Fltk on BBB

Hi all, I’m working on a porting from a custom linux distribution to a BBB environment.
If I use the BBB environment with debian user interface I have no problem, otherwise if I use the BBB environment without user interface I recieve the error “Can’t open display”.
What packages I need to install to use fltk without user interface?
Thank you in advance for all support.

tOn Thu, 20 Sep 2018 05:35:53 -0700 (PDT), Destroy
<dima80destroy@gmail.com> declaimed the
following:

If I use the BBB environment with debian user interface I have no problem,
otherwise if I use the BBB environment without user interface I recieve the
error "Can't open display".
What packages I need to install to use fltk without user interface?

  According to Wikipedia, https://en.wikipedia.org/wiki/FLTK
"Fast Light Toolkit (FLTK, pronounced fulltick)[3] is a cross-platform
widget (graphical control element) library for graphical user interfaces
(GUIs)" -- it requires an underlying/compatible graphical system to be
available... So on Linux, that means some form of X-Window (Wikipedia says
Wayland is under development).

  At a minimum, that likely means xlib and an environment definition to
an x-server (display).

Thank you so much for the information, now I’m go to search information about these topic.
If anyone have other useful information are very appreciated.