Re: Pixmaps & transparency

["Brian J. Tarricone" <bjt23 cornell edu>]

Peter Zhelezniakov wrote:

> Hello GTK developers,
>
> I have an external library that draws possibly transparent/translucent 
> images to a GdkDrawable only. I want to capture these images in a 
> pixbuf, but when i try to copy pixels from the drawable (a pixmap), 
> all information about transparency is lost.

GdkPixmap is essentially a wrapper around X Pixmaps.  You won't be able 
to keep transparency info in an X Pixmap unless you're using X.org 6.8.x 
(and I believe you need the composite extension enabled to get the 
required ARGB visual).  Then you need to create the Pixmap (or 
GdkPixmap) using the correct 32-bit ARGB visual that supports transparency.

I suppose you could subclass GdkPixmap to add some private fields to 
somehow keep track of transparency data (a mask, probably), regardless 
of what visual the backing Pixmap uses.  I'm not sure that this would 
get you exactly what you want.  Likely you'd be much better off using 
GdkPixbuf to hang on to image data with transparency.  The downside, of 
course, is that GdkPixbufs are stored client-side rather than 
server-side like Pixmaps are, so you might suffer a performance hit 
depending on what you're doing.

   -brian