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### 2.3. Layer masks

Overview

  A transparency mask can be added to each layer, it's called Layer mask. A layer mask has the same size and same pixel number as the layer to which it is attached. Every pixel of the mask can then be coupled with a pixel at the same location in the layer. The mask is a set of pixels in graytone on a value scale from 0 to 255. The pixels with a value 0 are black and give a full transparency to the coupled pixel in the layer. The pixels with a value 255 are white and give a full opacity to the coupled pixel in the layer.

  To create a layer mask start with a right click on the layer to call the context menu and select Add layer mask in the menu. If the menu item is grayed first select Add Alpha channel in the same menu. A dialog appears where you can initialize the content of the mask:

  - White: the mask has no effect, all layer pixels are full visible.

  - Black: the mask gives a full transparency to the layer which becomes invisible.

  - Layer's Alpha channel: the mask is initialized according to the content of layer Alpha channel. If the layer still contains transparency it's copied in the mask.

  - Selection : the mask is initialized according to pixel values found in the selection.

  - Grayscale copy of layer: the mask is initialized according to pixel values of the layer.

  When the mask is created it appears as a thumbnail right to the layer thumbnail. By clicking alternatively on the layer and mask thumbnail you can enable one or other. The active item is highlighted by a white border (which is not well visible around a white mask). To view mask content instead of its effect in image window, press Ctrl Alt and then on mask thumbnail. Its borber turns to red. To return to normal view redo last operation.

  | Caution
  The layer mask acts like a transparency mask on a particular layer. Since you have many layers it becomes different from the image Alpha channel which represents transparency applied to the whole image.

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