This filter takes an source image and a mask image and sets the alpha of the source image depending on the brightness of the mask. So if the mask is black, the source image will be transparent and if the mask is white then the source image will be opaque. If the mask is gray then the source image will be translucent.
This filter is similar to the Gradient Mask. It takes a source image and a mask image and sets the alpha of the source image according to the brightness of the mask. It offers two extra parameters, a transition parameter and a softness parameter. If the transition parameter is set then the source image will be transparent in places where the mask is brighter than the transition value. The softness parameter is used to soften the edges.