When you create a complicated image that contains dozens of layers, your project can start hogging memory, which in turn makes it difficult to manage all the layers. Every time you create a new layer and add something to it, Photoshop gobbles up more memory. Photoshop not only has to think about what’s on that layer, but also has to remember what’s below the layer (even if that information is completely covered by the information on the layers above).
Note: Once you’ve merged two layers, it’s awfully hard to get them apart—the only way to do so is to use the History palette. However, even with the History palette, you might lose all the changes you’ve made since you merged the layers.
Whenever possible, I try to simplify my image by merging layers. This combines the layers into a single layer, and thus saves memory (because Photoshop no longer has to remember the parts of those layers that were previously being covered). The side menu on the Layers palette and the Layer menu itself give you several ways to do this:
- Merge Down: Merges the active layer into the layer directly below it.
- Merge Visible: Merges all the layers that are currently visible in the main image window.
- Merge Layers: Merges all the currently selected layers.
- Merge Group: Merges all the layers that are within the active group.
- Flatten Image: Merges all visible layers into the background, discards hidden layers, and fills empty areas with white.
If you want to know how much extra memory the layers take up as you’re modifying your image, choose Document Sizes from the menu that appears at the bottom center of the document. The number on the left should stay relatively constant (unless you scale or crop the image); it indicates how much memory your image would use if all the layers were merged together. The number on the right indicates how much memory the image is using with all the layers included. This number changes as you add and modify your layers. Keep an eye on it so that you can see how memory-intensive the different layers are.
Note: In Photoshop CS2 the memory usage indicator also appears at the bottom of the Info palette.
The number on the right might get huge if you’re using a lot of layers; however, keep in mind that you’ll know exactly how large the image will be when you flatten the layers by glancing at the left number.