"Add Layer Mask" adds a mask to the layer, bringing up a dialog in which you can set the initial properties of the mask. If the layer already has a layer mask, or cannot have one because it lacks an alpha channel, the menu entry is insensitive. See the Layer Mask section for more information.
This command can be accessed from an image menubar as
→ →or from the pop-menu you get by right clicking on the active layer in the Layer Dialog.
The popup dialog allows you six choices for the initial contents of the layer mask:
This option will produce full opacity when the layer mask; that is, it will be as if there were no layer mask.
This option will produce complete transparency, so that you will need to paint on the layer mask in order to make any part of the layer visible.
This option will cause the layer mask to produce the same transparency that is produced by the layer's alpha channel. Note that the alpha channel itself will not be altered, so for partially transparent areas, the transparency will be "squared", as it were.
This option does the same thing as the previous option, except that it also resets the layer's alpha channel to full opacity. The effect, then, is to transfer the transparency information from the alpha channel to the layer mask, leaving the layer with the same appearance as before.
This option converts the selection into a layer mask, so that selected areas are opaque, and unselected areas are transparent. If any areas are partially selected, toggling the QuickMask button will help you predict what the effects are going to be.
This option converts the layer itself into a layer mask. It is mainly useful when you plan to add new contents to the layer afterwards.
If you check "Invert Mask" at the bottom of the dialog, then the resulting mask is inverted, so that transparent becomes opaque and vice versa.
When you click on "OK" the mask symbol appears at the right of the layer thumbnail in the Layer Dialog.