Ask HN: How to transform/distort screen in Linux Ubuntu?

I have a projector, which doesn't come with horizontal keystone correction built in, only vertical correction.

I've seen that it is possible to do it with xrandr, using the --transform parameter, but needs matrix coordinates and I'm lost. That deserves a GUI, but ARandR doesn't support it.

I've also checked MapMap project, but seems that v4l2 virtual video device is not well recognized to use it as source.

Do you know any other alternative to transform the screen?