Adobe Photoshop CS5 for Photographers - Martin Evening [31]
The History panel
The History panel displays the sequence of Photoshop steps that have been applied during a Photoshop session and its main purpose is to let you manage and access the history steps recorded by Photoshop. To revert to a previous step, drag the slider up the list of history steps or, alternatively, you can click directly on a specific history step. For example, in Figure 1.81 I carried out a simple one-step undo by clicking on the one but last history step.
Figure 1.81 A previous history step can be selected by clicking on the history step name in the History panel. In its default configuration, you will notice how when you go back in history, the history steps that appear after the one that is selected will appear dimmed. If you have moved back in history, and you then make further edits to the image, the history steps after the selected history step will become deleted. However, you can change this behavior by selecting Allow Non-linear History in the History panel options (see Figure 1.82).
History settings and memory usage
When the maximum number of recordable history steps has been reached, the earliest history step at the top of the list is discarded. With this in mind, the number of recorded histories can be altered via the Photoshop Performance preferences (Figure 1.83). Note that if you reduce the number of history states (or steps) allowed, any subsequent action will immediately cause all earlier steps beyond this new limit to be discarded. To set the options for the History panel, mouse down on the fly-out menu and select History Options… (Figure 1.82). I'll come on to the Snapshot settings shortly, but at this stage you may want to consider enabling ‘non-linear’ history. This will allow you to select a previous history step, but instead of undoing those steps between the earlier state and the latest and deleting them, non-linear history allows you to shoot off in a new direction and still preserve all the original history steps. ‘Make Layer Visibility Changes Undoable’ makes switching layer visibility on or off a recordable step in history, although this can be annoying when turning the layer visibility on or off prevents you from using undo/redo to undo the last Photoshop step.
Figure 1.83 The number of recorded history states can be set via the History & Cache section of the Performance preferences dialog.
Conventional wisdom would suggest that a multiple undo feature is bound to tie up vast amounts of scratch disk space to store all the previous image steps. However, proper testing of history indicates that this is not really the case. It is true that a series of global Photoshop steps may cause the scratch disk usage to rise, but localized changes will not. This is because the history feature makes clever use of the image cache tiling structure to limit any unnecessary drain on the memory usage. Essentially, Photoshop divides an image up into tiled sections and the size of these tiles can be set in the Performance preferences (see Chapter 2 for advice on how to optimize the cache tile size). Because of the way Photoshop images are tiled, the History feature only needs to memorize the changes that take place in each tile. If a brush stroke takes place across two image tiles, only the changes taking place in those tiles need to be updated (Figure 1.84). If a global change takes place, such as a filter effect, the whole of the image area is updated and the scratch disk usage rises accordingly. A savvy Photoshop user will want to customize the History feature to record a reasonable number of histories, while at the same time be aware of the need to change this setting if the history usage is likely to place too heavy a burden on the scratch disk. The Figure 1.85 example demonstrates that successive histories need not consume an escalating amount of memory. After the