Selecting the Contents of Layers

Every layer except the background includes a transparency mask . This mask tells Photoshop which pixels are opaque , which are translucent, and which are transparent. Like any mask, Photoshop lets you convert the transparency mask for any layer ” active or not ” to a selection outline. In fact, you use the same keyboard techniques that you use to convert channels to selections.

Cross-Reference

For more on converting a mask to a selection, see Chapter 4.

To convert the transparency mask for that layer to a selection outline: Ctrl+click (on the Mac, z +click) the thumbnail in the Layers palette.
To add the transparency mask to an existing selection outline: Ctrl+Shift+click ( z +Shift+click) the layer thumbnail. The little selection cursor includes a plus sign to show you that you're about to add.
To subtract the transparency mask: Ctrl+Alt+click ( z +Option+click) the layer thumbnail.
To find the intersection of the transparency mask and the current selection outline: Ctrl+Shift+Alt+click ( z +Shift+Option+click) the layer thumbnail.

If you're uncertain that you'll remember all these keyboard shortcuts, you can use Select Load Selection instead. After choosing the command, select the Transparency item from the Channel pop-up menu. (You can even load a transparency mask from another open image if the image is exactly the same size as the one you're working on.) Then use the Operation radio buttons to merge the mask with an existing selection.

Selection outlines exist independently of layers, so you can use the transparency mask from one layer to select part of another layer. For example, to select the part of the background layer that exactly matches the contents of another layer, press Shift+Alt+[ (Shift+Option+[ on the Mac) to descend to the background layer and then Ctrl+click (or z +click) the name of the layer you want to match.

The most common reason to borrow a selection from one layer and apply it to another is to create manual shadow and lighting effects. After Ctrl+clicking (or z +clicking) a layer, you can use this selection to create a shadow that precisely matches the contours of the layer itself. No messing with the Brush or the Lasso tool ” Photoshop does the tough work for you.

Now, you might think that with Photoshop's extensive range of layer styles (covered in Chapter 7), manual drop shadows and the like would be a thing of the past.

After all, you have only to choose Layer Layer Style Drop Shadow and, bang, the program adds a shadow to the layer. But the old, manual methods still have their advantages. You don't have to visit a complicated dialog box to edit a manual drop shadow. You can reposition a manual shadow from the keyboard, and you can modify a manual shadow with more precision than you can an automatic one. For an example of how to adding a cast shadow to a layer, see Chapter 8.