by Julieanne Kost
To add a mask to a layer, click on the layer or vector mask icon in the Masks panel. If you prefer to use the Layers panel “Add Layer Mask” icon, click once to add a layer mask, click again to add a vector mask, or if you want only the vector mask, Command (Mac) / Control (Win) -click the mask icon. To add a layer mask which automatically hides the selection (as oppose to reveals it as it does by default), Option (Mac) / Alt (Win) -click the icon. Of course with Photoshop CS4, you can always invert a layer mask using the new Invert button on the Masks panel!