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Stepping Back through Time

Since roughly the dawn of recorded time, folks begged, pleaded, and screamed at the top of their lungs for multiple undos in Photoshop. But it wasn't until Photoshop 5 that Adobe delivered what the masses craved. The payoff for the long wait was huge: Version 5 offered up the History palette, which remains the best implementation of multiple undos I've seen to this day.

Moving beyond simple backstepping, the History palette takes the whole reversion metaphor into Slaughterhouse Five territory. If you've never read the novel (or you've somehow forgotten), let me recap: Kurt Vonnegut, Jr. suggested that humans live from one moment to the next like a person strapped to a boxcar, unable to change the speed or direction of the train as it hurtles through time. In most programs that offer multiple undos, you can make the train stop and back up, but you're still strapped to it. The History palette is the first tool that lets you get off the train and transport to any point on the track — instantaneously. In short, we now have a digital version of time travel.

Here are just a few of the marvelous innovations of the History palette:

Undo-independent stepping: Step backward by pressing Ctrl+Alt+Z (z -Option-Z on the Mac); step forward by pressing Ctrl+Shift+Z (z -Shift-Z). Every program with multiple undos does this, but Photoshop's default keyboard equivalents are different. Why? Because you can backstep independently of the Undo command, so that even backstepping is undoable.

Before and after: Revert to a point in history to see a "before" view of your image and then fly forward to see the "after" view. From then on, Ctrl+Z (z -Z on the Mac) becomes a super-undo, toggling between the before and after views. The opportunities for comparing states and changing your mind are truly colossal.

Dynamic time travel: If before and after aren't enough, how about animated history? You can drag a control to slide dynamically forward and backward through operations. It's as if you recorded the operations to videotape, and now you're rewinding and fast-forwarding through them.

Sweeping away mistakes: Select a point in the history of your image and paint back to it using the history brush. You can let the mistakes pile up and then brush them away. This brush isn't a paintbrush; it's a hand broom. Want even more variety? Use the art history brush to paint back to the image using various artistic styles.

Take a picture, it'll last longer: You can save any point in the History palette as a snapshot. That way, even several hundred operations after that point in history are long gone, you can revisit the snapshot.

This is your life, Image A: Each and every image has its own history. So after performing a few hundred operations on Image A, you can still go back to Image B and backstep through operations you performed hours ago. The caveat is that the history remains available only as long as an image is open. Close the image, and its history goes away.

Undo the Revert command: Back in the days before Photoshop 5.5, you couldn't undo the Revert command. Now, the History palette tracks Revert. So if you don't like the image that was last saved to disk, you can undo the reversion and get back to where you were. Also notice that when you choose File Revert, Photoshop does not ask you to confirm the reversion. There's no reason for a warning because Revert is fully undoable.

The only thing you can't do through the History palette is travel forward into the future — say, to about three days from now when you've finished your grueling project, submitted it to your client, and received your big fat paycheck. Believe it or not, that's actually good news. The day Adobe can figure out how to do your work for you, your clients will hire Photoshop and stop hiring you.

So I ask you — Photoshop, Slaughterhouse Five, just a coincidence? Well, yes, I suppose it is. But the fact remains, you have the option of getting off the boxcar. How you make use of your freedom is up to you.

Using the traditional undo functions

Before I dive into the History palette, I should take a moment to summarize Photoshop's more traditional reversion functions. (If you already know about this stuff, skip to the next section.)

Undo: To restore an image to the way it looked before the last operation, choose Edit Undo or press Ctrl+Z (z -Z on the Mac). You can undo the effect of a paint or edit tool, a change made to a selection outline, or a special-effect or color-correction command. You can't undo disk operations, such as opening or saving. Photoshop does enable you to undo an edit after printing an image, though. You can test an effect, print the image, and then undo the effect if you think it looks awful. But to perform such an undo, you have to press Ctrl+Alt+Z (z -Option-Z on the Mac) to backstep through history.

Revert: Choose File Revert to reload an image from disk. In most programs, this is the last-resort function, the command you choose after everything else has failed. But in Photoshop, it's a very useful tool. Forget what the image looked like last time you saved it? Choose the Revert command. Don't like it? Press Ctrl+Z (z -Z on the Mac) to undo it. That's right, you can undo a reversion — what'll they think of next?

Selective reversion: To revert a selected area to the way it appeared when it was first opened — or some other source state identified in the History palette — choose Edit Fill or press Shift+Backspace (Shift-Delete on the Mac). Then select History from the Use pop-up menu and press Enter or Return.

The erasers: Drag in the background layer with the eraser tool to paint in the background color. You're essentially erasing the image back to bare canvas. Or apply the eraser to a layer to delete pixels and expose underlying layers.

Where warranted, I explain these functions in greater detail in the following sections. But first, the next few paragraphs look at the central headquarters for reversion in Photoshop, the History palette.

The History palette

Choose Window History to view the History palette, annotated with the palette menu in full view in Figure 7-25. 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.

Figure 7-25: The History palette records each significant event as an independent state. To return to a state, just click it.

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. Although it is similar in usage, don't confuse this type of state with the states in Photoshop CS's new Layer Comps palette. We'll be digging into the Layer Comps palette in Chapter 12.

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 (z -Z on the Mac) or choose Edit Undo State Change.

That one action — clicking 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. To change this behavior, choose Edit Preferences General or press Ctrl+K (z -K on the Mac), which opens the Preferences dialog box, and then enter your preferred number of undoable operations in the History States option box. If your computer is equipped with 128MB of RAM or less, 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 — say, 1GB or more — turn it up, baby. Should you be so inclined, the History palette can hold up to 1,000 states. That's probably more than you want to use — after all, some states take up an awful lot of memory — but when working on a single image, 100 states may on rare occasions be comfortable.

Undone states: When you revert to a state by clicking it, every subsequent state is dimmed to show that it's been undone. You can redo a dimmed state simply by clicking it. But if you perform a new operation, all dimmed states disappear. You have one opportunity to bring them back by pressing Ctrl+Z (z -Z); if you perform another new operation, the once-dimmed states are gone for good. For an exception to this behavior, see the very next paragraph.

Working with nonsequential states: If you don't like the idea of losing your undone states — every state is sacred, after all — choose the History Options command in the palette menu and select the Allow Non-Linear History check box (see Figure 7-26). 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.

Figure 7-26: Choose History Options from the History palette menu and select Allow Non-Linear History to permit Photoshop to keep states that you have undone.

Stepping through states: As I mentioned earlier, you can press Ctrl+Alt+Z (z -Option-Z) to undo the active step or Ctrl+Shift+Z (z -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. So bear 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 7-25) 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. To rename a snapshot after you create it, just double-click its name at the top of the History palette and enter a new one. Or you can name a snapshot as you create it by pressing the Alt key (or Option on the Mac), clicking the little page icon, and entering a name in the dialog box.

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 change this behavior by turning off Automatically Create First Snapshot in 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. This is useful if you find yourself venturing down uncertain roads from one save to the next, and want the ability to backstep not only to the last saved state (which you can do by choosing File Revert), but to the one before that and the one before that.

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 (z -Option-Delete on the Mac) fills a selection with the source state.

Trashing states: 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. To delete any state as well as those before it, 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.

Painting away the past

The History palette represents the regimental way to revert images in Photoshop. You can retreat, march forward, proceed in linear or nonlinear formation, capture states, and retire them. Every state plays backward in the same way it played forward. It's precise, predictable, and positively by the book.

But what if you want to get free-form? What if you want to brush away the present and paint in the past? In that case, a palette isn't going to do you any good. What you need is a pliable, emancipated, free-wheeling tool.

As luck would have it, Photoshop offers three candidates: the eraser, the history brush, and the art history brush. The eraser washes away pixels to reveal underlying pixels or exposed canvas. The history brush takes you back to a kinder, simpler state; the art history brush does the same but enables you to paint using special artistic effects. Although the functions of these tools overlap slightly, they each have a specific purpose, as becomes clear in the following sections.

The eraser tool

When you work with the eraser, you can select from three eraser styles, all available from the Mode pop-up menu in the Options bar, pictured in Figure 7-27. These are Brush, Pencil, and Block. Block is the ancient 1616-pixel square eraser that's great for hard-edged touch-ups. The other options work exactly like the tools for which they're named.

Figure 7-27: When the eraser is selected, the Mode pop-up menu offers a choice of eraser styles rather than the brush modes available to the painting and editing tools.

In addition to the Mode settings, the Options bar provides access to the Brush option, the Opacity and Flow values, and the airbrush icon, all of which work as described in Chapter 5. All options are available when using the Brush-style eraser, none are applicable to the Block style, and Flow and airbrush dim when painting with a Pencil-style eraser.

Although the eraser is pretty straightforward, there's no sense in leaving any stone unturned. So here's everything you ever wanted to know about the art of erasing:

Erasing on a layer: When you're working on the Background layer, the eraser merely paints in the background color. Big whoop. What distinguishes the eraser tool from the other brushes is layers. If you drag on a layer and deselect the Lock buttons for transparency and image pixels in the Layers palette, the eraser tool removes paint and exposes portions of the underlying image. The eraser tool suddenly performs like a real eraser.

Erasing lightly: Change the Opacity setting in the Options bar to make portions of a layer translucent in inverse proportion to the Opacity value. For example, if you set the Opacity to 90 percent, you remove 90 percent of the opacity from the layer and, therefore, leave 10 percent of the opacity behind. The result is a nearly transparent stroke through the layer.

Erasing versus using layer masks: As described in the "Creating layer-specific masks" section of Chapter 12, you can also erase holes in a layer using a layer mask. But unlike the eraser — which eliminates pixels for good — a layer mask doesn't do any permanent damage. On the other hand, using the eraser tool doesn't increase the size of your image as much as a layer mask does. (You can argue that any operation — even a deletion — increases the size of the image in RAM because the History palette has to track it. But even so, the eraser remains more memory-efficient than a layer mask.) With the speed and power of most modern computers, though, this is hardly the issue that it once was.

Erasing with the pencil: When you work with the pencil tool — not the Pencil mode but the actual pencil tool — Photoshop presents you with an Auto Erase check box in the Options bar. Turn it on to draw in the background color any time you click or drag on a pixel that is already colored in the foreground color. This technique can be useful when you're drawing a line against a plain background. Set the foreground color to the color of the line; set the background color to the color of the background. Then use the pencil tool to draw and erase the line until you get it just right. I use this feature all the time when preparing screen shots. Adobe engineers once called the Auto Erase check box their "ode to Fatbits," from the ancient MacPaint zoom function.

Erasing to history: Press Alt (Option on the Mac) as you drag with the eraser to paint with the source state identified by the history brush icon in the History palette. It's like scraping away the paint laid down by the operations following the source state. For example, in Figure 7-28, I used the rectangular marquee tool along with the eraser to paint a frame around a frame job inside this picture of a picture window. I began by creating a new layer and then selecting various rectangular areas and filling them with black or gray. In the second image, I selected a few more areas and filled them with white. At this point, however, I decided the white was too garish and elected to erase portions of it using history. I clicked in front of the last state before I added the white to make it the source state. Then I pressed Alt (or Option) and painted with the eraser to erase through the white rectangles while leaving the black and gray rectangles unchanged.

Figure 7-28: I created a geometric frame by selecting areas with the rectangular marquee tool and filling them with black or gray (left). Then I added a series of white rectangles (middle). By identifying the state just before the first white rectangle as the source state, I was then able to use the history eraser function to erase away the white and leave the black and gray unharmed.

Instead of pressing Alt, you can select the Erase to History check box in the Options bar. In this case, dragging with the eraser reverts and Alt-dragging (Option-dragging on the Mac) paints in the background color or erases the active layer.

The history brush

Painting with the history brush tool — which you can select from the keyboard by pressing the Y key — is like painting with the eraser when Erase to History is turned on. Just drag with the history brush to selectively revert to the source state targeted in the History palette. You also can vary the translucency of your strokes using the Opacity setting in the Options bar. But that's where the similarities end. Unlike the part-time history eraser, the dedicated history brush lets you take advantage of brush modes. By choosing a different brush mode from the Mode pop-up menu in the Options bar, you can mix pixels from the changed and saved images to achieve interesting, and sometimes surprising, effects.

I advise you to get in the habit of using the history brush instead of using the eraser's Erase to History function. Granted, with Pencil and Block, the eraser offers more styles. But when weighed against brush modes, these styles aren't much of an advantage. The history brush is also more intuitive because its icon matches the source state icon in the History palette.

STEPS: Brushing to a Parallel Time Line

Open the image you want to warp into the fourth dimension. I began with an embellished map of Japan, shown in Figure 7-29. Japan is a wacky combination of 17th-century cultural uniformity, 1950's innocence, and 21st-century corporate imperialism, so it struck me as a perfect subject for my compound-time experiment.

Figure 7-29: I created this image by compositing a stock art image of Japan against a soft cloud background.

Apply a couple of filters. I chose Filter Pixelate Mosaic and set the Cell Size value to 20 pixels. Then I applied Filter Stylize Emboss with a Height of 5 pixels and an Amount of 200 percent. Figure 7-30 shows the results of each.

Figure 7-30: The results of applying the Mosaic (left) and Emboss (right) filters. Both effects are overstated, so I decided to undo them and then paint them back in with the history brush.

Choose the History Options command from the History palette menu. Then turn on the Allow Non-Linear History check box and press Enter or Return.

Click the Open item in the History palette. This reverts the image to the state at which it existed when you first opened it. But thanks to nonlinear history, Photoshop retains the filtered versions of the image just in case you'd like to revisit this timeline in the future.

Click in front of the first filter effect in the History palette to make it the source state. In my case, I clicked in front of the Mosaic item.

Select the history brush and start painting. As you do, you'll paint with the filtered version of the image. For my part, I set the blend mode to Darken and painted around the island country to give it a chunky digital edge, as in the first example of Figure 7-31.

Figure 7-31: Having restored the unfiltered version of the image, I set the brush mode to Darken and painted in the Mosaic state with the history brush (left). Then I changed the brush mode to Overlay and brushed in the Emboss state (right).

Switch the source state by clicking in front of the second filter effect. Naturally, I clicked in front of the Emboss item.

Paint again with the history brush. This time, I changed the brush mode to Overlay and painted randomly over Japan and the surrounding ocean. The result appears in the second example of Figure 7-31.

After you finish, you can toss the filtered states. This alternate timeline has served its purpose. Or keep it around as a snapshot to come back to later.

The art history brush

The art history brush lets you create impressionistic effects with the aid of the History palette. To get a sense of how it works, open any old file. Me, I opened the simple still life featured in Figure 7-32. Press D to get the default foreground and background colors, select the standard brush tool, and paint willy-nilly all over your image. That's right, make a total mess of it, as I did in the first example in Figure 7-33. You think it's ruined? Aha, it is not. It is now ready for you to turn into an impressionistic masterpiece.

Figure 7-32: A simple still life that I shot a few years back using what was then a state-of-the-art digital camera but is now considered so low-res, it makes the digital angels cry.

Figure 7-33: After painting a random series of black brushstrokes all over my image (left), I selected the art history brush and painted in a rough translation of the original (right).

Select the art history brush, which shares a flyout menu and keyboard shortcut (Y) with the history brush. Bring up the History palette and make sure the first snapshot is identified as the source state (assuming that you haven't made any unauthorized changes to the image since you opened it). Now paint inside your black image. Each stroke reveals a bit of your original photograph in painterly detail, as illustrated in the second image in Figure 7-33.

Like the history brush, the art history brush paints from the source state specified in the History palette. But it does so by painting tens or even hundreds of tiny brushstrokes at a time, swirling and gyrating according to settings you select in the Options bar. Many of these settings you've seen several times before. As shown in Figure 7-35, you have the standard Brush controls, a reduced Mode option, and the tried-and-true Opacity value. But starting with the Style option, the art history brush goes its own way:

Figure 7-35: Choose an option from the Style menu to change the type of strokes applied by the art history brush.

Style: The art history brush paints with randomly generated worms and corkscrews of color. You can decide the basic shapes of the creepy crawlies by selecting an option from the Style pop-up menu, displayed in Figure 7-35. Combine these options with different brush sizes to vary the detail conveyed by the impressionistic image. Tight styles and small brushes give you better detail; Loose styles and big brushes produce less detail.

Area: This value defines the area covered by a single spot of corkscrews. Larger values generally mean more corkscrews are laid down at a time; reduce the value for a sparser look. You can get some very interesting effects by raising the Area value to its maximum, 500 pixels, and mousing down inside the image without moving the cursor. Watch those worms writhe.

Tolerance: This value limits where the art history brush can paint. A value of 0 lets the brush paint anywhere; higher values let the brush paint only in areas where the current state and source state differ dramatically in color. High Tolerance values are especially useful for achieving the black velvet look that's always in vogue because it's perpetually on the verge of experiencing a major revival. Figure 7-36 shows a couple of examples painted against black using different Style settings, namely Dab and Tight Long, combined with high Area and Tolerance values. For effect, I applied the Texturizer filter set to Bricks to the second image.

Figure 7-36: To get the black velvet effect, fill your image with black and raise the Area and Tolerance values in the Options bar. I set the brush size to 10 pixels and clicked maybe a dozen times inside each image with the art history brush.

If impressionism interests you, I encourage you to experiment. If not, give this brush the slip. I happen to think it's pretty nifty (and surprisingly well implemented), but it definitely falls under the heading of Whimsical Creative Tools to Play with When You're Not under a Deadline.

Source state limitations

Photoshop displays the cancel cursor if you try to paint with the history brush or art history brush using a source state that's a different width or height than the current image. One pixel difference, and the source state is a dead issue. This same restriction applies to Edit Fill, Ctrl+Alt+Backspace (z -Option-Delete on the Mac), and any other history technique.

You may also see the cancel cursor if the layer is locked or the source state lacks an equivalent layer. To find out exactly what the problem is, click the image with the cancel cursor to display an explanatory alert message. If the problem relates to the source state, move the source state icon in the History palette to a point after you modified the width or the height of the image. The crop tool and the Image Image Size, Canvas Size, Rotate Canvas, and Crop commands can mix up the history brush. If you applied one of these operations in the very last state, you have to backstep before that operation or find some alternative to the history brush.

It's not a big deal, though. Give it some time and you'll learn to anticipate this problem. In the case of my experiment with Japan a couple sections ago, I made sure to resample and crop the image before I began applying filters. Get the dimensions ironed out, and then start laying down your time trails.