Add Layer Mask

Will add a layer mask to the currently active layer. A layer mask is a one to control the transparency of the current layer . The mask itself is a grayscale "image" where black pixels will make the layer totally transparent and white pixels will make it opaque.

Adding a mask to a layer has the same effect like if you could add a second Alpha Channel for that Layer. In fact, both are applied to the layer in the same way, first Alpha, then Mask.

The "Fill" frame enables you to choose the initial fill of the layer mask. Black which will make the layer totally transparent, White which will make the layer totally opace or the alpha channel of the layer. When you choose the alpha channel the transparency of each pixels will be mapped to a grayscale value in the mask. If you want to further manipulate the transparency in a layer this can be a very good option (e.g. maybe you have softly erased some parts of the image and want to continue to manipulate the transparency).

To paint on the layer mask you have to activate it, you do that by clicking on the thumbnail image of the mask. However you will not see the mask, you will just see the result of your mask applied to the layer. To see the mask you have to Alt-click on the thumbnail. If you want to see you layer without the mask applied, Ctrl-click on the mask thumbnail. Remember to click once again with the same modifier key to exit the current "preview" state.