You can change the curvature of the cropping-mask via the curve window.
By clicking on the curve, additional adjustment points can be added so that the curve can be variably adjusted.
This makes it possible to adjust the display-strength of the foreground-pixels and the background-pixels as required.
The foreground-pixels are displayed on the right side of the curve and the background-pixels on the left side.
The vertical axes determine the strength or transparency with which the respective pixel-area is displayed in the image.
If a curve point is moved downwards in the vertical direction, the transparency of this pixel area increases..