Some time ago Zach Lieberman posted code on the oF forums which allows you to show the pixels directly below the openFrameworks app window in a texture. This little app Satoru Higa (4nchor5 la6/rhizomatiks) uses the same method to create a music sequencer. Each time app is ran, the transparent window uses the desktop as the source for sound. The colour and the position of the colours below decides the pitch, tone and tempo. As you drag the window around the desktop or add additional ones you can further create compositions.
In the video below Satoru also shows what happens when you load a webpage underneath and use that as a source. Watch below.
To see more experiments by Satoru, see his GitHub.