MaskBrush
Reference to an instance of a Brush (#Prim_vs.Brush) applied to a control to selectively apply transparency. This allows for controls, particularly images, to fade out and blend in to the background.
In the case of a linear brush with a gradient from any color to transparent, the control will be become increasingly transparent. If using a Visual Brush, transparency will be applied based on the transparency of the image used in the Visual Brush.