rectangles and squares . . .

Yes, it is true that some filters work right ONLY with square images, and if used with rectangular images, the part that gets outside of the square gets deformed or acts weirdly sometimes.

I do not think that normally the filter can be changed to be able to use it on non-square images, as probably the problem is not of the filter, and is of the effect that gives and the components used that must have a square photo.

Although I am not any expert in filter forge, so really I really do not know if it would be possible or not, probably depends on what filter you want to use.

Lets see what other real experts can say about this