I have a c++ program in which a gdk-pixbuf is created. I want to output it as an image, so I call gdk_pixbuf_save_to_stream(pixbuf,stream,type,NULL,&err,NULL). This works fine when "type" is png or tiff, but with jpeg or bmp it just produces a black square. The original pixbuf consists of black-on-transparent (and gdk_pixbuf_get_has_alpha returns true) so I'm guessing that the problem is with the alpha mask.
GdkPixbuf has a function to add an alpha channel, but I can't see one that removes it again, or (which might be as good) to invert it.
Is there a simple way to get the jpeg and bmp formats to work properly?
(I should say that I'm very new to proper programming like this.)