Re: transparent glass layer over windows

["Timothy M. Shead" <tshead k-3d com>]

Havoc Pennington wrote:
> Martin Portman <mrp cre canon co uk> writes:
>
> > i want to put a transparent 'glass' frame over a window, to intercept
> > mouse events, possbily draw on the glass, and pass mouse events on to
> > what's below the glass.
> >
> > think of a gesture recogniser, receiving mouse events/signals, drawing
> > the stroke, but sending key events down, and removing the stroke.
> >
> > in java i'd use the glassPane part of root panes to do this.
> >
> > how do i do this using gtk?   
>
> To intercept events you could use a GDK_INPUT_ONLY GdkWindow, which is
> invisible. However, there is no particularly good way to implement
> drawing on the pane in GTK. (To support it cleanly GTK would need to work
> more like Java/Swing or GnomeCanvas, and avoid having any X windows
> inside the GTK toplevel window.)
>
> The best hack I can think of is to draw your stroke on the GtkWindow
> using GDK_INCLUDE_INFERIORS for the subwindow mode, then to clear the
> stroke call gtk_widget_queue_draw() on the whole window. But this will
> break if any widget happens to redraw during the stroke.

SDPGTK, a C++ wrapper for GTK+, provides the sdpGtkScreenOverlay class 
for doing this - internally, it is implemented as a drawing area inside 
a top-level window, with a mask bitmap which controls the transparency 
of the drawing area through a call to gdk_window_shape_combine_mask(). 
Separate GCs are maintained, so you can draw on the drawing area or the 
mask.  I use it to provide an animated pen-stroke effect (much like 
MacOS help, if you're familiar) for highlighting widgets in tutorials. 
See sdpGtkWidget::InteractiveHighlight() for the code.

http://www.k-3d.com

Regards,
Timothy M. Shead