On Thu, Apr 6, 2017, at 09:57, Stefan van der Walt wrote:
On Wed, Apr 5, 2017, at 19:02, Serge Shakhov wrote:
 Just want to clarify. Am I right that your solution isn't applicable to PiecewiseAffineTransform at all?
Do you think it may be possible to find a workaround?

Sorry, I should have been clearer.  With "defined" I meant implemented.

Upon closer inspection, it turns out that the PiecewiseAffineTransform *does* have an inverse implemented.  It was the "+" operator causing the problem.

Still, I have found cases now where I get invalid answers for valid-seeming sets of coordinates.  I'm investigating further, but I think I have a solution to your problem.

Stéfan