A mask is a very important tool when working with a picture with a number of layers. With the mask it is possible to cause certain parts in a layer to be hidden from view (transparent) or partially hidden (partially transparent). The mask is very useful when you want to present only certain parts in a layer but do not wish to erase the parts you do not wish to present, but rather hide them. Since the hidden parts have not been erased (they exist in the layer, but are hidden), it will always be possible to present them again whenever you wish. The mask is part of the layer which is why it is relevant only if the layer is displayed (Visibility On) and the mask is active (Mask On). A normal picture is made up of three color channels, RGB. The mask adds to the picture a fourth channel (called Alpha channel) and is displayed by a separate picture with 256 hues. If a layer has a mask, it is possible to see its reduced picture (its thumbnail) in the Layers Window under the Layer Properties card-index. The colored areas on the mask represent the visible parts on the layer and the dark areas represent the transparent parts (the darker the color of a mask is in a certain area, the more transparent that part will be. The darkest parts, the black areas, represent the areas on the layer that will not be visible at all-they will be hidden, that is entirely transparent).
* Masks can be applied to Parent Layers only. Masks Menu