10.5 Editing History

An extremely useful feature in Photoshop is the ability to view your editing history, and to step back to any previous version of an image from the current editing session (note that there is a limit to how far back you can step, but this limit can be modified via a configuration window).  This feature isn’t just useful for undo-ing your mistakes.  The editing history window (see the figure below) allows you to rapidly compare different versions of an image, to see if the various operations you’ve applied in producing one from the other have resulted in an overall better or worse image. 

Fig. 10.5.1: The editing history window in Photoshop.  Clicking on any operation
temporarily reverts the displayed image to the state it was in just after completion
of the listed operation.  Editing histories are not saved with the file, so if you’re
not sure which version of an image you prefer, you should save them to separate
files before closing the original image file.  This includes history snapshots.

     In the figure above you can see the history window and the processing operations that have been applied to produce the final image.  Clicking on any of the operations listed will (temporarily) revert to the image as it was just after that operation was performed.  At any point you can also click the camera icon at the bottom of the history window to create a snapshot.  Snapshots appear in iconic form at the top of the history window, and provide a way to rapidly compare different version of an image.  You can, for example, take a snapshot after having applied many postprocessing operations, then revert to the original image, and apply some different sequence of operations to produce an alternate version of the image.  At that point you can again create a snapshot.  Now you can compare the two versions by alternately clicking on either of the two snapshots.  You can create as many snapshots as you want.
    Generating and comparing alternate versions of an image is an extremely powerful technique.  A common pitfall among novices is to forget to regularly compare the current version of the image to previous versions, to see if things are getting better or worse.  Eyes and brains both get weary after long hours at the computer, and it’s not uncommon to become so engrossed in analyzing and manipulating the current version of an image that the user forgets what the original image looked like and exactly what defects he or she was trying to fix.  The human brain quickly loses context when immersed in a visually engaging computer environment, and can easily fail to notice that the current version of an image is in some way inferior to the original (this is especially true for image qualities such as saturation and contrast).
    Since the editing history in Photoshop is lost when an image file is closed, any unresolved decisions regarding history snapshots should be archived by explicitly saving versions of the image to separate files (or as layers in a single file).  These can then be compared later as distinct images rather as than snapshots within the editing history of a single image.