Draw in a GdkPixbuf

[Luis Menina <liberforce freeside fr>]

Hi list!

I'm trying to find a way to draw in a GdkPixbuf (for example draw a 
rectangle on the pixbuf) and save the result to a file afterwards.

I found in devhelp the gdk_draw_* functions, but to use them I need a 
GdkPixmap. gdk_pixmap_new requires a GdkDrawable. The problem is that 
I'm developping a command line application, so I don't use GTK, only 
glib, gdk, and gdk-pixbuf. Even creating the GdkPixmap with a NULL 
GdkDrawable, I can't workaround the problem, because the documentation 
of gdk_draw_pixbuf tells me that "All windows have a colormap, however, 
pixmaps only have colormap by default if they were created with a 
non-NULL window argument. Otherwise a colormap must be set on them with 
gdk_drawable_set_colormap()". Trying to get a colormap requires other 
stuff, which requires other stuff, and so on...

As this seems to be quite old-fashioned way of doing things, I tried to 
see if cairo could do the trick. But it seems there's no way to draw to 
a GdkPixbuf with cairo (I'm unfamiliar with cairo).

So, does anyone have an hint on how to draw and save an image from a 
GdkPixbuf, with gdk or cairo ?

Thanks for you help,

Luis