The preferences file (which you can customize by going to Edit > Preferences on a PC, or Photoshop > Preferences on a Mac) holds Photoshop settings and can often become corrupted.

The location of the preferences file depends on the operating system and version of Photoshop you are using. For Photoshop CS3, the preferences file is named Adobe Photoshop CS3 Prefs.psp.The preference file for other versions of Photoshop will have a similar name.

Backing Up Your Preferences File
It’s a good idea to back up your preferences file by copying and pasting it into a location outside of the Photoshop settings folder. Then, if the preferences file Photoshop is using becomes corrupted, you can copy your backup back into the settings folder to replace the corrupted file, without losing any of your settings.

To reset the preferences file, locate the current preferences file, delete it (while Photoshop is closed), and restart Photoshop — it will recreate the preferences file using default settings. Creativepro.com provides a detailed tutorial that explains how to find and replace your preferences file, and includes preference filenames for different versions of Photoshop.

If Photoshop continues to act up, restart it while holding down the Shift-Ctrl-Alt keys (Shift-Command-Option on a Mac), and click OK when asked if you wish to delete the Photoshop settings file. Unfortunately, this will also delete your custom actions, tools, and other settings, but the good news is that it should fix your Photoshop problems.

Summary

This chapter provided an overview of the Photoshop and Fireworks interfaces and common tools, and also explained a few basic tasks such as creating new documents and saving files for the Web. You also took a quick tour of handy keyboard shortcuts and other time-saving tips. Even if you haven’t used Photoshop before, you now have the tools that you’ll need to work with the examples we’ll discuss throughout the rest of this book.

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