7.5. Transparency

The Transparency submenu of the Layer menu

The Transparency submenu of the Layers menu contains operations that use or affect the alpha channel of the active layer.

Add Alpha Channel

This command can be accessed from an image menubar as Layer->Transparency->Add alpha Channel.

"Add Alpha Channel" adds an alpha channel to the active layer. Without an alpha channel, a layer cannot have transparency or a layer mask. If the active layer already has an alpha channel (and unless it is the bottom layer of the image, it must), then the menu entry will be insensitive.

Color to Alpha

This menu entry, accessed from an image menubar as Layer->Transparency->Color to Alpha, is an alternative way to activate the Color to Alpha filter.

Semi-flatten

This menu entry, accessed from an image menubar as Layer->Transparency->Semi-flatten, is an alternative way to activate the Semi-flatten filter.

Threshold Alpha

The "Threshold Alpha" dialog

This operation can be accessed from an image menubar as Layer->Transparency->Threshold Alpha.

"Threshold Alpha" converts partial transparency into all-or-none transparency, by applying a user-specified cutoff threshold to the layer's alpha channel. It only works on alpha-channel-posessing layers from RGB images: if the image is Grayscale or Indexed, or the layer lacks an alpha channel, the menu entry will be insensitive. If "Keep transparency" has been toggled on in the Layers dialog, the command will show an error message and refuse to operate.

Alpha to Selection [or Add, Subtract, Intersect]

These commands can be accessed from an image menubar as Layer->Transparency, followed by Alpha to Selection, Add to Selection, Subtract from Selection, or Intersection with Selection.

"Alpha to Selection" transforms the active layer's alpha channel (which encodes transparency) into the image's selection. Opaque areas become fully selected, transparent areas unselected, and translucent 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 alpha channel, they either add the two selections, subtract the alpha-selection from the existing selection, or create a selection that is the intersection of the two.