You can use layer masks to hide or show parts of a layer. Layer masks are very flexible because they make it possible to show some or all of the hidden parts of your object again at any time you choose.
To add a layer mask to a layer, click on a layer to activate it, then do one of the following:
Click the Add layer mask button at the bottom of the Layers panel
Click one of the Add Mask buttons in CS4's fancy new Masks panel
After either of the above options, you can then add black to the mask to hide corresponding parts of the layer (make them transparent), add gray to partially hide corresponding parts of the layer (make them partially transparent), or add white to make the corresponding parts of the layer visible again.
Make a selection of the area you want to remain visible on the layer, then click the Add layer mask button at the bottom of the Layers palette. This automatically puts black on the layer mask that hides everything on the layer that was not selected.
You must change a background layer to a regular layer to add a layer mask to it (double-click the Background layer in the Layers palette, then click OK).
The top image in Figure 8.21 shows a layer mask that is filled with white, allowing all the layer's contents to show. The bottom image in Figure 8.21 shows the same layer mask filled with black in areas that correspond to the greenery behind the butterfly. The greenery is not visible because the corresponding parts of the layer mask are black. ...