Capturing events in X11 even after the loss of focus

Question

I am trying to develop an application which responds to multiple digital pens (IRIS Pens) so that if any of the pen writes on paper; I relay the output to a single screen. Thus making a multi-input whiteboard for myself.

In Ubuntu these pens are recognized as mouse and thus can be handled in a similar manner as mouse events are handled.

So now what I plan to do is to handle these events in C/C++ using XLib and pass these events to a Java Swing application using JNI callback. I am able to do this but when the X11 window looses focus no events are transferred to the Swing frame. I also tried to use the root window in X11 but it does not seem to work.

Any help would be really appreciated. Thanking you in advance.

Answer 1

How about maximizing the C/X11 window in front of the Java one, and making it transparent? You should be able to see the Java window while still focusing on the C/X11 one.

Since you are using Ubuntu, you can achieve this using the "Opacity, brightness and saturation" plugin for Compiz. It is in the compiz-plugins-main package, and you can activate it with Compiz Settings Manager (from the compizconfig-settings-manager package). When you activate the plugin, alt+wheel is bound by default to change the transparency of the focused window.