Photoshop’s incredible selective Undo

Photoshop’s incredible selective Undo

Here’s one major difference between Photoshop and other programs. Almost
all programs have some form of Undo, enabling you to reverse the most
recent command or action (or mistake). Like many programs, Photoshop
uses the Ô+Z/Ctrl+Z shortcut for Undo/Redo and the Ô+Option+Z/Ctrl+Alt+Z
shortcut for Step Backward, which allows you to undo a series of steps (but
remember that you can change those shortcuts, as described in Chapter 3).
Photoshop also has, however, a couple of great features that let you partially
undo.
Painting to undo with the History Brush
You can use Photoshop’s History Brush to partially undo just about any filter,
adjustment, or tool by painting. You select the History Brush, choose a history
state (a stage in the image development) to which you want to revert,
and then paint over areas of the image that you want to change back to the
earlier state.
You can undo as far back in the editing process as you want, with a couple of
limitations: The History panel (where you select the state to which you want
to revert) holds only a limited number of history states. In the Photoshop
Preferences➪General pane, you can specify how many states you want
Photoshop to remember (to a maximum of 1,000). Keep in mind that storing
lots of history states takes up computer memory that you might need for processing
filters and adjustments. That can slow things down. The default of 20
history states is good for most projects, but when using painting tools or
other procedures that involve lots of repetitive steps (such as touching up
with the Dodge, Burn, or Clone Stamp tools), a larger number (perhaps as
high as 60) is generally a better idea.

The second limitation is pixel dimensions. If you make changes to the image’s
actual size (in pixels) with the Crop tool or the Image Size or Canvas Size
commands, you cannot revert to prior steps with the History Brush. You can
choose as a source any history state that comes after the image’s pixel
dimensions change but none that come before.
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