Matrix on Multiscreen

i am going to use the multiscreen node to project with 2 projectors that will be placed at 90 degrees so I need to rotate both of the renderer.
The center of rotation is still the centre of the whole renderer, how would i make so that each screen gets its own real center as center of transformation?
Actually the rotation doesn t really bothers me too much since it is quite easy to put back in place the image with some transformation, but it is the homography transform that behave weirdly due to the new original points.
How can I fix that on th ehomography node?

sounds like the hidden Viewport Transform is what you need