For OpenGL-based images, you're limited by the maximum dimensions of your display. Hopefully that restriction will be eliminated before too much longer...
Actually the restriction is how big a window your window manager supports,
which can be bigger than your display. The png commands still captures the
entire window even if it's not entirely visible.
I was able to get some big pictures using this trick in window maker.
--
All generalizations are false, including this one.
-- Mark Twain
