A round window: one picture in the middle, the other around the edges.
Distance from the centre of the frame decides which you see, and raising that distance to a power reshapes the falloff. A low power spreads the centre picture across most of the frame; a high one keeps it small until close to the border.
hardness decides whether it is a window or a fade. At zero, the transition is a gradient stretched across the entire frame. Near one, it becomes a crisp boundary.
How distance is measured changes the shape entirely. Measure it the ordinary way and you get a circle. Measure it by whichever direction is furthest and you get a rectangle; add the two directions together and you get a diamond.
The two pictures are blended together first, and the mask then chooses between the centre picture and that blend.