The History Palette
The History palette, at its most basic, remembers what you've done to your file and lets you either retrace your steps or revert back to any earlier version of the image. Every time you do something to your image-paint a brush stroke, run a filter, make a selection, and so on-Photoshop saves this change as a state in the History palette (see Figure 6). At any time, you can revert the entire image to any previous state, or-using the History Brush tool or the Fill command, which we'll discuss in a moment-selectively paint back in time.
There is, however, an itty-bitty problem with the History palette: it can take up a lot of RAM. Sorry, did we say "a lot"? We meant "vast, awe-inspiring, mind-boggling quantities" of memory. Photoshop can require as much as 10 to 20 times your file size in RAM-or more-to perform efficiently when Photoshop is saving history states (that's 200 to 400 MB of RAM for a 20 MB image). Performing simple tasks such as opening, rotating, sharpening, and saving may take significantly longer when the History feature is turned on (which it is by default in Photoshop 6).
Tip: Turning Off History.
If you're doing straight-laced production work all day (the kind of work for which a single Undo is perfectly adequate), you may want to avoid the History feature's RAM overhead by changing the History States value to 1 in the Preferences dialog box (under the Edit menu or press Command-K). Similarly, you can turn off "Automatically Create First Snapshot" in the History Options dialog box (which you can find on the History palette's popout menu). You might also want to turn off these functions if you're going to batch-process a number of images using actions or the Automate "wizards" (because in these cases, History isn't necessary).
The History palette has two sections: snapshots and states. Let's take a look at each of these and how you can use them.
Snapshots. Early versions of Photoshop let you save a single snapshot of your document, representing a moment in time for your image. The History palette lets you save any number of snapshots so that at any time you can return to a speciÞc state. There are two main differences between snapshots and states.
Photoshop records almost everything you do to an image as a state. By default, snapshots are only recorded when you Þrst open an image and when you click the New Snapshot button in the History palette.
When the number of states recorded on the History palette exceeds the Maximum Remembered States value (set in the History Options dialog box), the oldest states start dropping off the list. Snapshots don't disappear until you close the document.
Tip: What's in the Snapshot.
When you click the New Snapshot button on the History palette (or select New Snapshot from the palette's popout menu), Photoshop saves the whole document (individual layers and all). Depending on how many layers you have and how large your document is, this might require a lot of RAM. If you Option-click the button, Photoshop offers two other less-memory-intensive snapshot choices: a version of the image with merged layers, or just of the currently selected layer. (If you Þnd yourself Option-clicking the button a lot in order to get these options, then turn on the Show New Snapshop Dialog By Default checkbox in History Options. That way, you don't have to press the Option key anymore.)
Stepping through states. As we mentioned above, Photoshop saves every brush stroke, every selection, every anything you do to your image as a state on the History palette (though the state only remains on the palette until you reach the maximum number of states or you close the document). There are several ways to move among states of your image.
To revert your image back to a state, you can click on any state's tile in the History palette.
You can also move the active state marker to a state on the History palette.
You can press Command-Z to step back to the last state (just as you've always been able to do). But you can also press Command-Option-Z to move backward one state at a time, and Command-Shift-Z to move forward one state at a time.
In general, when you move to an earlier state, Photoshop grays out every subsequent state on the History palette, indicating that if you do anything now these grayed-out states will be erased. This is like going back to a fork in the road and choosing the opposite path from what you took before. Photoshop offers another option: if you turn on the Allow Non-Linear History checkbox in the History Options dialog box, Photoshop doesn't gray out or remove subsequent states when you move back in time (though it still deletes old states when you hit the maximum number of states limit).
The History Brush. Returning to a previous state returns the entire image to that state. But Photoshop's History feature lets you selectively return portions of your image to a previous state, too, with the History Brush and the Fill command. Before painting with the History Brush, first select the source state in the History palette (click in the column to the left of the state from which you want to paint). For instance, let's say you sharpen a picture of a face with Unsharp Masking and find that the lips have become oversharp. You can select the History Brush, set the source state to the presharpened state, and brush around the lips (though you'd probably want to reduce the opacity of the History Brush to 20 or 30 percent by pressing 2 or 3 first).
The History Brush tool (press Y) is very similar to the Eraser tool when the Erase to History checkbox is turned on in the Options bar, but the History Brush lets you paint with modes, such as Multiply and Screen.
Tip: Snap Before Action.
If you run an action in the Actions palette that has more steps than your History States preference, you won't be able to "undo" the action. That's why before running the action you should either save a snapshot of your full document or set the source state for the History Brush to the current state. The latter works because Photoshop never "rolls off" the source state in the History palette, so you don't have to worry about its getting deleted after reaching the maxiumum number of states.
Fill with History. One last nifty technique that can rescue you from a catastrophic "oops" is the Fill command on the Edit menu (press Shift-Delete). This lets you fill any selection (or the entire image, if nothing is selected) with the pixels from the current source state on the History palette. We usually use this in preference to the History Brush or Eraser tools when the area to be reverted is easily selectable. Sometimes when we paint with those tools, we overlook some pixels (it's hard to use a brush to paint every pixel in an area at 100 percent). This is never a problem when you use the Fill command.
You've always been able to press Option-Delete to fill a selection or layer with the foreground color. In version 4, Photoshop added the ability to automatically preserve transparency on the layer when you add the Shift key (slightly faster than having to turn on the Preserve Transparency checkbox in the Layers palette). Similarly, you can fill with the background color by pressing Command-Delete (add the Shift key to preserve transparency). To fill the layer or selection with the current history source state, press Command-Option-Delete. And, of course, you can add the Shift key to this to fill with Preserve Transparency turned on.
Tip: Persistent States.
Remember that both snapshots and states are cleared out when you close a document. If you want to save a particular state or snapshot, drag its tile over the Create New Document button on the History palette. Now that state is its own document that you can save to disk. If you want to copy pixels from that document into another image, simply use the Rubber Stamp tool (you can set the source point to one document and then paint with it in the other file).
Tip: Revert When Revert Doesn't Work.
Deke McClelland taught us a trick at a recent Photoshop conference that has already saved David's buttocks several times. Because David has a tendency to type fast and loose, he'll often press Command-S (Save) when he meant to press Command-A (Select All) or Command-D (Deselect). Of course, this saves over his file on disk, often ruining his original scan. The History palette to the rescue! Remember that the default preference for the History palette is to create a snapshot of the image when you first open it. If you save over your original image, you can drag the snapshot's tile over the Create New Document button in the History palette to recreate the original data in its own file.
Tip: Copying States.
Although Photoshop lets you copy states from one document to another simply by dragging them from the History palette onto the other document's window, we can't think of many good reasons to do this. The copied state completely replaces the image that you've dragged it over.
Tip: When History Stops Working.
Note that you cannot use the History Brush or the Fill from History feature when your image's pixel dimensions or color mode has changed. Pixel dimensions usually change when you rotate the whole image, use the Cropping tool, or use the Image Size or Canvas Size dialog boxes.
Tip: Purging States.
As we said earlier, the History palette takes up a lot of memory. If you find yourself needing more RAM, you might try clearing out the History states by either selecting Clear History from the popout menu on the History palette or choosing History from the Purge submenu (under the Edit menu). The former can be undone in a pinch; the latter cannot. Curiously, neither of these removes your snapshots, so you have to delete those manually if you want to save even more RAM. Remember that closing your document and reopening it will also remove all snapshots and history states.