[gtkmm] Why gdk::Pixbuf makes every image to 24bit or 32 bit ?

[Ramachandra Putti <ramachandrap dpsl net>]

Hi ,
   I am currently working on a Image Processing tool. I am using
Gnome::Canvas to display the image with the help of pixbuf.
   The situation in which I am strucked is on creating Pixbuf from file
or data , even though I have 1 bit image, gdk:: pixbuf will
   convert it into 24 bit image or 32 bit image.

        Glib::RefPtr<Gdk::Pixbuf>  pixBuf;  
        pixBuf = Gdk::Pixbuf::create_from_file( fileName );
 
        Glib::RefPtr<Gdk::Pixbuf> newpicture;
        
        newpicture = Gdk::Pixbuf::create(Gdk::COLORSPACE_RGB, true, 1,
width, height);   //  This to be checked ???
        newpicture =  Gdk::Pixbuf::create_subpixbuf (pixBuf, ax1, ay1,
width, height);
  
          I have to implement one feature which changes 24 bit image to
 1 bit , 8 bit , 32 bit and vice versa. How can I do that ?
     Question is why gdk::pixbuf changes every image into 24 or 32 bit ?
don't it makes image manipulation slow too ?

 Any help will be appreciated.

Regards ,
Ramachandra