Back to the Future: Fixing Editing Mistakes with the Photoshop History Brush
I'm a big advocate of non-destructive editing in Photoshop. But there are things that don't lend themselves to such techniques, and even when you can use do so, you might not always avail yourself. Some edits seems so simple and only too late do you realize you should have paid more attention. But once you've messed your image up, what do you do? The History Brush may just be able to save things.
While trying to learn Photoshop, you may have already encountered the history brush when trying to click on the regular paintbrush in the tools palette. In the standard arrangement, there's the paintbrush, then below that the clone tool, then the history brush. Both brush icons look similar and being so close below the regular paintbrush you may have clicked on it by mistake. But if you haven't done so yet, it's time to learn what it can do for you.
To understand the history brush, we have to start with understanding the history panel. This panel keeps a list of all the edits you have made to your open document. Every time you make a change, a new entry gets added to the list. If you make enough changes, the old ones will roll off the end, but within limits it keeps track of what it can. You can change how many history states Photoshop will maintain by going to Edit >> Preferences >> Performance. Each entry eats up memory of course so don't go hog wild but if you have enough RAM feel free to increase the default. If you want to make sure some particular state gets remembered, you can create a snapshot of that state by clicking on a small icon at the bottom of the history panel. It's the one in the middle, just to the left of the trashcan icon. If you want to go back to one of these prior states, either a regular entry or a snapshot, you merely need to click on the name of that state's entry in the history panel.
But if you do go back in history this way, you lose everything you've done since then. Bam!, it's almost like its 1975 again and everyone's wearing bellbottoms. All your history after the point you click on is gone and your image looks like it used to. You're back in the past now. In extreme cases, that may be what you want, but what if all you want is to undo changes so some part of your image, blending back in part of what it used to look like with what it looks like currently. This is where the history brush comes in.
The history brush gives you selective access to what is in your history panel, allowing you to paint on the current document layer using a prior history state as the source. In a way, you can think of it as being somewhat like the clone tool that sits between it and the regular paintbrush on the tools palette. But whereas the clone tools copies one portion of the current image to another portion, the history brush essentially uses a time machine to copy from the past to the present. And unlike the clone tool, you have no control over the location in the image used for the source. Source and target location are always one and the same with the history brush. You get to control the when, not the where.
To use the history brush, you first need to find the point in the past you want to copy from. To help find what you are after, just start clicking around on the entries in the history panel. With each click you will see various versions of how your image used to look. After you find the state you want, remember which one it is, and then click back on the current history state entry to get back to the present. Now click on the history brush icon in the tools palette and check its options in options bar. Here you can adjust the bush opacity, size and so forth. Generally, an opacity around thirty percent is a good starting point in order to easily blend your work but this will of course vary based on your needs. Your brush size will also depend on what you want to bring back from history. Smaller objects will call for a smaller brush. Now click on the left hand column in the history panel next to the state or snapshot you want to copy from. Then just start painting with the brush. As you paint, details from the past as it was in the selected history state will be copied onto the current layer.
There are limits though. The history brush works on individual layers, not on the image as a whole. Don't think of it as affecting the whole image of you will get confused. Because of this, you can't paint with the history brush using a state where the target layer doesn't even exist yet. That is, you can't go back in time for any given layer using a state where that layer hadn't yet been created. But on the flip side, if you do revert an image to a prior state by clicking directly on it in the history panel rather than on the left hand column, you can actually paint with any of the now "future" states on what is now the current state. In other words, those states that will be discarded when you go back to a prior state aren't entirely lost. Everything set to be lost will be grayed out in the panel but you can actually pick any one future state and paint with it on the reverted image. Pretty weird, but it works. Also, if you use any Edit >> Transform options on a layer it would probably not lend itself to use with the history brush past that point since the pixels on the resized layer wouldn't line up any more.
So the history brush can't do everything, but it does give you great tool to copy portions of the way your image used to look back to the current image. Think of it as being "back to the future" I guess. Give it a try when the occasion presents itself.