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### 7.4. Mask

The Mask submenu of the Layers menu

The Mask submenu of the Layers menu contains operations that involve a layer mask: creating one, applying one, deleting one, or converting one into a selection. See the Layer Masks section for more information on layer masks and how to use them.

#### Add Layer Mask

The "Add Layer Mask" dialog

This command can be accessed from an image menubar as Layer->Mask->Add Layer Mask or from the pop-menu you get by right clicking on the active layer in the Layer Dialog.

"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.

The popup dialog allows you six choices for the initial contents of the layer mask:

White

  This option will produce full opacity when the layer mask; that is, it will be as if there were no layer mask.

Black

  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.

Layer's alpha channel

  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.

Transfer layer's alpha channel

  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.

Selection

  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.

Grayscale copy of layer

  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.

When you click on "OK" the mask symbol appears at the right of the layer thumbnail in the Layer Dialog.

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.

#### Apply Layer Mask

This command can be accessed from an image menubar as Layer->Mask->Apply Layer Mask. or from the pop-menu you get by right clicking on the active layer in the Layer Dialog.

"Apply Layer Mask" causes the layer mask to be merged with the active layer, with the transparency set by the mask applied to the layer's alpha channel. The layer mask is removed. If the active layer does not have a layer mask, the menu entry is insensitive. See the Layer Masks section for more information.

#### Delete Layer Mask

This command can be accessed from an image menubar as Layer->Mask->Delete Layer Mask or from the pop-menu you get by right clicking on the active layer in the Layer Dialog.

"Delete Layer Mask" deletes the active layer's layer mask. If the active layer does not have one, the menu entry is insensitive.

#### Mask to Selection (or Add; Subtract; Intersect)

These commands can be accessed from an image menubar as Layer->Mask, followed by Mask to Selection, Add to Selection, Subtract from Selection, or Intersection with Selection or from the pop-menu you get by right clicking on the active layer in Layer Dialog.

"Mask to Selection" transforms the active layer's layer mask into a selection mask, that is, into the image's selection. White areas of the mask become completely selected, black areas unselected, and gray areas partially selected. The other operations in this group do similar things, except that instead of completely replacing the existing selection with the selection produced from the layer mask, they either add the two selections, subtract the layer mask selection from the existing selection, or yield a selection that is the intersection of the two. The mask is not deleted.

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