Thanks a lot, Kristian, you are right! gdk_pixbuf_new_from_file helps to keep the alpha channel from being ruined. however my job not accomplished yet, now I can use gdk_pixbuf_composite do an overall alpha blending either by cairo_paint or gdk_draw_pixbuf .  But precisely what I need is to make alpha blending pixel by pixel, is there any api like gdk_pixbuf_composite without an overall_alpha parameter? Can cairo help to render an rgba pixbuf as my wish? Laowb <div>2006/6/30, Kristian Høgsberg <<a href="mailto:krh@bitplanet.net">krh@bitplanet.net</a>>:<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"> On 6/27/06, lao wb <<a href="mailto:laowenbo@gmail.com">laowenbo@gmail.com</a>> wrote: > Hi >    I am using GTK+ to do with my application. I have a bitmap without alpha > channel as my background, another bitmap with alpha channel to show above > through to see the background. At first, I tried to use gdk to do this, > still not succeed yet, always  show only the last one gdk_draw_pixbuf > invokes in the expose handler. To do this with cairo, you can say something like:   cr = gdk_cairo_create (widget->window);   gdk_cairo_set_source_pixbuf (cr, pixbuf, 0, 0);   cairo_paint (cr);   cairo_destroy (cr); But I suspect the problem is you're loading an xpm file to a drawable and converting that to a pixbuf.  Consider storing the overlay image as a PNG file and load it using gdk_pixbuf_new_from_file(). cheers, Kristian </blockquote></div>