You can add a layer mask to a layer in a number of different ways, including the following:
n Select the layer; then click the Add Layer Mask icon at the bottom of the Layers palette
(indicated in Figure 9.10). This creates a fully white layer mask (so no pixels of the layer
are masked) and attaches it to the layer. Alternatively, choose Layer ➪ Layer Mask ➪
Reveal All.
n Select the layer, then Alt+click (Option+click on the Mac) the Add Layer Mask icon. This
creates a fully black layer mask; all pixels of the layer are masked, effectively making the
layer completely transparent (or hidden—whichever way you prefer to look at it). To do
the same thing via the menu, choose Layer ➪ Layer Mask ➪ Hide All.
n Make a selection using one of the selection tools (or choose Select ➪ Load Selection),
then select the layer and click the Add Layer Mask icon. Photoshop creates the layer mask
based on your selection, so any selected pixels of the layer remain opaque, while unselected
pixels are masked off and appear transparent in the layer. You can also create a
layer mask this way by choosing Layer ➪ Layer Mask ➪ Reveal Selection. To do the
opposite and create a layer mask that hides the selected areas of your selection, Alt+click
(Option+click on the Mac) the Add Layer Mask icon. Alternatively, choose Layer ➪ Layer
Mask ➪ Hide Selection.