Mapio has a perspective correction so it compensated for angle change, there are a few other options for adjusting linearity in the projected image. I'm sure
As [jhoepffner](http://troikatronix.com/troikatronixforum/profile/15/jhoepffner) suggested in isadora, I use the x and y rotation first so that you can have equidistant vertical and horizontal lines, then adjust corners. It's not perfect, and you may need to offset the anchor point of the plane, but it is doable. I was able to do projector blending with setups like that, it's just takes a bit of time. h