Hi Sixburg!
one of the ways to create a rectangular mask is:
Clone your image,
load PixelMath process, write 0 (Zero) in the top line and drag the triangle
over the clone image. This results in a black image.
Then create a preview in the original image with your mouse, resize and move it
whereever you want to have it.
Than start the script: SubstitutewithPreview,
in the Source line you enter the imagename where the preview is located
in the Target you enter the name of the black image.
press ok
So you get a black image with your rectangle. Either you use the
rectangle that way or soften it with RangeMask,
or set it to binary white with PixelMath using the expression:
iif($T > 0, 1, $T) or set it to gray by using PixelMath with
iif($T > 0, 0.5, $T)
Hope this helps
Gerald