gdk:window-set-opacity (window opacity)

window -- a toplevel gdk:window object
opacity -- a double float for the opacity

Request the windowing system to make window partially transparent, with opacity 0 being fully transparent and 1 fully opaque. Values of the opacity parameter are clamped to the [0,1] range.

On X11, this works only on X screens with a compositing manager running.

For setting up per-pixel alpha, see the gdk:screen-rgba-visual function. For making non-toplevel windows translucent, see the gdk:window-composited function.