As you apply changes to your document, the histories feature creates snapshots and stores them in RAM or on your hard disk that you have assigned as the scratch disk. Each history state is given a name to correspond with the command applied to the document. When the maximum number of history states has been reached, as specified in Preferences ^ General, in order to make room for the next history state any references to the first history state is erased from the History palette, as well as from memory. However, these snapshots, or history states, are only temporary. When you close the document or quit Photoshop, all history states are erased from memory.
You might ask, what is the point of keeping snapshots of your work in progress? Well, as long as the document is open, its history states can be accessed from the History palette and used as an aid to editing. For example, when you select a history state, the document reverts to that state—not only visually but also with respect to all other properties, such as layers and channels and visibility; these are restored.
You can make use of any one of the available history states to paint from by electing it as the source for the History Brush tool. To give another brief example, you can run the Smart Sharpen filter (Filters ^ Sharpen ^ Smart Sharpen) on a layer containing a portrait, select the History Brush tool, elect the previous state—thus undoing the effect of Smart Sharpen—and paint selectively over the eyes, nose, and lips from the Smart Sharpen state. This way, only the areas you paint will take on the sharpening, thus avoiding the sharpening of the skin or hair. You also have control over the opacity of the brush strokes, the blend mode, and, if you employ a pen and tablet, pressure sensitivity.
To make the most of the history feature, here are a few hacks relating to the History palette.
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Adobe Photoshop can be a complex tool only because you can do so much with it, however for in this video series, we're going to keep it as simple as possible. In fact, in this video you'll see an overview of the few tools and Adobe Photoshop features we will use. When you see this video, you'll see how you can do so much with so few features, but you'll learn how to use them in depth in the future videos.