When your image has multiple layers (as I discuss in Chapter 10), you can partially hide layers with layer masks. Layer masks and alpha channels have much in common: Layer masks are selections saved as channels, you can paint in the layer mask, you can apply filters and adjustment to the layer mask, and so on. Just keep in mind that a layer mask appears in the Channels palette only when you select its layer in the Layers palette. (In the Layers palette, you'll see the layer mask thumbnail to the right of the layer's thumbnail.) To edit a layer mask, click its thumbnail in the Layers palette, and then edit it as you would an alpha channel. (Remember to click the layer thumbnail afterward to reactivate the layer itself.)
The easiest way to add a layer mask is to make a selection of the pixels that you want visible on that layer and then click the Add Layer Mask button at the bottom of the Layers palette (third button from the left). You can also make a selection and choose LayerOLayer MaskOReveal Selection or LayerOAdd Layer MaskOHide Selection. That menu also offers Reveal All and Hide All as well as commands to disable (hide) or delete the layer mask. You can also apply the layer mask, which deletes any hidden pixels on that layer.
HBEfl A layer named Background (in italics) in the Layers palette can't have any transparent areas, so you can't add a layer mask. Not a problem! Simply double-click the layer name in the Layers palette and rename the layer. That converts it to a regular layer, which would be more than happy to accept your layer mask.
In the Layers palette, a layer mask is linked to the layer — that is, if you drag the layer, the mask moves right along with it. You can unlink/relink the mask and layer by clicking the icon/space between their thumbnails in the Layers palette.
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