Photoshop has been criticized for its lack of a "pyramid-style" file format, such as Live Picture's IVUE or xRes's LRG. Both IVUE and LRG store an image several times over at progressively smaller and smaller image sizes, called downsamplings. For example, the program would save a full view of the image, a 50 percent view, a 25 percent view, and so on. Live Picture or xRes can then load and edit only the portion of the image visible on screen, greatly accelerating functions.
Photoshop's alternative is image caching. Rather than saving the downsamplings to disk, Photoshop generates the reduced images in RAM. By default, the Cache Levels value is set to 4, the medium value. This means Photoshop can cache up to four downsamplings — at 100, 50, 25, and 12.5 percent — which permits the program to apply operations more quickly at reduced view sizes. For example, if you choose a color correction command at the 50 percent view size, it previews much faster than normal because Photoshop has to modify a quarter as many pixels on screen.
However, Photoshop must cache downsamplings in RAM, which takes away memory that could be used to hold the image. If you have lots of RAM (128MB or more)
and you frequently work on large images (20MB or larger), you'll probably want to raise the value to the maximum, 8. The lost memory is worth the speed boost. If you have little RAM (say, 16MB or less) and you usually work on small images or Web graphics (4MB or smaller), you may want to reduce the Cache Levels value to 1 or 2. When files are small, RAM is better allocated to storing images rather than caching them.
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