Controlling Photoshop History
These days, almost all applications include some form of support for basic Undo and Redo operations. Some provide more than others, but it's almost expected to have at least something. With its History Panel, Adobe Photoshop excels at Undo / Redo but very few of us take full advantage of its capabilities.
Over on the right-hand side of the default workspace in Photoshop you'll find a column of panels to help you manage what you are working in. Since Photoshop is a complex product, not everything fits at the same time. In order to access the History panel you'll need to click on its icon in panel Dock. Most of what's in the History panel consists of a list of what you've done during your current edit session. This list of History States allows you not only to see how your image got from where it started to what it looks like now, but also to undo things back to any previous state on that list. Click on a state and everything you did after that point is instantly undone. Until you do something else and start rewriting history, you'll still see what you've undone grayed out in the list. If you want to redo all or some of what you previously did, just click on that state in the list. You can freely click on any state in the list to step forward or back in what you've done. It makes for a very visual way of dealing with undo / redo.
Most people know this much about the History panel. But it can do a lot more than just this. At the very top of the list of History States is a thumbnail of what your image looked like when you first opened it for edit. Technically known as a Snapshot, you can somewhat think of this as being a History State that won't roll off the edit list no matter how many changes you make. You can create additional Snapshots if you want to also. Just click on the New Snapshot icon at the bottom of the History palette. If you hold down the Alt key (Option key on Mac) when doing this you'll get a dialog allowing both to name the Snapshot and to base it on either the full document with all its layers, just the current layer, or a merged version of all layers. The default is the full document with layers. Snapshots can be quite useful but as with everything, they don't come for free and do occupy memory (real memory or Scratch Disk). As such, you should only create one when needed.
You can also use the entries in the History panel as the basis for creating new documents. To do so, simply drag a History State or Snapshot to the New Document from Current State icon at the bottom of the History Panel. Don't make the mistake of dragging it to the New Layer icon on the Layers panel; you want the New Document icon in the History panel.
There's also a Delete (trashcan) icon as well to let you delete history but generally you won't actually need it. In a default configuration, it will delete not only the History State you select, but all those that come after it as well. To change this behavior, you'll need to enable something known as "Non-Linear History," which brings me to the History Options dialog.
In the upper right corner of the History panel you'll find a none-too-obvious button that when clicked reveals a fly-out menu. Most panels in Photoshop have them. Once you know where Adobe hid them you'll start spotting them all over but if you're new to Photoshop you may not have even noticed they were there yet. When you click on the one for the History panel you'll get a menu with alternate ways of doing most of what I have already described. At the bottom of this menu though is something you can't get to any other way: the History Options dialog.
There aren't a lot of choices in the History Options dialog, but there are a couple that are worth pointing out.
The first of these is "Allow Non-Linear History." Turning this on allows you to change or undo commands you have executed without everything that came after. Go to any History State and change whatever you want and rather than obliterating everything below this in the list, a new entry will be created at the bottom of the list to represent your changes. With Non-Linear History you can also delete a State without disrupting what you've done since. Just drag it to the trashcan icon at the bottom of the panel as you normally would and marvel at how Photoshop preserved everything but that. Pretty slick. But you probably won't want to leave Non-Linear turned on all the time since generally going cleanly back to a prior state makes most sense.
The second is "Make Layer Visibility Changes Undoable." By default, simply making a layer visible or invisible doesn't constitute a new State in the History panel. This works well most of the time but can sometimes seem confusing if do a lot of toggling layers on and off. Be careful though since this can consume space in the History panel undo list.
Speaking of space, one setting that affects the History panel isn't in the History Options at all. For historical reasons, Edit >> Preferences >> Performance settings is where you'll need to go to control how many entries are on the History panel. By default, you get 20 which will be enough for most of us, but if you do a lot of editing and have the memory to do it you can increase this to whatever you'd prefer (well, anything up to 1000 entries). Before you go bumping this up though, remember that you can save States as Snapshots too.
History was never one of my favorite classes when I was in school. A lot of memorizing dates and names basically. When working with Photoshop though, it pays to learn about history.