Masks are used to selectively display portions of a layer. In Fable, this is done by associating another layer as the mask.
You can establish a mask relationship between layers in the timeline. Hover over the layer you want to be the mask, and find the mask icon (half-filled circle). Click the icon and drag the arrow that appears to the layer you would like to mask. When you let go of the arrow, there will be two things will change in your timeline:
The masking layer's mask icon will be solid yellow
The masking layer's visibility will be turned off
By default, Fable will establish an additive mask association, causing the masked layer to only display the portions that overlap with the masking layer.
Even though the masking layer is hidden, it can still be animated or have effects associated with it. Use a combination of keyframes and effects on masking layers to create complex designs.
If you would like the masked layer to be invisible in portions it overlaps with the masking layer, you need to invert the mask. In Fable, this is done by hovering over the masking layer's mask icon (should be solid yellow) and clicking the invert mask icon (square with a white circle inside) that appears in the pop-up. If you would like to reset the mask to an additive mask, simply click the icon again (square with a grey circle inside).
If you would like to remove a mask, hover over the masking layer's mask icon (should be solid yellow) and click the
x icon that appears in the pop-up.
In other software, masks are sometimes referred to as mattes (luma/alpha matte) or are properties within a layer.