Choose Window ^ Show History to view the History palette, annotated with the palette menu in full view in Figure 7-26. The History palette records each significant operation — everything other than settings and preferences (for example, selecting a new foreground color) — and adds it to a list. The oldest operations appear at the top of the list with the most recent operations at the bottom.
Snapshots Opened state Palette menu
Snapshots Opened state Palette menu
Each item in the list is called a state. That's not my word, it's Adobe's, and several have voiced the opinion that the term is too stiff and formal. But I think it's dead on. Each item in the palette represents a stepping stone in the progression of the image, a condition at a moment in time — in other words, a state.
Photoshop automatically names each item according to the tool, command, or operation used to arrive at the state. The icon next to the name helps to identify the state further. But the best way to find out what a state is like is to click it. Photoshop instantaneously undoes all operations performed after that state and returns you to the state so that you can inspect it in detail. To redo all the operations you just did in one fell swoop, press Ctrl+Z or choose Edit ^ Undo State Change.
That one action — clicking on a state — is the gist of what you need to know to travel forward and backward through time in Photoshop. If that's all you ever learn, you'll find yourself working with greater speed, freedom, and security than is possible in virtually any other graphics application. But this represents only the first in a long list of the History palette's capabilities. Here's the rest of what you might want to know:
* Changing the number of undos: By default, Photoshop records the last 20 operations in the History palette. When you perform the 21st operation, the first state is shoved off the list.
'J In Photoshop 6, you set the number of operations that the History palette tracks in the Preferences dialog box. Choose Edit Preferences General or press Ctrl+K to open the dialog box and enter the value you want to use in the History States box. If your computer is equipped with 32MB or less of RAM, you might want to lower the value to 5 or 10 to maintain greater efficiency. On the other hand, if you become a time-traveling freak (like me) and have plenty of RAM, turn it up, baby, all the way up!
* Undone states: When you revert to a state by clicking on it, every subsequent state turns gray to show that it's been undone. You can redo a grayed state simply by clicking on it. But if you perform a new operation, all grayed states disappear. You have one opportunity to bring them back by pressing Ctrl+Z; if you perform another new operation, the once-grayed states are gone for good.
* Working with non-sequential states: If you don't like the idea of losing your undone states — every state is sacred, after all — choose the History Options command and select the Allow Non-Linear History check box (see Figure 7-27). Undone states no longer drop off the list when you perform a new operation. They remain available on the off chance that you might want to revisit them. It's like having multiple possible time trails.
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Figure 7-27: Choose the History Options command to permit Photoshop to record states out of order.
The Allow Non-Linear History check box does not permit you to undo a single state without affecting the subsequent states. For example, let's say you paint with the airbrush, smear with the smudge tool, and then clone with the rubber stamp. You can revert back to the airbrush state and then apply other operations without losing the option of restoring the smudge and clone. But you can't undo the smudge and leave the clone intact. Operations can only occur in the sequence they were applied.
Ij-1 If you revert back to a state and then apply an edit, the reverted state and all actions that fall between that state and the new edit are set off by horizontal lines running across the palette. The lines show you which operations you'll lose if you undo the first state in the group.
* Stepping through states: As I mentioned earlier, you can press -Ctrl+Alt+Z to undo the active step or Ctrl+Shift+Z to redo the next step in the list. Backstepping goes up the list of states in the History palette; forward stepping goes down. Keep in mind that if the Allow Non-Linear History check box is active, backstepping may take you to a state that was previously inactive.
* Flying through states: Drag the right-pointing active state marker (labeled in Figure 7-26) up and down the list to rewind and fast-forward, respectively, through time. If the screen image doesn't appear to change as you fly by certain states, it most likely means those states involve small brushstrokes or changes to selection outlines. Otherwise, the changes are quite apparent.
* Taking a snapshot: Every once in a while, a state comes along that's so great, you don't want it to fall by the wayside 20 operations from now. To set a state aside, choose New Snapshot or click the little page icon at the bottom of the History palette.
Ij-1 By default, Photoshop no longer displays the New Snapshot dialog box asking you to name the snapshot. If you want to name a snapshot, Alt click the New Snapshot icon. Or choose History Options from the palette menu and select the Show New Snapshot Dialog by Default option. Photoshop then presents the dialog box. In the dialog box, you also can specify whether you want to save all layers (as by default), flatten the image, or retain just the active layer. The new snapshot — as it's called — then appears in the top portion of the palette.
Tip If you turn on the Show New Snapshot Dialog by Default check box, you can circumvent the dialog box by Alt clicking the New Snapshot icon. (The state has to be active to convert it to a snapshot, so you can't drag a state and drop it onto the page icon, as you can drag-and-drop elements in other palettes.)
Photoshop lets you store as many snapshots as your computer's RAM permits. Also worth noting, the program automatically creates a snapshot of the image as it appears when it's first opened. If you don't like this opening snapshot, you can turn it off inside the History Options dialog box.
* Creating a snapshot upon saving the image: Select the Automatically Create New Snapshot When Saving box in the History Options dialog box to create a new snapshot every time you save your image.
* Saving the state permanently: The problem with snapshots is that they last only as long as the current session. If you quit Photoshop or the program crashes, you lose the entire history list, snapshots included. To save a state so you can refer to it several days from now, choose the New Document command or click the leftmost icon at the bottom of the History palette. You can also drag and drop a state onto the icon. Either way, Photoshop duplicates the state to a new image window. Then you can save the state to the format of your choice.
* Setting the source: Click to the left of a state to identify it as the source state. The history brush icon appears where you click. The source state affects the performance of the history brush, art history brush, Fill command, and eraser, if you select Erase to History. The keystroke Ctrl+Alt+Backspace fills the selection with the source state.
* Trashing states: To delete any state and those that follow, drag the state to the trash icon at the bottom of the palette. Your image updates accordingly. If the Allow Non-Linear History check box is on, clicking the trash can deletes just the active state.
If your machine is equipped with little RAM or you're working on a particularly large image, Photoshop may slow down as the states accumulate. If it gets too slow, you may want to purge the History palette. You can clear every state from the active state forward without affecting the image by Alt-clicking Clear History in the palette menu. You can also choose Edit ^ Purge ^ Histories to purge the list of states for all open documents.
You can't undo either purge command. So if you want to clear the states from the palette but have the option of choosing Undo to bring them back, choose Clear History without Alt-clicking.
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