Thanks EAdams!
Yes, you're right about it. I will try to check the components one by one, to understand what component is creating the problem.
My first idea was to turn around the problem deforming the original picture to a square, but it was not so easy and anyway the result is not realistic.
I'll try to find a different solution.
As you surely remember, you told me of the same problem in another of my filters too.
There are many others ( mine and not mine ). So, let's see where this thing happens and I'll be back to you about it.