Adobe Photoshop CS5 for Photographers - Martin Evening [32]
Figure 1.84 This picture shows the underlying tiled structure of a Photoshop image. This is the clue to how history works as economically as possible. The history stores the minimum amount of data necessary at each step in Photoshop's memory. So if only one or two tile areas are altered by a Photoshop action, only the data change in those tiles is recorded.
Figure 1.85 The accompanying table shows how the scratch disk usage can fluctuate during a typical Photoshop session. The image I opened here was 95 MB in size and 3 GB of memory was allocated to Photoshop. The scratch disk overhead is usually quite big at the beginning of a Photoshop session, but notice how there was little proportional increase in scratch disk size with each added history step.
If the picture you are working with is exceptionally large, then having more than one undo can be both wasteful and unnecessary, so you should perhaps consider restricting the number of recordable history states. On the other hand, if multiple history undos are well within the scratch disk memory limits of your system, then make the most of them. If excessive scratch disk usage does prove to be a problem, the Purge History command in the Edit Purge menu provides a useful way to keep the scratch disk memory usage under control. Above all, remember that the History feature is not just there as a mistake correcting tool, it has great potential for mixing composites from previous image states.
History brush
The history brush can be used to paint from any previous history state. To do this you need to leave the current history state as it is and select a source history state for the history brush by clicking the box in the column next to the history step you wish to sample from. In Figure 1.86 you can see how I set the Levels history step as the history source (the small history brush icon indicates which history step or snapshot is currently being used as the source). I was then able to paint with the history brush from this previous history state. The history brush therefore allows you to selectively restore the previously held image information as desired. In the Figure 1.86 example I was able to use the history brush to paint over the areas that had been worked on with the healing brush and use the history brush to restore those parts of the picture back to the previous Levels adjusted history state.
Figure 1.86 A previous history state can be selected as the source for the history brush by going to the History panel and clicking in the box to the left of the history step you want to paint from using the history brush.
Art history brush
The art history brush is something of an oddity. It is a history brush that allows you to paint from history but does so via a brush which distorts the sampled data and can be used to create impressionist type painting effects. You can learn more about this tool from the Photoshop CS5 for Photographers Help Guide that's on the DVD.
Use of history versus undo
As you will have seen so far, the History feature is capable of being a lot more than a repeat Edit Undo command. Although the History feature is sometimes described as a multiple undo, it is important not to confuse Photoshop history with the role of the Undo command. For example, there are a number of Photoshop procedures that are only undoable through using the Edit Undo command, like intermediate changes made when setting the shadows and highlights in the Levels dialog. Plus there are things which can be undone using Edit Undo that have nothing to do with Photoshop's history record for an image. For example, if you delete a swatch color or delete a history state, these actions are only recoverable by using Edit Undo. The Undo command is