Non-squared pictures and canvas resizing

I think that almost all of us had, using or making a filter, to face the problem that the filter perfectly worked on squared images ( 1x1, 2x2, 3x3, etc. ), but not on non-squared rectangular images ( 2x1, 3x1, 3x2, etc. ).

In the library there are many filters having this trouble, and giving different undesired effects.

The most typical case is when the filter splits the rectangular image in "a square + a piece of square".
It applies the effect on the first square. Then, instead to create a smooth connection with the first square, it abruptly restart the effect in the remaining space without even doing anything complete ( because the remaining space is not a square ).

Other times the filter smoothly continues in the remaining space but ( for different reasons ) without generating a complete effect.
I show the case of one of my last filters :



This and other filters trouble could be solved easily if we could resize the image canvas to a square area.
Example : having a 3x2 image, we cut away 1/3 of the image, obtaining a 2x2 picture.

I tried to find a solution, but I only succeeded to cut away an area of the picture, not of the canvas.
The result is still a 3x2 image, where the first 2x2 area contains the picture and the remaining 1x2 area is empty.
That empty area should disappear, but how?