The Use All Layers option enables you to create a selection based on pixels from different layers (see Chapter 12 for more about layers). Returning to my previous landmass example, suppose you set Europe on one layer and North America on the layer behind it so the two continents overlap. Normally, if you clicked inside Europe with the magic wand, it would select an area inside Europe without extending out into the area occupied by North America on the other layer. Because the wand doesn't even see the contents of other layers, anything outside Europe is an empty void. We're talking pre-Columbus Europe here.
If you select Use All Layers, though, the situation changes. Suddenly, the wand can see all the layers you can see. If you click on Europe, and if North America and Europe contain similar colors, the wand selects across both shapes.
Mind you, while the Use All Layers option enables the wand to consider pixels on different layers when creating a selection, it does not permit the wand to actually select images on two separate layers. Strange as this may sound, no selection tool can pull off this feat. Every one of the techniques explained in this chapter is applicable to only a single layer at a time. Use All Layers merely allows the wand to draw selection outlines that appear to encompass colors on many layers.
What good is this? Well, suppose you want to apply an effect to both Europe and North America. With the help of Use All Layers, you can draw a selection outline that encompasses both continents. After you apply the effect to Europe, you can switch to the North America layer — the selection outline remains intact — and then reapply the effect.
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